ORNL-TM-3884.txt

 

ORNL-TM-3884

 

THE MIGRATION OF A CLASS OF
FISSION PRODUCTS (NOBLE METALS) IN
THE MOLTEN-SALT REACTOR EXPERIMENT

R. J. Kedl

  

 

 

 

 
 

 

 

 

 

This report was prepared as an account of work sponscred by the United
States Government. Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights.

 

 

 
 

   

. NOTICE
This report was prepared as an sccount of work
sponsored by the United States Government, Nejther
the United States nor the United States Atomic Energy
& } Commission, nor any of their employees, not any of
their contractors, |ubcontrsctoril. 1; ;hek employees, |
makes any warranty, express or implied, or assumes any |
Jegal liability or responsibility for the aceuracy, com- | ORNL—TM—3881£
i‘:‘ pleteness or usefulness of any information, apparatus, |
_ product or process disclosed, or represents that its use
would not infringe privately owned rights, -
-

 

Contract No. W-T405-eng-26

Reactor Division

THE MIGRATION OF A CLASS OF FISSION PRODUCTS (NOBLE METALS)
IN THE MOLTEN-SALT REACTOR EXPERIMENT

R. J. Kedl

Molten-Salt Reactor Program

December 1972

OAX RIDGE NATIONAL LABORATCRY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION

ap"

; ‘ j For the
{ _ U.5. ATOMIC ENERGY COMMISSION

 

DISTRIBUTION OF THIS DOCUMENT IS UNUIMITED

-

 

 
 

 

 

 

 
 

@ 4 (

FOREWORD

ABSTRACT

1.

2.

N

L,

dy)

INTRODUCTION

 

DESCRIPTION OF THE MSRE

iii

TABLE OF CONTENTS

2.1 General Description

2.2 Description of the Reactor

2.3 Description of the Fuel Pump

FISSION PRODUCT EXPERIENCE IN THE MSRE

3.1 General Fission Product Disposition

3.2 Fission Product Disposition Measurements

3.2.1
3.2.2

3.2.3

3.3 The Difference Between the

Fuel Salt and Gas Phase Samples

Gamma Spectrometry of the Primary
Heat Exchanger

Core Surveillance Samples

ANALYTICAL MODEL

4.1 Physical Basis of Model

4.2 Analytical Model

L.2.1
- h,2.2
L.2.3
.2k
h.2.5
h.2.6
h.2.7
4,2.8
L.2.9
L.2.10

Generation
Generation
Decay Rate
Deposition
Deposition
Deposition

Deposition

235 233

U and U Runs

from Fission

from Decay of Precursor

Rate
Rate
Rate
Rate

Equation for CS
Noble Metals on
Noble Metals on

on Heat Exchanger
on Graphite
on Rest of Fuel Loop:

on Liquid-Gas Interfaces

Solid Surfaces

Liquid-Gas Interfaces

1k

1L
17
17
19

21
23

29

29
31
32
32
33
33
33
33
3k
3L
35
36

 
 

iv.

5. RESULTS FROM THE MSRE

5.1 Introduction

5.2 (Comparison of Measured Deposition on Heat
- Exchanger to Theoretical

5.3 Comparison of Measured Deposition on Core Surveillance
Samples to Theoretlcal

5.4 Comparison of Fuel Salt Samples with c®
5.5 Comparison of Gas Samples with c®
5.6 Time Constant for Noble Metals on Interfaces
S.Tr.Miscellaheous Noble Metal Observations
5.7.1 Leminar Flow Core Surveillance Sample
5.7.2 Noble Metal Distribution in the MSRE

6. CONCLUSIONS AND RECOMMENDATTIONS

Conclusions

Recommendations
T. REFERENCES
APPENDIX A

AP?ENDIX B

43

18
61
65
68
68

- T3

T3
Th

76
79
&,

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FOREWORD

The behavior of the ™oble metal"” fission products in the fuel salt
is a subject of major importance to the design of molten-salt reactors.
A considerable amount of data bearing on the subject was obtained from
operation of the Molten-Salt Reactor Experiment, All or part of the data
has been studied with various degrees of thoroughness by many people with
different backgrounds and different viewpoints. Unfortunately, the chemi-
cal and physical situations in the reactor were complex and the data ob-
tained are not very accurate or consistent. Consequently, no one has de-
veloped an explanation of the detalled behavior of noble metals that is
acceptable to a majority of the knowledgeable observers. It is generally

agreed that most of the fission products from niobium through tellurium

are reduced to metals in the fuel salt, that they migrate to metal and

graphite surfaces and to salt-gas interfaces, and that they adhere to the
surfacés with varying degrees of tenacity. The details of the processes
involved and the manner in which the noble metal particles may be affected
by other processes in the reactof are subjects of frequent debate in which
opinions vary widely. This report describes the author's interpretation of
the data and explanation of some aspects of the operation of the reactor.
Although others would analyze the data differently and would reach differ-
ent conclusions concerning some of the mechanics,.we believe that publica-
tion of this report will provide information helpful to the design of
molten-salt reactor systems and to the development of a better understand-

ing of the behavior of fission products in those systems.

 
 

vi
ABSTRACT

The Molten-Salt Reactor Experiment (MSRE) is a fluid-fueled experi-
mental nuclear reactor; consequently, fission products are dispersed
throughout the entire fuel circulation system. One group of fission
products, referred to as noble metals, exists in the fuel salt in the
reduced metallic state. They are insoluble and unwet by salt. They de-

posit on surfaces exposed to salt such as the Hastelloy_N-piping,and the

moderator graphite. They appérently accumulate in a stable form on the

liquid-gas interface in the fuel pump. The amounts of noble metals on

these surfaces and on other deposition sites have been measured. These

‘measurements have been analyzed within the framework of mass transfer

theory. The analysis has been found to correlate the data from these

gsources in a unified manner. It is therefore concluded that the noble

~ metals do migrate in accordance with mass transfer theory, although some

parameters still remain unevaluated. A hypothesis is presented to ex-
piain some of the dramatic differences in reactor operating characteris-
tics between the ??°U and ?33U runs. It is recommended that additional
noble metal deposition experiments be conducted in a circulating salt

loop.

Keywords: Fission Products + Noble Metals + Mass Transfer
+ MSRE + Experienée + Bubbles + Foaming + Mist + Physical Properties
+ Entrainment + Off-Gas System + Void Fractions + Corrosion Products
+ Fluid Flow

 
 

 

 

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1. INTRODUCTION

The goal of the Molten-Salt Reactor Program at Oak Ridge National
Laboratory is to develop the technology for an efficient power producing,
thermal-breeding (based on the Th-233y cycle) nuclear reactor for commer-

cial use., The fuel in a molten-salt breeder reactor is fluid, and con-

- sists of UF, and ThF, dissolved in a carrier salt mixture of LiF and BeF>.

The liquidus temperature ranges from 800 to 9406F, depending on the exact
carrier salt composition, with the nominal reactor operating temperature
being ~1200°F. The fuel salt is pumped through a graphite-moderated core
and then through a heat exchanger where it transfers heat to a secondary
salt system, which then'genérates steam in another heat exchanger. One of
the unique features of this concept is that the fuel is in the liquid
state, This gives rise to many advantages, the principal one being poten-
tially very low fuel cycle cost. The fluid fuel also creates a few lia-
bilities, one of which is that the fission products are spread throughout
the entire fuel loop and other hydraulically connected regions (e.g., off-
gas system, dump tanks, etc.).

A 7.3-MW(t) experimental reactor based on this concept was built and
operated at Oak Ridge National ILaboratory. This reactor, the Molten-Salt
Reactor Experiment (hereafter called the MSRE), first went critical in
June 1965, Nuclear operations wére terminated in December 1969. Being
an experimental reactor, it was subjected to a good deal of testing and
observation. One of the principal efforts was to determine the distribu-
tion of the various fission products in the fuel loop and connected re-
gions, This-information is critical in the design of large central power
stations [1000 MW(e)] where the heat geherated by fission products is sub-
stantial. _ , o

Fission products in molten fuel salt'can be grouped intb three prin-

cipal types where .the mechanics of migration is the diétinguishing fea-

ture — (1) salt seekers, (2) noble gases, and (3) noble metals, The salt-

seeking fission products (which include Sr, Y, Zr, I, Cs, Ba, and Ce) are

- the best behaved. They are soluble in a fuel salt and remain with the

fuel salt in inventory amounts. The nobie gases are Kr and Xe, A great

 
 

 

2 —

deal of work has been done to understand ndblé gas migration; particu-
larly 135Xe because of its thermal neutron cross section of over 106
barns. The third group, the so-called "noble metals " Nb, Mo, Ru, 5b,

and Te, is the subject qf this report. The noble metals are reduced by
the UF3 in the fuel salt, and therefore exist in salt in the metallic
state, They are insoluble in fuel salt and are unwet by it. Because of
their incompatibility with salt they migrate to various surfaces (graph-
ite and Hastelloy N) and adhere to them, They apparently also migrate to
_gas-liquid interfaces and adhere to these in a stable manner. Noble met-
als have been found and measured in fuel salt samples and gas phase sam-
.ples on the surfaces of Hastelloy N and graphite surveillance specimens
in the core, and on the fuel loop and heat exchanger surfaces. In this
vreport we shall present a theory of noble metal migration based on conven-
tional mass transfer concepts. We shall then analyze data from the above
mentioned samples and measurements in the framework of this theory, and
show that noble metals apparently do migrate from the fuel salt to their
various depositories in accordénce_with the theory. A major thesis of ¥
this analysis is that noble metals migrate and adhere to liquid-gaé inter-
faces, As such, they apparently have properties similar to insoluble sur-
face active agents. This idea will be used to explain many of the obser-
vations on fission product behavior in the reactor. It will also be used
to suggest an explanation for the rather dramatic difference in reactor

operating characteristics between runs made with 235U and 233U fuels.

2. DESCRIPTION OF THE MSRE

2.1 General Description

 

The purpose of the MSRE was to demonstrate, on a pilot plant scale,
the safety, reliability, and maintainability of a molten-salt reactor. -
The operating power level was 7.3 MW(t). Because of its small size and
other considerations, it was not intended to be a breeder, and no thorifim
was added to the fuel. The fuel consisted of UF,; and UF; dissolved in a

mixture of LiF, BeF,, and ZrF,. Its composition and physical proPertiés

-

are given in Table 2.1. All fuel loop components are constructed from " ;

 
 

 

 

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Table 2.1. MSRE Fuel Salt Composition and Physical Properties

Composition — LiF, BeF;, ZrF,, UF,
(65.0, 29.1, 5.0, 0.9 mole %)
Liquid type — Newtonian

 

 

English Units ' Metric Units
Liquidus temperature g13°F 434°C
Properties at 1200°F (650°C)
Density - 141 1b/ft3 2.3 g/cm?
Specific heat 0.47 Btu/1b«°F 2.0 x 103 J/kg+°C
Thermal conductivity 0.83 Btu/hreft+°F  1.43 W/m+°C
Viscosity 19 1b/ftehr 28 kg/h°m
Vapor pressure <0.1 mm-Hg <1 X 10™% bar

 

80.3 mole % 25U and 0.6 mole % 238y,

Hastelloy N, which is essentially unwet by fuel salt under normal operat-
ing conditions. The'nominal operating temperature was 1200°F with a 40°F
temperature change.across_the core and primary heat exchanger.

Figure 2.1 ié a schematic flow diagram of the MSRE, which will be
described briefly here. More detailed descriptions of the reactor and
the concept are available in Refs. 1, 2, and 3, The fuel loop consisted
essentially of a centrifugal pump, a heat exchanger, and the reactor
vessel. The nominal flow rate was 1200 gpm. The heat exchanger was a
conventional U-tube type with the fuel salt on the shell side. Heat was
transferred to a secondary coolant salt that in turn dumped it to the
atmosphere via a large radiator."Dfiring periods of shutdown, the fuel
salt was drained into either of two drain tenks. In addition, a third

“drain tank contained a load of flush sait for rinsing the fuel loop pridr

to any maintenance that was required. Note the use of freeze flanges in
the priméry and secondary salt loops for easy disconnection of main compo-
nents should they need replacement, and also the use of freeze valves in

the drain lines. Off-gas from the pump bowl passed through & volume holdup,

" charcoal beds, and then absolute filters before it was discharged up the

stack. Characteristics of the reactor core and fuel pump influenced the

 
OFF-GAS
HOLOUP

ABSOLUTE
FILTERS

 

 

STACK  FAN b et etagessas

 

 

=
w
<
w0
=
m
x

 

 

 

 

SAMPLE
ENRICHER PumP

 

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M

 

 

 

 

WA'I"ER ST

 

 

 

 

 

FIGURE 2.1. SCHEMATIC FLOW DIAGRAM OF THE MSRE

 
    
    
 
  
 
  
 
   
   

ORNL-DWG 65-H410R

LEGEND
S FUEL SALT
wmrve— COOLANT SALY .
............. HELIWM COVER GAS
mwem = RADIOACTIVE OFF -GAS

SAMPLER

TO
ABSOLUTE
FILTERS

COOLANT
ORAIN
TANK

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. SODtUM
FLUORIDE BED

 

 
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mechanics of noble metal fission product migration, so they will be des-
cribed in somewhat more detail in the following sections.

The MSRE went critical June 1, 1965, and nuclear operations were
terminated December 12, 1969. The reactor was critical for a total of
17,655 hr. Figure 2.2 is a brief histérical outline of the MSRE's power
operation. Note particularly that for the first 2 1/2 years the reactor
was operated with 23°U fuel. The fuel was chemically.processed and the
235y replaced with 233y, Then for the last 1 1/2 years, the reactor was
operated with 233U fuel, Most of the results presented in this report

will be from data obtained during the 233U rums.

_ 2.2 Description of the Reactor

A detailed view of the MSRE core and reactor vessel is shown in
Figure 2.3.  Fuel éalt entered the reactor vessel through a flow distribu-
tion volute near the top of the vessel. It then flowed down through a
l—in.—thick annular passage bounded by the reactor vessel and reactor
core can and into the lower vessel plenum. The fuel then passed up
through the graphite moderator region and out the top outlet pipe. The
moderator assembly was composed of graphite stringers about 5 ft long and
2 in. square. The stringers had grooves cut longitudinally in the four
faces, so that when the stringers were stacked together vertically, the
grooves formed the fuel channels. The graphite was grade CGB (trade name
of Union Carbide Corporation). It was unclad, in intimate contact with
the fuel, and unwet by fuel salt under normsl MSRE operating conditions.
In the bulk of the fuel channels (95 percent of them), the fuel salt
velocity was about 0.7 ft/sec,_yielding a Reynolds number of about 1000.

. The entrafice_to-the fuel channels through the moderator'support grid

structure was rather tortuous and turbulence was generated that persisted
for some distance up into the fuel channels. Nevertheless, the flcw‘is
thought to have béen eésenfiially laminar ifi most of the length of the
fuel channel. ' | | | | L

Located near thercenter line of the core in a square array was an
arrangement of three control rods_afid 6ne surveillance specimen holder.
Details of the specimen holder are shown in Figure 2.k, It fias positioned
vertically in the reactor and extended the entire height of the moderator

region. At times when the reactor was shut down and drained, the

 
 

 

FUEL POWER (Mw)

REPLACE VALVES
AND FILTERS

6 l RAISE POWER
REPAIR SAMPLER
ATTAN FLL POWER

CHECK CONTAINMENT

   
  

,5 FULL - POWER RUN

TEST CONTAINMENT

n .....9 ROD QUT OFFGAS LINE
CHECK CONTAINMENT

i‘ 10 Aragu.d:yPguwNER

}RE?LACEAIRLNE

ll SUSTAINED OPERATION
AT HIGH POWER

} TEST CONTAINMENT

FLUSHI':]

' =— MAIN BLOWER FALURE

REPLACE MAIN BLOWER
MELT SALT FROM GAS LINES
REPLACE CORE SAMPLES

 

f 8 RUN WITH ONE BLOWER
‘ > WNSTALL SECOND BLOWER

REPLACE CORE SAMPLES

 

 

 

 

0 2 46 8 ¥

POWER (Mw)}

 

 

ORNL-DWG 69 - 7293R2

XENON STRIPPING
EXPERIMENTS

1] WSPECTION AND
MAINTENANCE

RE{‘LACE CORE SAMPLES

TEST AND MODIFY
FLUORINE DISPOSAL
SYSTEM

PROCESS FLUSH SALT
PROCESS FUEL SALY

LOAD URANIUM =233
REMOVE LOADING DEVICE

233y ZER0 - POWER
PHYSICS EXPERIMENTS
INVESTIGATE FUEL

SALT BEMAVIOR
CLEAR OFFGAS LINES

REPAIR SAMPLER AND
CONTROL ROD DRIVE

233); DYNAMICS TESTS
INVESTIGATE GAS

N FUEL LOOP

HIGH-POWER OPERATION
TO MEASURE 233y o, /g

ANVESTIGATE COVER GAS,

XENON, AND FISSION
PRODUCT BEHAVIOR

ADD PLUTONIUM
IRRADIATE ENCAPSULATED U

MAP F.P. DEPOSITION WITH
GAMMA SPECTROMETER
MEASURE TRITIUM,
SAMPLE FUEL

REMOVE CORE ARRAY
PUT REACTOR IN STANDEBY

FIGURE 2.2, HISTORICA.L OUTLINE OF MSRE'S OPERATION

L ¥

 
 

 

 

 

 
 
   
     
 

  
    
    
 
 
   
    
  
  

  
 
 

  
  
  

  
 

  
  

 

 

 
    
 
 
 

 
 
  

2
ORNL-LR-DWG GI097R1A
-
¢
_ FLEXIBLE CONDUIT TO
GRAPHITE SAMPLE ACCESS PORT /; sy CONTROL ROD DRIVES
’ COOLING AIR LINES
. ACCESS PORT COOLING JACKETS
FUEL OUTLET R it REACTOR ACCESS PORT
CORE ROD THIMBLES A »-!‘«, figfuaED%mmggg SAMPLES
LARGE GRAPHITE SAMPLES il |'15 OUTLET STRAINER
CORE CENTERING GRID A ]
FLOW DISTRIBUTOR
VOLUTE
s
GRAPHITE ~ MODERATOR
STRINGER
R
-
FUEL INLET -/ :
,_ ~— CORE WALL COOLING ANNULUS
REACTOR CORE CAN ] '
REACTOR VESSEL —
TR
) fl"\“’l-ll . ‘\ Q
.511 r" $
\ > \f‘ .
ANTI-SWIRL VANES 4 e
_ DERATOR
VESSEL DRAIN LINE ggpFEORRT GRID
FIGURE 2.3. MSRE CORE AND REACTOR VESSEL
S

 
 

ORNL-DWS 68-12146R

TOP GUIDE

~HASTELLOY N TENSILE
SPECIMENS ROD

  
 
 
 
 
 
   
 
 
 
  

_ENTIRE SPECIMEN ASSEMBLY

IS ENCLOSED IN A PERFORATED
4 BASKET AS SHOWN IN PLAN
™ VIEW A-A

FLUX MONITOR TUBE -

///1/81m 0.D.
7”7

   
   

 

PERFORATED BASKET
3/8 din. DIAMETER HOLES
ON 9/16 in. CENTERS
TRIANGULAR PITCH

 

 

 

 

 

2 in.
TYPICAL

 

 

 

 

 

 

 

 

 

OUTLINE OF GRAPHITE
CHANNEL CONTAINING CORE
SURVEILLANCE SAMPLER

 

LOWER GUIDE

G-GRAPHITE SPECIMENS
N-HASTELLOY N TENSILE
SPECIMENS ROD

SECTION A-A

INCLUDING BASKET AND
SHOWING FLOW AREA

FIGU?E 2.4. MSRE CORE SURVEILLANCE SPECIMENS
AND CROSS SECTION OF FLOW GEOMETRY

 
 

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9

surveillance specimen holder was removed from the core and taken to a
hot cell where the samples could be removed. Samples consisted of
graphite specimens, Hastelloy-N rods for tensile strength tests, and
Hastelloy-N flux monitor tubes. The specimen assembly was made so that
only part of the samples need to be removed, and the remaining samples
along with new replacements could be reinserted into the reactor for
further irradiation. Fission product deposition measurements were then
made on the graphite and Hastelloy-N specimens, which had been removed

from the sample holder.

2.3 Descriptioh of the Fuel Pump

The fuel pump turned out to be a very important component in under-
standing noble metal migfation in the MSRE. A detail drawing of the pump
is shown in Figure 2.5. Its rated capacity is 1200 gpm of fuel salt at
418.5 ft of head. The volute is completely enclosed in another vessel
referred to as the pump bowl, which served a variety of purposes. It
contained the only free liquid surface in the system and therefore
served as an expansion volume for salt; It also contained the 135%e
stripping system, a salt level indica@or, and‘thé‘fuel salt sampling
facility. The fuel expansion capacity of the pump bowl was more than
adequate for the normal operating rangés of the MSRE, but was not adequate
for some postulated accident conditions. Thérefore, the pump bowl was
provided with an overflow tank that would fi;l if the salt level in the
pump bowl reached the level of'the.ovérflcw pipe. The normal operating
helium pressure in the pump bcfil, which was dléo the pump suction pres-
sure, was about 5 psig. [ - B |

The xenon stripper was a gas-liqui& contacting device. A toroidal
spray ring containing many small holes'(1h6 - 1/8 in. holes, and 145 -
1/16 in. holes), sPr&yed:the salt through the gés-phase.' The salt flow,
which resulted in a mesn jet velocity of T.2 ft/sec, was estimated (not
measured) to bé at a rate of about 50 gpm. The xenon rich cover gas was
purged from the pump bowl by a continuous flow of clean helium from
(1) the pump shaft purge, (2) two bubbler level indicators, and (3) the
reference pressure line purge for the bubblers. The purge gas then went
to the off-gas system described earlier. The overflow tank also had a

bubbler level indicator and its purge also went to the off-gas system.

 
 

10

. ORNL-OWG €9-10172R

 

 

 

      

 

 

 

 

 

 

 

gHAFT
URGE
aTs
OFF-GAS ' .
LINE 1 BUBBLER LEVEL
: O inorcaToR
["U Xe STRIPPER
100 (B W SPRAY RiNG
v Q] A [T5 RO
wof|l__ " , [ _\\g":”
40 N o ’ - I e
2 W 3O | -;").l A
3 0 o x 7
SAMPLE = LEVEL Sl L2 DISCHARGE
CAPSULE SCALE PUMP A _
CAGE (%) BOWL O < SALT
L, <— RADIOACTIVE GAS
SUCTION <= CLZAN GAS
)
f 7\
PUMP
SUCTION
PIPE

OVERFLOW TANK

 

 

 

 

 

JL

DIMPLE_A

 

\UJ__ \_/

 

 

 

 

FIGURE 2.5. MSRE FUEL PUMP AND OVERFLOW TANK

 
 

 

1

a0

i

‘were measured in this region.

11

The spray jets impinging on the salt surface generated large amounts of
helium bubbles and fluid turbulence in the pump bowl., These bubbles

manifested themselves in several ways in the operation of the MSRE. 1In

- one sense their présence was unfortunate because it complicated the under-

standing of many observations in the reactor, but in another sense it was
fortunate because it led to the suggestion of efficient ways of removing

fission products (particularly noble gases, but possibly also noble

‘metals) from future molten-salt reactor systems.

Let us examine the behavior of gas bubbles in the salt in the pump
bowl by first considering the carryunder of bubbles. Note from Figure 2.5
that there are two bubblers and they are at different depths. By differ-

ence in reading between the bubblers, one can deduce information on the

average fluid density between them. Void fractions as high as 18 percent

(&)

and would rise to the surface, but some of them were small and would be

Certainly most of the bubbles were large

carried down into the pump suction to circulate with the fuel salt.
Estimates of the size of the small bubbles indicate they were less than
0.010 in. in diameter. The amounts of bubbles circulating with fuel salt
could be estimated by analysis of (1) level changes in the pump bowl,

(2) nuclear reactivity balances in the core, (3) sudden pressure release
tests, (4) small induced pressure perturbations, and (5) other less
direct observations. The general conclusion from these analyses is that
the volume fraction of circulating bubbles in the fuel loop during the
233U runs was 0.0002 to 0;000h5,(h?5)and the void fraction during the
233y runs was 0.005 and 0.006;(h)A hypothesis is presented in Section 3.3

to explain this rather large change in circulating void fraction between

‘the 235U and 233U runs. The same hypothesis is used to explaln the dif-

ference in overflow rates discussed next.
Now consider the bubbles that rise to the surface in the pump bowl.
There are strong indications that they produced a froth with a high liquid

. content on - the salt surface. For instance, there was a constent flow of

fuel salt from the pump bowl to the overflow tank even though the indi~
cated salt level in the pump bowl was well below the overflow pipe. |
Periodically this salt was forced back into the pump bowl by pressuriz-

ing the overflow tank with helium. This transfer must have been due to

 
 

12

froth flowing over the lip of the overflow pipe, or to a mist of salt

drifting into the overflow pipe, or to both. Overflow rates during the
235U runs ranged from 0.4 to 1.5 1b/hr. During the 233U rums they ranged

- from L to 10 1b/hr with excursions up to TO 1b/hr.(h) A simplé comparison

u (W)

indicates that a rain of "more than the hardest torrential downpour
would have been required to match the lowest overflow rate during the

233y runs. A reasonable conclusion is that froth with a high liquid con-

tent flowing down the overflow pipe accounted for the high overflow rates.

There may also have been mist in the pump bowl. It is well known that the

mechanical action of a high velocity jet impinging on a liquid surface

- will generate a mist. Bursting bubbles are also known to produce mist.

There are physical indications from the reactor that a mist was present
in the pump bowl. PFor example, the off-gas line plugged periodically
vhich was probably due to freezing of salt from a mist. Strips-bf metal
suspended in the gas phase of the pump bowl were covered with small -drop-
lets when retrieved (see Figure I of Ref. 4). Later when we discuss the
analysis 6f gas samples taken from the pump bowl, we will explain the
results by theorizing that a salt mist was probably being sampled. .

‘A facility was provided for taking salt samples from the bowl of the

fuel pump. This sampling facility was used for a variety of purposes

including (1) taking fuel salt samples, (2) teking gas phase samples,

- (3) adding uranium to the fuel salt, (4) adding chemicals to the fuel

‘salt to control its oxidation-reduction state, and (5) exposing materials

to fuel salt for short periods of time. The sampling facility was quite

complex,'&nd included.a dry box, isolation valves, remote handling gear,

- shielding, instrumentation, etc. A detailed description of the entire

'facility is not necessary for the purposes of this report,'and only the

sample station in the pump bowl is shown in Figure 2.6. Note that the
sample station is enclosed by an overlapping shield arrangement. It was
desired to take the salt and gas samples under rather quiescent con-
ditions, and this shield was intended to prevent the mist, foam, and
fluid turbulence generated by the 5pray ring from penétrating too aggres-
sively into the sampling region. The shield does not, however, prevent
free movement of salt through the sample region. The overlapping portiofi
of the shield is not closed, and also, the shield is elevated.off the

s ]

 
 

 

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NORMAL OPERATING
SALT LEVEL -

CAGE FOR

SAMPLE CAPSULE——

 

FIGURE 2.6.

 

 

'¢?:=flr--

Di

Taa.

'

%

 

 

Y

~3 I~ SHIELD

13

ORNL-DWG 67-10766R

SAMPLE TRANSPORT CABLE

TRANSPORT PIPE

LATCH ASSEMBLY NORMAL POSITION
TOP OF FUEL PUMP

  

//SAMPLE

W CAPSULE ==
A CanLE fi¢i;;;£;;5§§:»-CAPSULE

it
! 00 i
1

CAPSULEQAQXQ;;L' Y

~SALT INTAKE
CAGE" S TN O

1 SHIELD SECTION A-A
T‘SAMPLE CAPSULE (LADLE TYPE)

 

 

3/16 in. MAX |
;{ 1001 2 3 4

- INCHES-

PUMP BOWL SAMPLE.STATION,

 
 

1L

bottom of the pump bowl. The sample capsule shown is typical of thg first
kind of capsulés used in the MSRE. It is referred to as a "ladle
capsule." It is simply a container with open ports in the side. During
normal éampling5 it was lowered with the sample transport cable until it
was well below thé salt surface. After filling, it was lifted out of the
salt and suspended for & period of time a short distance up in the trans-
port pipe in order to freeze the salt. It was then withdrawn, isolated
from the fuel system, and taken to a hot cell for analeis. During the
sampling period, a small purge of helium was maintained down the trans-
port pipe. Fig. 2.7 shows other sample capsules used which will be des-

cribed lsater.

3. FISSION PRODUCT EXPERIENCE IN THE MSRE
3.1 General Fission Product Disposition

In this section we will discuss in a qualitative way the general .
disposition of fission products in the MSRE, and the techniques used to
measure this disposition. The emphasis®will be on noble metals. We will
also discuss the rather dramatic differences in the reactor operating
characteristics when fueled with 235U end 233U, and propose a reason for
this difference,

A systematic way of classifying fission products in the MSRE, based
on their migrational characteristics, is as follows:

(1) salt seekers,

(2) noble gases, and

(3) noble metals.

As a group and under normal MSRE operating conditions, the salt
seekers are the best behaved of all fission products. Examples of salt '
seekers are Sr, Y, Zr, I, Cs, Ba, Ce and Nd. Unless affected by
migrational characteristics of their precursors, they remain dissolved
in the fuel salt in inventory quantities. Consider the following gen-
eralized beta decay scheme of fission pioducts:

2) 1) @ @ @ & (3
*r - Rb - S > Y »> Zr > Nb = Mo =

(3)  (3)  (3)  (3) (3) (&) (1)
Te - Ru - Rh - Pd~» Ag - C4d > In -~

 
 

 

 

 

-l

b

"

&

VOLUME

20cc-—'_\_i_.

 

N
w o Li, BeF,

 

15

ORNL~-DWG 67-4784 ' ‘ ORNL-DWG 68-6079

| .——STAINLESS STEEL
CABLE

   

AL ERLRRRY

/3/4-in.-oo NICKEL TUBE 0.020 -in. WALL

Tl

| _.—Y4~in. OD NICKEL

T N T S T T T T T T YT
TITLTTLTST

TTOTTTTINTS

TSI eI T T
TTLITT

 

 

| _— Yg-in. Ni TUBING

LT ITTRIY

 
   

 

 

 

/ NICKEL CAPILLARY
46 ~in. WELD ROD

S

 

—— FLARED CUP

{(a) Two Modifications of Freeze Valve Capsules

ORNL-DWG 70-6758

CUT FOR ,
SAMPLE REMOVAL - he—— NICKEL OUTER TUBE

s
' > VACUUM

- COPPER INNER CAPSULE

 

=

| BALL RETAINER

 

-BALL WITH SOFT SOLDER SEAL

 

 

 

 

\ l//
“|| ——COPPER NOZZLE TUBE

ABRADE FOR )
SAMPLE REMOVAL:

 

 

{b) Double-Wall Freeze Valve Capsule .

FIGURE 2.7. FREEZE VALVE SAMPLE CAPSULES

 
 

 

16

3y (3) (3 @) (&) (1) (1)

"Sn > Sb > Te > I » Xe -+ Cs -+ Ba =
(1) (1) (1) (1)
la » Ce -+ Pr - NdA -

where (1) - salt seeker,
(2)
(3)
(L)

noble gas,

noble metal, and

noble metal or salt seeker depending on
oxidation-reduction state of fuel salt.

Note that Kr (a noble gas) is a precursor of Rb, Sr, Y and Zr (salt
seekers) and also that Xe (a noble gas) is a precursor of Cs, Ba, La and
Ce (salt seekers). Also note that Sb and Te (noble metals)-ére precursors
of I (a salt seeker). The behavior of precursors of?éalt seekers may

dramatically affect the ultimate disposition of the salt seekers them-

' selves. TFor instance 2%Zr has a yield of sbout 6.2 percent but its pre-

cursor, 95Kr, has a yield of only 0.007 percent and is very short lived.
Therefore any migrational tendency of the 95Kr will have little effect

on the ?°Zr. Indeed the complete 35zr inventory was found in the fuel
salt.(6) As opposed to this, consider the salt seeker 13705 with a
cumulative yield of 6.15 percent. Most of its yield comes from the decay
of 137Xe that has a yield of sbout 6.0 percent and a half life of

3.9 min. Xenon can be transferred to the off-gas system and also can
diffuse into the porous structure of the graphite. Accordingly, only

80 to 90 percent of the 137Cs inventory was found in the fuel salt(

and significant quantities were found deep inside the graphite where.it

(7,8)

deposited upon decay of its precursor.¥ In conclusion, the salt
seeking fission products are well behaved and remain dissolved'in the
circulating fuel salt in inventory quantities except when influenced
by the behavior of their precursors. | '

There are only two noble gas fissicfi products, Kr and Xe. They
appear, however, in over 30 mass number decay chains and therefore
significantly affect the general fission product disposition. Notable

among these fission products is 135%e with its large thermal neutron

 

¥The amownts and concentration profiles of salt seekers inside the
graphite have been correlated with theory quite well in Ref. (9) for

the case where the noble gas precursor is short lived.

 
 

 

 

n

wcl

-

o

K9

 

17

cross section of 3 x 106 barns. Because of 135Xe, a great deal of wofk
has gone into quantitgtively understanding noble gas migration in the
MSRE. This work has been reported in the semiannual reports and other
documents. The noble gases, particularly xenon, are very insoluble in
fuel salt; therefore, they are readily transported into any available
gas phase such as the circulating bubbles, the gas space in the pfifip
bowl, and the pores of the moderator graphite.

The third group of fission products is the so-called "noble metals."
They are reduced by UF3 in the fuel salt and exist in the reactor
environment in the metallic state, hence the name. Examples are Mo, Ru,
Ag, Sb, Te and sometimes Nb. If the fuel salt is in a well reduced
state, then the Nb exists as a noble metal, but if the fuel is more oxi-
dized, it exists as a salt seeker. Noble metals have been found throughé
out the entire reactor fuel salt loop. They have been found in large
gquantities in fuel salt and gas phase samples from the pump bowl. 'They
have been found in large quantities on the Hastelloy-N and graphite~core
surveillance samples, and on the primary'heat exchanger tube surfaces and -
loop piping surfaces. They have been found at various locations in the
off-gas system. In this report we will look quantitatively at the noble
metals in these depositories, within the framework of mass transfer
theory, and try to developla unified‘model of noble metal migration in the
MSRE.

3.2 TFission Product Disposition Measurements

—3;2.1 Fuel Salt and Gas Phase Sampies

Three typés of capsules were fised to obtain fuel salt samples from

the MSRE pump bowl sample station, and these are shown in Figures.216 and

2.T. Numerous other sampling devices were used for special tests but only
thoée_illustrated were used on a routine basis. The ladle capsule is the
simplest and was the fifst used for sampling fuel salt. This capsule is
illustrated in Fiéufie 2.6 in the process of taking a salt sample. It is

simply a small container with open ports on the side to allow salt to

enter. During the course of fuel salt sampling from the reactor, it was

found that s mist of salt, which was heavily contaminated with noble

metals, existed over the salt pool. Presumably this mist was generated by

 
 

 

18

the mechanical action of the spray impinging on the salt surface and by
bursting bubbles. When the ladles were lowered to take a salt sample,

the mist adhered to the capsule end grossly contaminated the sample.
"Freeze vg1ve“ capsules, which had previously been developed for taking
gas samples, were therefore used to take salt samples in the hope of
allevigting this problem. Two modifications of these freeze valve
capsuies are shown in Figure 2.Ta. Each capsule contains a vacuum which

is held by a frozen salt sesl around the capillary entrance tube, The
selt seal is designed to melt after a delay time long enough to allow the
capsule to be completely submerged in fuel salt before 1t opens. The
inside surface of the capsule is therefore not contamlnated W1th the salt
-mist when taking a fuel salt sample. Freeze valve capsules for taking
salt samples were used first in run 14. The noble metal concentration
measured in salt from these capsules was generally'abofit two orders of
magnitude less than in salt from the ladle capsules.. Note that in both
modifications of the freeze valve capsule, the sealing salt remains inside
the capsule, and puddles over the capillary nozzle. When taking gas phase
samples, the capsule is lowered into the pump bowl until the valve thaws
and the sample is taken, then it is withdrawn to & cooler region so the
salt puddle may again freeze and contain the sample. These freeze valve
capsules did not exclude mist and scum on the salt surface from adhering
to the outside surfaces of the capsule. During capsule processing the
outside surfaces were always well leached to remove this material. Never-
theless, the question of transfer of contamination from the outside sur-
face to the inside materisls during chemical processing always remained.
This worry led to the development of the double walled, freeze valve
capsule.shown in Figure 2.Tb. It is basically a freeze valve capsule but
it is doubly contained, and can be used for salt or gas phase sampling.
During processing, the nozzle and top of the outer container are cut
through. The inner capsule containing the sample then falls away from

the outer container and is free from mist and scum., There was little

difference in the measured noble metal concentration between samples taken =

with freeze valve and with double-walled freeze valve capsules.

1§ ]

 
 

 

 

Xy

)

 

19

These’three kinds of capsules were routinely used for taking fuel
salt and gas phase samplee. In the analytical work presented in this
report, only data from the freeze valve and double-walled freeze valve
capsules will be used, and they will be used with equal weight. .Ladle
samples will not be used because of the problem of mist and scum contami-
nation. On all plots that follow, the data points will specify whether
the capsule was a freeze valve or a double-walled freeze valve type.
Another point to note is the difference in volume of the two capsule
types. The freeze valve capsule when full holds 50 to 60 grams of fuel
salt, but the double-walled capsule when full holds only 14 to 15 grams.
During the sampling procedure, a purge of helium was maintained down the
sample transport pipe. This should make little difference to the salt
samples but could be a significant parameter for the gas samples. The
helium purge varied from 575 to T5 standard cm3 /min depending on the
sample. All samples were taken with the higher purge rate before sample
19-16; after that the salt samples had the higher purge rate and the gas.
eamples the lower purge rate.

- Only four freeze valve samples of fuel salt were taken during the
235y yuns (during run 14) but meny freeze valve and double-walled freeze
valve samples were taken during the 233y puns. The 233U runs will there-
fore be discussed most extensively. Many ladle samples of salt were taken
during the 233y runs because they continued to give good results on salt-
seeking fission products. Both the salt and gas samples were analyzed
radiochemically. This technique was used to determine both the identifi-

cation and the amount of the isotopes present

3.2.2 Gamma Spectrometry of the Prlmary Heat Exchanger

 

‘The amounts of certain noble metal fission products dep051ted on the
pr;mary.heat_exchanger tube surfaces were measured by gamme ray spec-
trometry. The technique consisted of looking at & spot on the heat ex-

changer with a colllmated detector and measuring the gamma energy spectrum

| emitted from that spot. The resultlng spectra were used to 1dent1fy and

measure quantitatively the fission products present.
~ The first gamma scans were made after run 1k (last 23%U run) and were,

to an extent, exploratory.in nature; i.e., an attempt was made to see if

 
 

 

20

specific fission prodfict information could be extracted quantitatively
from the tremendous amount of background gemma radiation emitted by an
operating reactor system. Briefly, the equipment consisted of a highly
collimated lithium-drifted germanium diode detector that was coupled to
a 400 channel analyzer to determine the energy Speétra. All scans were
takenrwith‘the reactor shut down and either drained 6r filled with flush
salt. The equipment was mounted ofi a portable maintenance shield, so
gammé scans‘could be taken at many locations on the heat exchanger. All
together ébout 100 spectra were determined. Most of these were from the
heat exchanger but a few were from other components. The equipment was
calibrated so that the measured count rate could be reduced to'atqms of
fission,product per unit area of tube surface. Data processing was done
by hand and'was quite tedious. It was possiblé to isolate quantitatively
four noble metals from a typical spectrum (2%Mo, !03Ru, 132Te, and 95xD).
More details of the equipfient and procedures éan be obtained frbm Ref. 10.
At the end of the experiment it was concluded that the quality of the
data and'its potential applications were sufficiently promising to warrant
improving the system and repeating the experiments during later runs.
After considerable equipment and calibration procedure improvement,
the gamma spectrometry measurements were repeated after run 19 (233U). An
improved Ge(li) detector and a 4096 channel analyzer were used. Data
processing was done with a computer program developed for this purpose.
Precise alignment was achieved by use of a laser beam and surveyor's
transit.- Altogether_some 1000 spectra were measured, many of which were
teken with the reactor at different power levels (a few watts to full _
§0wer); Another 400 spectra were taken for calibration purposes. Details
of the equipment, calibration procedures, and data analysis can be
obtained from Ref. 11. Gamme spectra were obtained from other fuel loop
components besides the heat exchanger, such as the pump bowl, off-gas
line, and main loop piping. The principal intent of the experiment, how-
ever, was to measure fission product deposition quantitatively in the
primary heat exchangef. This was the only component for which an
absolute calibration of the detector was made. Therefore, these are
the only data that will bé analyzed in this report. The principal sPectfa

of the primary heat exchanger were obtained after run 19, although some

 

 
 

»%

&)

 

21

prelimihary data were taken after run 18. The data were taken with the
reactor shut down and drained. Altogether, quantitative information was
obtained for 10 noble metal isotopes from these data. Figure 3.1 is
typical of the kind of data obtained from this experiment. It shows the
amount of 132Te per square centimeter of tube surface plotted against a
longitudinal representation of the heat exchanger. The total range in
concentration is from 0.3 x 1012 t0 0.75 x 1012 disintegrations/min—cmz.
The higher value seems to be associated with the baffle plates and their
windows, whereas the lower value is associated with the crossflow part of
the heat exchanger. In this report the lower value was considered to be
most representative of the heat exchanger and, as an example, 0.3 x 1012
disintegrations/min-0m2 was chosen for !32Te., The quality of data from
the gamma scens after run 14 were not nearly as good as shown in Figure

3.1, and a sort of weighted average of all the data was used.

3.2.3 Core Surveillance Samples

The core surveillance specimens were briefly described (see Sect. 2.2)
and illustrated (see Figure 2.4) earlier. Periodically, when the reactor
was shut down and drained, all graphite and Hastelloy-N specimens were
removed and the amounts of fission products deposited on the surfaées and
the interior of some were measured. Radipchemical techniques were used

to determine the identity and amount of the isotopes present. Unfortunate-

ly, the fluid dynamic conditions in the surveillance specimen holder are
not well known because the flow passages are so complicated. These fluid
dynamic difficulties are discussed in detail in Appendix B where we
estimate the mass transfer coefficient. Briefly, let us just say that the
surveillance specimens feature inside corners, outside corners, fluid
entrance and éxit_regions, and possible stagnation areas. Superimposed
on these is a flow that is only marginally turbulent (Re ¥ 3000), These
difficulties were & result of the_many different kinds and geometries

of specimens that had to be incbrporated into a-vefy confined space. The
measured noble metal deposition'data on graéhite thatiwill be anal&zed in
this report came from the.surfacegfexposed'to fuei salt, although some

noble metals were also found on the inside surfacés that presumably weré

 
 

 

ORNL-DWG 70-7365

 

 

 

 

 

 

 

 

 

 

 

 

  

SPACEFI?'

 

'_ .
5 .LIJ ‘ 1 T T
T 3 0.800E12 |- | S S
% = ° — ,_i [ .:.I . . * . Jr_
o » . “. & e ® ° *
o & 0600ER |— —- e e s e et
o & —— %W . - s . [ - . ° T e I
© $ 040012 |- -Ly—peul } ol A e b T e } §
£ s | — SR e |
wu - } - !
T 1
' | : |
HTR. PLUG I-IITR. PLUG I HTR. PLUG ! l

SPACER

 

 

   

 

 

FUEL SALT

FLOW —»

 

 

 

Ie— - —— | !

 

FIGURE 3.1.

F1SSION PRODUCT DEPOSITION;

POSITION HOLE IN
SHIELD PLUGS

MSRE HEAT EXCHANGER

1 32¢e01)

cc

 

 
 

 

 

=y

2y

o

a)

 

23

not exposed to salt. The measured noble metal deposition data on

Hastelloy N came from the dosimetry wires. Noble metal deposition measure-
ments were also made dfi‘the perforated Haséelloy N cage but these will not
be analyzed because of the almost impossible job of estimating th%;mass
transfer coefficient. In Appendix B, fihe mass transfer coefficient to
both the graphite and the Héstelloy N is estimated to be about 0.25 ft/hr.
There is a Significantrunéertainty associated with this value for the
reasons described above, The mass transfer coefficient from the fuel salt
to the heat exchanger surfaces is a much better known number than to the
surveillance specimens. Figures 3.2 and 3.3 show typical data obtained

from these specimens.

3.3 The Difference Between the 235U and 233U Runs

 

Following run 1k the 235U fuel was removed from the carrier salt (and
flush salt) and the reactor was refueled with 233U, Chemical processing

of the salt was done at the MSRE site, and the process is discussed in

 detail in Ref. 12. The basic process was fluorination of the fuel salt

and removal of the uranium as a volatile fluoride. The corrosion rate on
the process tank during processing was high and has been estimated to be

sbout 0.1 mil/hr. The tank was constructed of Hastelloy N and the corro-
sion produéts were-Nin, FeF,, and CrFy. Following the fluorination step,

it was therefore necessary to remove the corrosion products from the salt.

This was done by reduction with hydrogen and zirconium powder and subse-

quent filtration. Removal of 23°U from the salt was essenfiially complete.

~ The carrier salt was then returned to the rea¢tor system and loaded with

233y, BRwn 15, the first 233y run, was concerned with the ZEerO-power

physics experiments’with thié new fuel,

During run 15 a significant change in 0perating'characteristics of
the reactor occurred and persisted.until the reactor was pérmanently shut
down."During the 239U runs, the volume fraction of circulating bubbles in
the fuel loop was determined to be between 0.0002 and 0.00045 (see Ref. 5),

end the overflow rate from the pump bowl to the overflow tank ranged from

0.4 to 1.5 Ib/hr, with essentially no overflow excursions following

beryllium additions. Beryllium was periodically added to the fuel salt,
primarily to reduce some UF, to UF3; and control the oxidation state of the

fuel. During the 233U runs the volume fraction of circulating bubbles

 
 

 

DEPOSITION (dpm/cm2)

1012

o1t

1010

]010

10°

108

2L

ORNL-DWG 70-15013

CGB IMPREGNATED

CGB

POCO

PYROLYTIC PARALLEL TO PLANES
PYROLYTIC PERPENDICULAR TO PLANES
WIDE FACE

NARROW FACE

DOUBLY EXPOSED

O CWax

PYil o —3

O PY]

 

10 - 20 30 40 - 50 60 4 70
DISTANCE FROM BOTTOM OF CORE (in.)

99 29

FIGURE 3.2. DEPOSITION OF 2°MO AND '2oTE
ON GRAPHITE CORE SURVE!LLANCE SPECIMENS

 
&}

-t

&)

 

 

»

Y]

 

DEPOSITION (dpm/cm?2)

 

25

i ORNL-DWG 70-15014

11
10

 

]010

10°

 

o w0 2 30 40 50 . 60 70
- DISTANCE FROM BOTTOM OF CORE (in.) |

FIGURE 3.3. DEPOSITION OF ~omo, '*%7e, '2%¢, 103y, 196y anp Pzr-

ON HASTELLOY N CORE SURVEILLANCE SPECIMENS AFTER 32,000 MW HRS

 
 

 

26

in the fuel loop was 0.005 to 0.006, and the overflow rate ranged from b
to 10 1b/hr with excursions up to 70 1b/hr following a beryllium addition.

The mean cireulating void fraction had gone up by & factor of about 20

“and the overflow rate had gone up by a factor of about 10 in the 233U runs

as compared to the 235y runs. Reference 4 provides a detalled discussion

of these'operatihg variables and others. The composition of the fuel salt

remalned nominally the same for both fuels, although the uranium concentra-.

tlon was reduced from 0 .9 mole percent to about 0.2 mole percent prlmarlly

because the 233U was not diluted with 238U, This resulted in a lowering

 of salt density by 3 to 4 percent. Other physical properties (viscosity,

surface tension, etc.) might be expected to change an equivalently small
amount. The rate of bubble injection from the pump bowl to the loop, and
the overflow rate are certainly functions of these varisbles. In my
opifiibn,.the change in reactor operational parameters is much too great to
be explained by such small changes in physical pfoperties. Actually,
there is evidence that the bubble ingestion phenomenon was near a thresh- -
old region. This was indicated by a steep change in void fraction when
the pump speed was changed s small amount.* It has been speculated that
the small changes in physical properties were coupled in some way to the
1ngest10n threshold to yield the high void fractions during the 233y
runsg ) This suggestion may account for part of the increased void frac-
tion, however, I believe that it accounts for only a sfiall amount of the

increase,

The question then is - Why the difference in the above parameters

when fueled with 23°U and 233U. A clue is given during the initial history

of the 233y runs. Prior to the start of run 15, flush salt was circulated
in the fuel loop for about 40 hr. There was no sbnormal behavior during

this period. Both the circulating void fraction and the overflow rate

. were consistent with what had been expected from past opérating history.

The flush salt was drained end the fuel salt was added. Circulation was.

started and again no abnormal behavior was noted. After about 14 hr of

 

*The fuel pump was powered by a variable frequency unit for a period
of time during the 233U runs to 1nvest1gate the effects of circulating
voids on !3%Xe behavior.

h

 
 

"

e

4+

€)

3

 

27

circulation, a beryllium rod was added for the purpose of reducing part
of the U to U3*, About 2 hr later, the salt level in the pump'bofil
began to rise indicating increased bubble ingestion into the fuel loop.
When it peaked out, the circulating void fraction was about 0.5 percent.
A short time later the overflow rate spparently went up to a highér than
normal value (v L 1b/hr). Detailed records are not available because the
data logger was not fully operable during this period. When the beryllium
rod was removed after 12 hr of exposure to salt, 10.1 grams of beryllium
had been dissolved. The circulating void fraction and overflow rate-
remained high for 10 hr of circulation until the fuel salt was drained
into the dump tanks. The above occurred before the reactor went critical
with 233y,

A suggested explanation for this behavior is as follows. During the
early operational history of run 15, the fuel salt was in a more oxidized
state than expected.(l3) The direct evidence that these fluorides were
there and that the fuel oxidized is as follows:

(1) The corrosion rate on the fuel loop was high during run 15.

(2) During the 23°U runs, 2°Nb behaved as & noble metal. During
the initial 233U runs, it behaved as a salt seeking fission product, indi-
cating the fuel was much more oxidized;(lh)

(3) When the beryllium capsule was removed from the salt, a thick
crust was found on the nickel cage enclosing the capsule. The crust was
predominantly salt, but the residue after extracting the salt was preé

(13) Similar

dominantly iron with small amounts of nickel and chromium.
crusts were found on other beryllium capsules added during run 15. The
perforated capsule for beryllium additions during the 235y yruns usually
came out of the pump bowl relstively clean and did not have iron deposits.
The beryllium then, rather than reducing the uranium ifi the fuel,
apparently reduced the iron and hickei corrosion'product fluorides to the

metallic state and they formed a scum on the surface of the fuel salt in

' the pump bowl. A hypothesis of this analysis is that the floating scum

possesses many properties of insoluble surface active agents. One
characteristic of such surface active materials is that they enhance’
froth stebility, and an effect would be produced in the pump bowl much

like in a froth floatation chamber. It was suggested earlier that a

 
 

 

28

mechanism involving heavy froth must be resorted to to explain the high

'overflow rates experienced dfiring’the 2337 runs. Another effect of sur-

face active agents is in the size of the bubbles generated. Development
(15,16) |
i

work with bubble generating devices n the form of a venturi or jet
pump has shown that cbnsiderably smaller bubbles are generated when a
surface active material is present than.when absent. It is unclear
whether they are attually generated smaller or whether the presence of
surface active agents prevents their coalescence in the immediate vicinity
of their generation. The result, however, is that they are considerably
smaller., Now to extrapolate'to the MSRE, one wofild say that with reduced
corrosibn products acting as surface active materials in the pump bowl,
more small Bubbles were generated during the 233U runs than the.235U |
rus. The smaller the'bubbles, the better their chance of being swept into
the fuel loop by the undér flow. This then would be.a mechanistic hypothe-

sis to explain the higher circulating void fractions and pump bowl overflow

- rates during the 233y runs. Of Course, it must be shown that surface

active materials existed in the pump bowl and fuel loop for all the 233y
runs. Although regular salt sampling capsules came out of the feactor
fairly clean, the beryllium addition capsules continued to show deposité_
of Fe and Ni in varying amounts during the remainder of the 233y runs.
In addition, magnets were periodically lowered into the pump bowl in an
effdrt to recover free metallic particles. They did recover these u
materials although the gquantities were small, less than a gram; however,
it doesn't take a large amount of surface active materials to have a
dramatic effect on surface behavior. o

The physical state or stability of thé froth in the pump bowl will
be referred to several times in this report, particularly with regard to
the differences in its--\sta.t_e between the 235U and 233U runs. In common
usage a froth or foam, such as the foam on a_glass of beer, often implies
a high degree of stability. In terms of bubble 1ifetime; the mean life
of a bubble in a head of beer is orders of magnitude greater than in
pure water. I do not intend to suggest an increase in bubble stability
in the pump bowl anything like this, rather that the increase in bubble
stability between the 235U and 233U runs was relatively minor. Let us.

see if we can extract something meaningful regarding bubble lifetimes in

 
 

 

[

"

)

i

»

 

29

the pump bowl. At steady state the bubble generation rate must equal the
bubble destruction rate. The bubble generation rate was determined by the
rate of gas carryunder from the xenon stripper salt spray. One might
expect that the carryunder was more or léss constant for the 23U and
233y runs; at least we will assume this to be the case. We will- also
assume that the bubble bursting rate is proportional to the total volume
of gas bubbles in the salt. This neglects many variables which are
certainly important, such as bubble size, depth of ffoth, salt drainage
from the'bubble swarm, etc., nevertheless, in the interests of continuing
with this discussion, the assumption probably isn't too bad. If it is

true then at steady state, we have
Bubble Generstion Rate = AV

where V is the total vqlume of gas entrained and A is the "bursting
constant" for the bubbles. The bubble half life would be 0.693/x. Note
that within these assumptions at a constant bubble generation rate, if the
bubble half life is doubled, then the total volume of gas entrained by
the salt would also be doubled. The froth height in the pump bowl would
then increase a corresponding amount. A mechanism like this, with an
increase in bubble lifetime of this magnitude, could easily account for
the higher overflow rates experienced during the 233y yuns., Note par-
ticularly that doubling or even tripling bubble lifetimes represents a
rather mild increase in bubble stability compared to more common notions.
This, then is a description of the differences in operating char-
acteristics between the 23°U and ?33U ruhs in the MSRE, and a suggested
hypothesis to explain the differehces. The essentials df this hypdthesis,

that noble metals and reduced corrosion products will adhere to liquid-

| gas interfaées, will be referred to meny times in the following sectionms.

4. ANALYTICAL MODEL
4.1 Phyéical Basis of Model

TFor reasons already discussed, and for.others which will bé@ome
apparent in the section on RESULTS FROM THE MSRE, one might expeét that

the transport of noble metals from the fuel salt to the various surfaces

 
 

30

where they deposit is controlled by the laws of mass transfer. Let us

review briefly the behavior of noble metal fission products and fuel

| sglt in the MSRE as follows:

1. Noble metals are born as ions from fission and decay of their
precursor, but become atoms very quickly. They are homogeneously dis-
persed in the salt.

2. Noble metals are unstable in fuel salt; i.e., they are present

_in the reduced metallic state and are quite insoluble. They may even

be unwet by salt. Massive metal objects display contact angles in the
renge of 90-150°. (17}

3. Noble metals deposit on Hastelloy N and graphite surfaces, and
large amounts are found there. _

4.. They also deposit on liquid-gas ihterfaces, and we infer that
they display some of the properties of surface active agents. This con-
cept has been used to explain the differences in 23U and 233y operation
of the MSRE, and will be used to explain many of the results observed
iater.

5. Fuel salt is known to behave as a conventional Newtonian fluid.
For instance, the primary heat exchanger was designed using conventional
heat transfer correlations and the measured overall heat transfer coef-
ficient was in good agreement with the design value. One would expect
the same degree of success in estimating mass transfer coefficients since
the transport phenomenon is the same in both cases. One must élWays_be
wary, of course, because sometimes physical and chemical phenomena come
info play that complicate the simple approach. |

The above points constitute the essential requirements for a material
to transport through the fluid boundary layer and deposit on the surfaces
according to the laws of mass transfer. Development work from other
reactor systems has shown that, if the chemistry of the fission products

permits, they will be transported according to these laws. See, for

~example, Refs. 18 through 21. Tt would therefore seem fruitful to attack

the noble metal migration question in the MSRE within the framework of
mass transfer theory in its simplest form and let the results speak

for themselves.

 
 

;
i
|
i
|
|
|
]
]

”

)

w

[

»

 

31

4.2 Analytical Model

 

First, we must write a rate balance on the noble metals in fuel salt
where they are born. The conditions of this rate balance are as follows.
Note from the generalized beta decay chain from Sect. 3.l-that noble metals
appear.in groups and we have a situation where noble metals can decay
into noble metals. ‘Note also from the generalized decay scheme that each
noble metal grouping may start‘off with a sait seeking precursor. The
analytical model will therefore consider the last salt seeking precursor
of the noble metal chain. We must do this primarily because of 95Nb whose
precursor is 95Zr with a half life of 65 days. The analytical model must
be for the unsteady state. This is because the MSRE, being an experimental
reactor, had a quite erratic power history. Seldom did any of the longer
lived isotopes reach steady state. The model will consider the entire
fuel loop to be a "well stirred pot." One might expect this assumption
to be adequate because the fuel circuit time around the loop is about
25 seconds and the isotopes we will be dealing with have half lives rang-
ing from hours to years. A better indication of the adequacy of this
assumption would be to compare the fuel circuit time to the cdmputed
residence time of a noble metal atom in salt before it deposits on a
surface. In Appendix B, it is shown that the longest residence time
(expressed as the half life of ncble metals in fuel'salt) is twice the
loop circuit time. Lastly, it is assumed that all noble metals migrate
independently of each other, even.when of the same chemical species.

Consistent with all the considerations discussed above, we can now

write a rate balance on the noble metals associated with fuel salt where

" they are born. The equation in words is as follows where the units of

each term is atoms/time.

S
v dcC

dt

generstion rate from fission + generation rate from
decay of precursor

- decay rete - deposition rate on graphite
~ deposition rate on heat exchanger | | (1)
- deposition rate on rest of fuel loop

- deposition rate on 1iquid—gas interfaces (bubbles).

 
 

32

‘See Appendix A for the Nomenclature. The two generation terms are func-

tions of the reactor power history and yields of the specific isotopes

involved. The decay rate is a function of the concentration of each
specific noble metal ahd its decay constafit. A1l the other terms are
deposition‘rates and are functions of the surface area, mass trfinsfer
coefficient and thé concentration potential. The deposition rates on the

graphite in the core and on the heat exchanger have been listed separately.

The rest of the loop is lumped into one term. Deposition on bubbles has

also been listed as a separate term because of its importance. Each term

will now be evaluated separafely and the equation will be integrated over

4.,2.1 Generation from Fission

 

' The generation rate direct from fission is simply as follows:
Generation from fission = yP | (2)

4.2.2 Generation from Decay of Precursor

 

A rate balence on the last soluble precursor before decaying into

a noble metal will be

 

pS |
dC  _ P p _ 3PcPS y - o (3)

V& =Y

Integrating and evaluating at the boundary condition

¢ =cPatt=0 (L)
we get
o - P | |
¢S - LR, (o PS_¥By (5)
APy © APy

The generation rate from decay of the precursor will be

Generation from decay of presursor = }PVCPS‘ (6)

P
2Pt
= PP + (WPve P° - yPp) e

o/

 
 

*

. "

&)

»

&

 

33

fi.2.3 Decay Rate

The decay rate of each noble metal isotope is simply as follows:
Decay rate =7)\VCs - o | (7)

4.2.4 Deposition Rate on Heat Exchanger

 

Written in the framework of mass transfer theory, the deposition rate

of noble metals on the heat exchanger surfaces can be expressed as

“follows:

Deposition rate on heat exchanger = n® a"e (c® - ct) (8)
At this point it will be necessary to make an assumption concerning c®t
(the concentration of noble metal in the fuel salt at the fluid-solid
interface). We will dssume that all solid surfaces involved (Hastelloy N
and graphite) behave as an infinite sink; i.e., if_a noble metal atom
migrates through the fluid boundary layer and contacts the surface, it
will stick there forever. This is, of course, one of the unknowns in
this analysis. The result of this assumption is that CSi effectively

becomes zero, and the equation reduces to

Deposition rate on heat exchanger = pe aPe ¢S (9)

h.2.5 Deposition Rate on Graphite |

By reasoning similar to that above we can arrive at the equation for
deposition of noble metals on the core graphite as follows:
gra ,gre cs (10)

Deposition rate on graphite = h A

We have again assumed a sticking fraction of 1.0,

4h.2.6 Dep051t1an Rate on Rest of Fuel Loop

Again by reasonlng similar to that above, we arrive at the equatlon

for the deposition of noble metals on the rest of the fuel loop (all
Hastelloy N). |

Deposition rate on rest of fuel loop ==2:(hA)rESt of fuel 1oopaS(yy)-

J

 
 

 

3

4.2.7 Deposition Rate on Liquid-Gas Interfaces

 

As noted before, there are small bubbles c1rcu1at1ng with the fuel

salt. Their amount is small but the product of their surface area and

'mass transfer coefficient is higher than the same product for all the rest

of the solid loop surfaces combined. Even if the sticking fraction to a
bubble is low, their effects will still be quite strong. If we assume
that noble metals deposit on liquid-gas interfaces in accordance with the

mass transfer theory and that the sticking fraction 1s 1. 0 we agaln

arrive at 8 similar equation for the rate of migration to bubbles.

Deposition rate on liquid-gas interfaces (bubbles) = pPUPPUbS (12)

Later we will deduce that the effective sticking fraction to bubbles is

"less than unity.

4,2.8 Equation for C°

Now the individual generation rate terms ahd decay and deposition

 

rate terms are substituted into the original rate balance around the fuel
salt; Eq. (1), and the equation is integrated. The constant of integration
is evaluated at the boundary condition

¢® = coS at t = 0 | (13)
This will yield the equation for the concentration (Cs) of a noble metal

in the fuel salt at any time (t). This equation is as follows:

_BGE 4 y) (14%)

S P PS _D P
P AT CT =y P/ At
s _Ply" +y) (L _o "' s
¢ = =g+ | ) e + [ Co VX

X - AP

AP cgs -yPe/v ] -xt
- (————) ] e

 

X - AP
where S '
gra ,gra he . he 'Z (hA)reSt of loop bub ,bub |
Loy s DETT ABTE "€ AR L h A (15)
: } vV vV vV Vv

This equation can be carried through the power history of the reactor. If

the power level is changed, then the value of Cs preceding the power change

will be the initial condition (Ci) at the new power level. Also note that

-
 

[

[

.oab i

»”»

 

35

X has units of time_l, end is the theoretical rate constant for migration

of noble metals from the fuel salt to their sinks.

4.2.9 Noble Metals on Solid Surfaces

Nofi'that we have an expression for the concentration of noble metals
in fuel salt as a function of time and the power history of the reactor, we
can compute the amount of noble metals deposited on any surface in the
reactor. To do this we must again set up an unsteady state rate balance
for that surface as we did for the fuel salt. The same assumptions and
considerations hold true here as they did for the fuel salt. In addition,
we will assume there is no interaction between the bubbles and solid
surfaces; i.e., the only source of noble metals for the surfaces is direct-
ly from the salt. The rate balance is as follows where the units of each

term is atoms/time-unit area.

0 |
g%—-= Deposition Rate - Decay Rate (16)

The individual term are as follows:

Deposition Rate = h" C° (17)
Decay Rate = ACT (18)
Substituting we have |
m
dC” _ .m.s m
== hC -ACc (19)

Now, substituting the value of c® determined in the previous section
(Eq. 1b4), integrating over time, and evalusting the constant of integration
at

 

 

m_ m : , '
¢ =C, stt=0 | - (20)
we get : '
o W (PgPE _ P ) -\"
e (v2 + v) ‘h (A"C" -y P/V)e m ot D
BBy, o - ol M) (o
(x-2®) (- AP) | a

 

 

PePS _ D © . ymp.ps _ P
APcPS _ yPpyy | - | PoPS _
- /V:]e " +{Cm_ ey +y) B (A0 - yR/V)

 

AVX
X - AP ,, (X-2F) (A-2F)
At
m P~ PS p
_h s _pGPey) MO -pR/V
A-X'| 7o VX T X i

p

 
 

36

This then is the equation for the concentration of a noble metal isotope
on & surface at any time as a function of power., It can be carried
through the power history of the reactor just as the equation for c®. The

equation is applicable to any noble metal isotope and to any solid surface

~in the reactor (Heat Exchanger, Graphite in the Core, Core Surveillance

Specimens,'étc.) by proper choice of the mass transfer coefficient.

4.2.10 Noble Metals on Liquid-Gas Interfaces

The principal difference befween.deposition on & solid surface treated
above and the ligquid-gas surface trested in this sectibn is the sink
terms once the noble metal has reached the surface. In the case of the
solid'surfaée,‘thé only sink term considered was decay.- In the case of a
liquid—gaé interface, migration of noble metals to the off-gas system repre-
sents another sink term that must be considered. |

Later on in the analysis of reSulté from the MSRE we will want to con-
sider the entire gas phase in the fuel loop as a well mixed pot. The
entire gas phase will consist of bubbles circulating in the lodp, the gas
phase in the pump bowl and also part of the gas phase in the overflow tank.
The gas phase is defined to include the liquid-gas interface, so that noble
netals attached to the interface will be considered as part of the gas
phaéé; The reasons will become more apparent later on, but for now let us
justlsay that it will be necessary to find a process by which noble metals
are removed from the reactor system gas phase as defined above with =
rather low rate constant. For example, noble metals transported to the
of f-gas system or the drain tanks would be considered as removed from the
gas phase as defined above. At any rate we will again write an unsteadyf
state rate balance around the gas phase. The same assumptions and con~-
siderations are still applicable as in the rate balance around the fuél
salt. This time, however,instead of using concentration units (atoms/vol),
we wiil use total inventory units (atoms) in the gas phase. The rate

belence is as follows:

al

i - Migration rate to bubbles - decay rate (22)

- migration rate to off-gas system.

it

 
 

 

"

-}

"

»

 

37

The individual terms are as follows.

Migration rate to bubbles - pPuP 4bub 8 (23)
Decay rate = AI 7 (2k)
Migration rate to off-gas system - ZI (25)

The transfer of noble metals from the pump bowl to the off-gas system is
dealt with in a very generaly way. We simply defined a rate constant

(Z) which says that the rate of transfer is proportional to the total
amount of a noble metal isotope. Now, substituting the individual terms
into the rate balance, then substituting the value of ¢® from the fuel
salt analysis (Eq. 1L), integrating over time and evaluating the constant

of integration at

 

 

 

 

I =1 att=0, (26)
we get . D
P~PS _ P -A
RPA%p(y® + y) I AC -y P/V e
I ( ) (27)
v (i+e) (A + 2 - AP) X - AP
pP-ps _ D
n° 8 fps _BGPay) Xl TVEN | xe
M B - X o VX x - 2P
PsPS _ P
o PP (y® 4+ v) ~ LP b e -y P/V)
VX (3 + ) (A+2-2F)  x-P
L o P.PS _ D
__w AP os RGP ry) MO “YENIL (e
O+ 28 - X) o vV X x - 2P

This then is the equation for the total amount of a noble metal isotope

in the gas phase of the fuel loop as a function of time and the power

history of the réactor,=andrthe rate constant (Z) for noble metal

transfer to the off-gas system or dump tanks.

 
 

 

38

5. RESULTS FROM THE MSRE
5.1 Introduction.

The general approach in the asnalysis of noble metal migration in the

- MSRE will be as follows. First, we will compare the measured to theoreti-

cal concentration of noble metals found on the primary heat exchanger by
gamma spectrometry. This measurement tedhnique.gafie quantitative results
for the greatest number of noble metal isotopes. The measurements were
made in situ, so there can be no question df contamination or problems
associated with hot cell processing. Also the fluid dynamic conditions in
this conventional U tube heat exchanger are fairly well ihown; This com-
parison then is quite good and seems to put the analyticael model on a firm
foundatibn. We will then look at the core surveillance sambies:and show
how they generally confirm the results from the heat exchanger. Then we
will look at the fuel saltkand gas phase samples. After each one of these
comparisons, observations will be made on the nature"offnoble'metai migra-~
tion. We will then mske a rather crude attempt to quantify the hypothesis '
that noble metals accumulate in the pump bowl and determine rate constants
for removal of noble metals from the pump bowl to the off-gas system to see
if they are physically reasonable. As noted previbusly, most of the.

233

analysis will be for measurements made during the U operation (runs 15-

235

20), although some data are available for the U operation (run 1-1%4).

5.2 Comparison of Measured Deposition on Heat Exchanger to Theoretical

| The theoretical amount of each noble metal isotope deposited on the-
surface of the heat exchanger tubes was computed with Eq. 21, and compared
to the measured amount as determined by gesmma spectrometry. The measured
values following Run 14 were obtained from Ref. 10 and after runs 18 and
19 from Ref. 11. Recall that the equipment and techniques used>to ‘
determine fission product deposition after run 14 were less sophisticated
than used after runs 18 and 19, therefore, the data following run 14 are
less certein than the newer data. The parameters used to calculate the
theoretical amount (e.g., mass transfer coefficients, isotope parameters,
ete.) are evaluated and tabulated in Appendix B. The computed concentra-
tion on the heat exchanger takes into account the entire power history of

the reactor. The only sink term for noble metals attached to the heat
 

”

»

o)

 

39

exchanger is decay. It should be noted that periodically the fuel loop
is cleaned out by circulating flush salt for a short period of time. The
computed concentration assumes that this process does not leach noble
metals from the surfaces. Some of the gamma spectra following run 1l were
made with and without flush salt in the loop and they indicate this aésump-
tion to be true. The theoretical amounts of noble metals on the héat
exchanger were computed on the basis that they do not adhere to liquid-gas
interfaces. Then, by comparison of the computed amounts with the measured
amounts, conclusions of a qualitative nature will be drawn to the effect
that noble metals apparently adhere to liquid-gas intérfaces.

 The results of this comparison are shown in Figure 5.1 where the ratio
of measured to computed amounts on the heat éxchanger is plotted against ,
the noble metal half life. First let us look at the curve measured during

233U operation. These measurements were made with the improved gamma

spectrometry equipment and.are Judged to be the best. The following

observations can be made.

1. The curve is madé up from 10 isotopes; 3.; rutheniums,
3 - telluriums, 2 - antimonies, 1 - molybdenum and 1 - niobium, some of
which are duplicated measurements. They seem to be rather tightly grouped

132Te and l03Ru. One would conclude

around the line, except perhaps
therefore that each noble metal isotope migrates as a function of its
own concentration in salt and is not ififluenced by other elemental species
of noble metals or even isotopic species of the same element.

2. Note that the curve is & straight line and has a slope very close
to zero;'i.e., ndble-métai migration is not an unaccounted-for function

of its own half life. It is important to note at this time, because

later we will see that this is not true for the fuel salt end gas phese

samples.
3. Because the sldpe.pf-therline is almost zero and bécause_of the

tight gfouping of data around this liné, some credence must be given to
the hypothesis that noble metals migrate according to the simplest form

of mass transfer theory. The good correlation 81s0 speaks well fbr the
quality of the gamma spectrometry deta. Something is still missing,
however, because the measured to theoretical concentration ratio is con-
siderably less than unit for the 233U run determinations, and this will be

discussed shortly.

 
 

MEASURED AMOUNT ON HEAT EXCHANGER (dpm/cmfi)
THEORETICAL AMOUNT ON HEAT EXCHANGER {dpm/cm?)

 

Lo

ORNL-DWG 70-15021
10

THEORETICAL AMOUNT COMPUTED FOR CASE WHEN NOBLE METALS DO
5.0 DEPOSIT ON LIQUID-GAS INTERFACE

O MEASURED AFTER RUN 14 (235u)
A MEASURED AFTER RUN 18 (233y)
@ MEASURED AFTER RUN 19 (233y)

2.0

oy kgt
1.0 ' lloflnu MEASURED DURING 235U OPERATION
95Nb
0.5

DURING 233y OPERATION
0.2 106Ry v

1
0.1

o
»

o
o

0.02 -

 

0.01
1 2 5 10 2 5 102 2 5 103 2 5 10 2 5 105

NOBLE METAL HALF LIFE (hr)

FIGURE 5.1. COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS
OF NOBLE METALS ON THE PRIMARY HEAT EXCHANGER
 

 

o

«f

.

n

 

Y yuns than the

L1

L. Note the good agreement between 95Nb and the other noble metals.
95

It was pointed out previously that ““Nb sometimes behaves as 8 salt seeking
fission product and sometimes as a noble metal, depending on the oxidation-
reduction state of the fuel salt. The fuel was in a reduced state at the

end of runs 18 and 19 so this 95Nb behavior is as expected.

35U operation. The same

Now consider the data measured during the
observations and conclusions can, in general, be made for this curve, but
on & lesser scale because there are fewer isotopes involved. The magnitude
of the measured to calculated ratio is different and is in the vieinity of

95Nb and the other noble metals.
235

1.0. Again note the good agreement between
The fuel salt was generally in a more reduced state during the U oper-
ation and Nb behaved as a noble metal.

Now - why is the magnitude of the measured to theoretical concentra-
tion ratio on the tube surface close to unlty when measured after run 1k
(last 235

(233U.runs)? As noted in section 3.3, there was a dramatic difference in

235U and 233U.

U run) and 0.15 = 0.20 when measured during runs 18 and 19

operational characteristics of the MSRE when operating with
The difference was most apparent in the amount of bubbles;circulating with
the fuel salt. As noted earlier the void fraction of bubbles during the

235y runs was 0.02 - 0.045%, while during the 233y runs, it was 0.5 to

. 0.6%, up by a factor of about 20. The theoretical amount in the denomina-

tor of the ordinate of Figure 5.1 is computed assuming that noble metals
do not adhere to liquid-gas interfaces (circulating bubbles). It has been
hypothesized that noble metals do deposit on these interfaces, so the
bubbles will compete with the heat exchanger as a noble metal defiository.
If this concept is included in the‘calculation, then the computed amount
on the fieat exchahger will diminiéh and the value of the ordinate will
increase. It is also assuméd'that there is no interaction between the
bubbles and the heat exchanger surfaces, so if a noble mefal atom migrates
to a bubble, it is no longer available to deposit on the heat exchanger.

Since the fuel salt contained many times more bubbles during the.?33

35U runs , 1nclud1ng bubbles in the calculation,

35U curve. Qualitatively

the 233U curve will move up much more than the
then this will explain the difference in elevation of the two curves.

Quantitetively, it is a bit more difficult, because uSing the best

 
 

 

 

ho

_‘estimated values of bubble surface area and mass transfer coefficient

(seé Appendix B), both curves are pushed upward parallel to themselves to
a value considerably greater than unity. If the empirical but nevertheless
mechanistic "sticking fraction" is brought into the calculation, we can
agéin‘bring both curves down to a value very close to unity. The sticking
fraction is defined as that fraction of atoms that contact thg_interface

and adhere to it. If the sticking fraction of noble metals to solid

- surfaces is maintained at unity, then a sticking fraction to iiquid—gas

interfaces of 0.1 - 0.2 is required to bring the value at the ordinate
of both curves close to unity.
A sticking fraction of noble metals to a liguid-gas interface con-

(22)

siderably less than 1.0 was not expected. For instance ,.noble metals

were found in significant quantities in helium passed over the surface of

'quiescent molten fuel salt samples in a hot cell experiment. - The vapor

pressure of noble metals is diminishingly small at these temperatures

and cannot possibly account for the chcentrations observed. The indivi-
dual noble\mgtal atoms or very small clusters of them, are apparently
spontaneously expelled from the surface. Considerations of the interfacial

(23)
Therefore, one would

energies involved indicate this to be possible.
expect a sticking fraction to bubbles of unity. A rationalization of
this apparent paradox is as follows. An observation from the reactor is

that many of the smaller bubbles circulating with the fuel salt completely

135

dissolve in the higher pressure part of the fuel loop.
observation is a result of an analysis to explain the Xe poisoning
effects observed in the reactor. Note that the surface tension of molten
salt is quite high, about 200 dynes/em. When a bubble is pressurized by
the pump and begins to dissolve, its diameter decreases. Thé internal
pressure sbove ambient, as generated by the surface tension (4o/d) becomes
quite high (0.8 psi for 0.00S in. bubble)rand further enhances the dis-
solution rate. This continues to the limit and the bubble dissolves.

The process is very rapid ‘and analysis indicates only a few seconds are

required. If this bubble contained some noble metalS'before it dissolved,

we would end up with a noble metal cluster associated with the salt again,

rather than a bubble. Qualitatively then, here is & mechanism where the

i

O
 

)

L 1]

 

L3

the sticking fraction of noble metal atoms to a liquid-gas interface
can be unity but where it actually appears to be less than unity.
Possibly other mechanisms cofild also be devised.

Conclusions. The conclusion from examination of noble metal deposi-
tion on the primary heat exchanger as determined by gamma spectrometry, is
that noble metals apparently do migrate and deposit on these surfaces in

accordance with the laws of mass transfer in the simplest form. Deposi-

tion on liquid-gas interfaces (bubbles) must be included in the calculation

to‘force the calculated concentration to equal the measured concentration
on the tubes. The exact mechanism of noble metal deposition on liquid-

gas interfaces is not known and had to be handled in a semiquantitative

‘way involving apparent sticking fractions to bubbles, Essentially these
same conclusions will be reached after each analysis section in this

-report.

5.3 Comparison of Measured Deposition on Core Surveillance Samples
to Theoretical

 

The theoretical amount of each noble metal isotope deposited on
the surface of'the graphite and the Hastelloy-N core surveillance samples
was computed and compared to the measured amount. The measured values were
obtained from semiannual reports.(26’ 2T, 28 and 29) The same introduc-
tory remarks are applicable here as in the first paragraph of the last
section. Most important, recali'that the theoretical amount will be com-
puted on the basis that noble metals do not deposit on liquid-gas inter-
faces. The mass transfer coefficient was estimated in Appendix B to be
abofit 0.25 ft/hr for both the gfaphité and Hastelloy-N. The uncertainty
in this mumber is considerable. |

The results of this comparison are shown in Figure 5.2 for the
graphite specimens and Figfire 5.3 for the Hastelloy-N specimens. Again,

the ratio of the measured to theoretical amount is plotted against the

noble metal half life. In drawing lines through the data, no weight

95

was given to the ““Nb points for reasons to be discussed later. The

'following observations can be made :

 
 

Ly

ORNL-DWG 70-15015

|

THEORETICAL AMOUNT COMPUTED FOR CASE WHERE NOBLE METALS
NOT DEPOSIT ON LIQUID-GAS INTERFACE

10

 

SAMPLES REMOVED FROM INTEGRATED
CORE AFTER : EXPOSURE . FUEL

 

2 RUN 7 7,800 Mw hrs 235y
RUN 11 24,000 Mw hrs 235y
RUN 14 32,000 Mw hrs 233y
RUN 18 20,000 Mw hrs 233y
RUN 18 76,000 Mw hrs 235 AND 233y

129m
Te

103Ry

—
o

o
»
wn

IOGRU

o
Y

SPECIMENS EXPOSED
TO FUEL SALT
DURING:

o
—

233) OPERATION

o
o
o

235 AND 233y
TION

 

MEASURED AMOUNT OF GRAPHITE SPECIMEN (dpm/cm2
THEORETICAL AMQUNT ON GRAPHITE SPECIMEN {dpm/cm

0.02

33y OPERATI

 

0.01
1 2 5 10 2 5 102 2 5 103 2 5 10
NOBLE METAL HALF LIFE (hr)

FIGURE 5.2. COMPARISON OF THE MEASURED TO THEORETICAL AMOUNTS
OF NOBLE METALS ON THE GRAPHITE CORE SURVEILLANCE SPECIMENS
 

¥

”)

0.2

0.1

0.05

MEASURED AMOUNT ON HASTELLOY-N SPECIMEN (dpm/cm?
THEORETICAL AMOUNT OF HASTELLOY-N SPECIMEN

 

 

45

ORNL-DWG 70-15016

 

THEORETICAL AMOUNT COMPUTED FQR CASE WHERE NOBLE METALS
DO NOT DEPOSIT ON LIQUID-GAS .INTERFACE
SAMPLES REMOVED FROM INTEGRATED
CORE AFTER EXPOSURE FUEL
RUN 14 32,000 Mw hrs 235y
2 RUN 18 20,000 Mw hrs 233))
L] g
_ Te
1.0, Mo fE L32Te ISNbHILO3RU
0.5 SPECIMENS
. , EXPOSED TO

SALT DURING:
33y OPERATION

235y OPERATION

 

1 2 5 0 2 - 5 102 2 5 103 2 5 104
NOBLE METAL HALF LIFE (hr)

FIGURE 5.3. COMPARISON OF THE MEASURED TO THEORET | CAL AMOUNTS
OF NOBLE METALS ON THE HASTELLOY-N CORE SURVE!LLANCE SPECIMENS
| (DOSIMETER TUBES)

 
 

L6

7~
-

1. Three different sets of graphite samples (Figure 5.2) were ex-

235

posed to fuel salt during only the U operations. All three are con-

sistent with each other and the data points are rather tightly grouped

around a single line. An exception is 95Nb which lies well above the
| 235

line. Recall that the fuel salt was in a reduced state during the U

95

operations and ““Nb behaved as a noble metal and would therefore be ex-

pected to fall on the line. More will be said about this below.

2. One set of graphite samples was exposed to fuel salt during only
233 ' '

U opérations. This curve falls below and parallel to the previous

235,

curve for U operation. This is consistent with observations of noble

metals in the heat exchanger in the last section and the same conclusions

can be drawn.

3. The curves for graphite may have a significant slope at the low
half life end which is not consistent with the gamma spectrometry data
from the heat exchanger. The slope seems to be zero however for half
lives over about 500 hours. This will be discussed further below.

L, The relative elevation of the curves thréugh the deposition data
on the Hastelloy-N surveillasnce samples (Figure 5.3) do not confirm the
previous observations from the heat exchanger and graphite. There is
however more scatter in the data. |

5. Concerning the absolute value of the measured to theoretical ratio
of the noble metal concentration. An overall average of the 233U line for
Hastelloy-~N (Figure 5fl3) seems to agree with the previous value from the
heat exchanger, however, the data from the 235U runs do not agree.

6. The measured to theoretical concentration ratio for all the
graphite samples seems to fall somewhat below the data from the heat ex-
changer‘and Hastelloy-N samples. There are two explanations for this.
First, inadequate knowledge of the mass transfer coefficient, although
this probably isn't enough to account for the entire discrepancy. Second,
the more plausible explanation is that'the sticking fraction for graphite
is less than unity. More will be said about this below.

Let us consider some of the above discrepancies with graphite'ahd
see if we can bring them into line with previous observations and conclu-

sions from the heat exchanger analysis. First let us hypothesize that the

‘noble metal sticking fraction to graphite is less than unity. We have

of
 

*}

)

 

 

k7

noted that this is 1likely in observation number 6 above. This will be con~
firmed more directly in Section 5.7 when we look at the results of a
special 1aMinar flow core surveillance test during the last MSRE operation-

al period. Nofi, there is evidence(BO) that Nb forms a stable carbide with

graphite under MSRE operating conditions. The graphite could therefore act

as an infinite sink for Nb, i.e., its sticking fraction could be unity.

95

An effective sticking fraction of ““Nb to graphite of unity, and a sticking

fraction of the other noble metals of less than unity, would explain the

95Nb points being so much higher than the other noble metals in Figure 5.2.

In fact, the ratio between the curves and the‘95Nb data cluster could be a

direct-measure_of'the sticking fraction of the other noble metals relastive

to 7°Nb. We can carry this discussion one step further. w5 (half life =

2.4 min) is a precursor of 9 Mo. Its half life is high enough so that it

is a slgnlflcant mlgratlng species in this decay chain. If then, amounts

99Nb migrate to the graphite with a sticking fraction of unity and decay

99 -~ 99

into ““"Mo, the measured amounts of ““Mo would also be relatively higher
then the other noble metals. Therefore, the left side of the curves in
Figure 5.2, which are controlled to an extent by 99Mo might tend to be
higher than the other noble metals. Another effect fihat must be considered
is that during the final week of run 18, the fuel pump was operated at a
slightiy reduced speed. The principal effect was that during this period

235

the c1rcu1ating void fraction was more equlvalent to the U runs than

the 233

during this period the nobie metal deposition rate on solid surfaces would .

U runs. Since there were fewer bubbles,.one might expect that

be greater. Since the time perlod was. relatlvely short it would elevate

only the low half life end of the curve. Thls last point may also help

explain why the short hsalf life end of the 33U curve for dep051tion on

| Hastelloy-N (Figure 5. 3) is higher than expected.

Conclusion. The conclusions then, from examination of noble metal
dep051tion data from the core surveill&nce samples,’ is that they generally
confirm the conclusions arrived at after examination of the heat exchanger
in Sectlon 5.2. It appears ‘that the sticking fraction of noble metals to
grephite is less than‘finity, We have had to form a hypothesis to explain
the behavior of Nb. |

 
 

 

 

~is sensitive and a small error in the

48

5.4 Comparison of Fuel Salt Samples with C°

The theoretical amount of each noble metal isotope contained in the
ffiel salt (Eq. (14) in Section 4.2) was computed and compared to the
measured amount in the fuel salt samples. Again the théorétical amount
Was-computed on the basis that noble metals do not deposit'on liquid-
gas interfaces. The measured and computed‘concentration differed by
1-L4 orders of magnitude, the measured concentration always being higher.
In an attempt to resolve this discrepancy, the measured-to-theoretical
concentration ratio was plotted against the fraction'of its total capacity
thét each sample capsule.was filled. These plots seem to be unique and

are shown in Figures 5.4 through 5.9 for 103Ru, 106Ru, 99Mo, 129mTe,

l32'I'e' and 95Nb, respectively. The kind of sample freeze valve or double-

‘wall freeze valve,is distinguished on the plots. Recall that a freeze

valve capsule contains 50-60 grams of salt then full and a double wali
freeze valve capsuie holds about 15 grams of salt when full. Ladle sample
data are not included on these plots for reasons pointed out in Section
3.2. All reported freeze valve capsule data are on these plots exgept
those taken at zero power. It is a characteristic of the analytical
model that the noble metal concentration ifi salt goes to zero shortiy
after the reactor power goes to zero. The measured to theoretical con-
centration ratio wduld therefore approach infinity if any noble metals at
all were measured in the sample. The following observations can bé made
about these curves.

95

1. All curves, except that for ““Nb, behave in a similar manner.
For samples that were mostly empty, the measured to theoretical concentra-
tion ratio is orders of magnitude higher than for those samples that were

mostly full.
5 95

Nb is being carried along for comparison, since it can behave

either as a noble metal or a salt seeking fission product, depending on

the oxidation-reduction state of the salt. Many of the reported o

concentrations in fuel salt are negative. This is because the reported

95

Nb concentration was extrapolated back from the time of measurement to

95

the time of sampling.

95

Zr with a 65-day half life is a precursor of

Nb, and therefore it must also be counted. The back calculation in time

95

Zr concentration measurement

=
 

 

| )]

MEASURED CONCENTRATION IN SALT (dpm/g—salt)
THEORETICAL CONCENTRATION IN SALT (dpm/g—salt)

 

100

 

L9

ORNL-DWG 71-1876A

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    

 

 

  
 

 

 

 

 

 

 

104 —
—— THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE
| METALS DO NOT DEPOSIT ON LIQUID—GAS INTERFACE
5 }— |
| ® 17-31 FREEZE VALVE CAPSULE
L O 17-32 DOUBLE-WALL FREEZE VALVE CAPSULE
L—SAMPLE NUMBER TAKEN DURING
2 — L—THIS RUN NUMBER
- SAMPLE GROUPING NO. 1 (SEE TEXT)
0O 19-44
017-32
102
\\
5 N\,
A’
@177 N
2 - \\
@17-10 __ ®17_2
o l \ "14-20 18219 L
101 18-44 *18—12 ::o. 17-31
~—~=19-36
5
: ®17-22 — ®14-66
‘@14—63
2 >
SAMPLE GROUPING NO. 2

 

 

 

 

 

 

 

 

 

 

 

 

0 01 02 03 04 05 06 07 08 09 10
FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

FIGURE 5.4. COMPARISON OF THE MEASUREb I03RU CONCENTRAT I ON

IN FUEL SALT TO THE THEORET!ICAL CONCENTRATION

 
 

 

MEASURED CONCENTRATION IN SALT {dpm/g—salt)

 

THEORETICAL CONCENTRATION IN SALT (dpm/g—salt)

50

ORNL-DWG 71-1877A

THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE
METALS DO NOT DEPOSIT ON LIQUID--GAS INTERFACE

©® 17-31 FREEZE VALVE CAPSULE

O 17-32 DOUBLE-WALL FREEZE VALVE CAPSULE
SAMPLE NUMBER TAKEN DURING

THIS RUN NUMBER

122

SAMPLE GROUPING NO. 2

® © 146
14-20

\

 

0 0.1 02 03 04 05 06 0.7 08 09 1.0

| FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

FIGURE 5.5. COMPARISON OF MEASURED lO6RU CONCENTRAT I ON

IN FUEL SALT TO THE THEORETICAL CONCENTRATION
 

 

pat

»

ORNL-DWG 71-1878A

THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE METALS
DO NOT DEPOSIT ON LIQUID-GAS INTERFACE

® 17-31 FREEZE VALVE CAPSULE
O 17-32 DOUBLE-WALL FREEZE VALVE CAPSULE

SAMPLE NUMBER TAKEN DURING
THIS RUN NUMBER

SAMPLE GROUPING NO. 1 (SEE TEXT)

[ 13

 

MEASURED CONCENTRATION IN SALT (dpm/g-salt)
THEORETICAL CONCENTRATION IN SALT (dpm/g-salt)

7-2
l9- C]}?'36
018-2 - 14-20 "17-31

' < O1s-
18-4 18-19 = :
®:1.30 SAMPLE GROUPING NO. 2

017-32 7 |

| ’ . ]4-66

 

0 0.1 0.2 0.3 0.4 05 0.6 0.7 0.8 0.9 1.0
FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

FIGURE 5.6. COMPARISON OF THE MEASURED °°MO CONCENTRATION

_RIN FUEL SALT TO THE THEORET ICAL CONCENTRATION

L]

 
 

MEASURED CONCENTRATION IN SALT (dpm/g-salt)
THEORETICAL CONCENTRATION IN SALT (dpm/g-sa

#1103

 

52
ORNL-DWG 71-1879A

THEORECTICAL CONCENTRATION COMPUTED FOR CASE WHERE NOBLE
METALS DO NOT DEPQSIT ON LIQUID-GAS INTERFACE

@ 17-31  FREEZE VALVE CAPSULE

O 17-32 DOUBLE-WALL FREEZE VALVE CAPSULE

b SAMPLE NUMBER TAKEN DURING
THIS RUN NUMBER

SAMPLE GROUPING NO. 1 (SEE TEXT)

O 20-1

102

10

1.0
o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY
FIGURE 5.7. COMPARISON OF THE MEASURED |29mTE CONCENTRATION
IN FUEL SALT TO THE THEORETICAL CONCENTRATION

 

"
 

"

"

i

+

MEASURED CONCENTRATION IN SALT (dpm/g-salt)

 

2
104
5
2
’E .
w103
&
~—
E
o
T 5
—
-
<X
[ 7]
-
=
s
2
—
2 102
=
el
)
=
S
-
=T
OO
=
o
o
=
’.—
10
.
2

1.0

FIGURE 5.8. COMPARISON OF THE MEASURED
IN FUEL SALT TO THE THEORETICAL CONCENTRATION

 

53

ORNL-DWG 71-1880A

 

| ® 17-31  FREEZE VALVE CAPSULE

- _ THIS RUN NUMBER

— THEORETICAL CONCENTRATION-COMPUTED FOR CASE WHERE
— NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE

— O 17-32  DOUBLE-WALL FREEZE VALVE CAPSULE
— SAMPLE NUMBER TAKEN DURING

 

 

 

 

 

 

 

 

 

®138-6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SAMPLE GROUPING NO. 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
 

 

 

(SEE TEXT)
@ 17-317
©17-22 ‘,‘.7'2
. _SAMPLE —
S, GROUPING NO. 2 —
. S0 ~d—]
O18-44 _ ~ \]%20 @ 4- ]
i | -36 — |
o17-32 Y
T O 18-4 l______,
18-12 O
- 19-76
{

 

, o o | 0.23 '
0. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY

|32

TE CONCENTRATION

 
 

 

 

2
104
5
2
o 3
R}
210
+ | ©
~— w
o 1
v | O
| S .
] E 5
~1 Q.
E|lw
o]~
o
e | p—
-
— |=<C
-l [
& |=
= 2
=
— | =
o
= | —
Q- 2
= =g
<C |-
o | =
=~ |w
= |
=
O IO 5
= 1O
O
[ iy Q|
<L
010
L | —
o |~
= Ll
v | o
< |O 2
L]l
=|x
-
10
5
2
‘ 1.0

54

ORNL-DWG 71-1882A

 

8.7 x 10°
@17-2
19-76
@ 12-66
O 18-19
THEORETICAL CONCENTRATION COMPUTED FOR CASE WHERE
NOBLE METALS DO NOT DEPOSIT ON LIQUID-GAS INTERFACE
O 17-31  FREEZE VALVE CAPSULE
‘@17-32  DOUBLE-WALL FREEZE VALVE CAPSULE
SAMPLE NUMBER TAKEN DURING
THIS RUN NUMBER |
- F
5
o
o 0.1 0.2 03 0.4 0.5 0.6 0.7 0.8 0.9 1.0
FUEL SALT IN CAPSULE AS FRACTION OF TOTAL CAPSULE CAPACITY
FIGURE 5.9. COMPARISON OF THE MEASURED ~°NB CONCENTRAT ION

IN FUEL SALT TO THE THEORETICAL CONCENTRATION
 

 

.

9

 

55
95

can result in a negative ““Nb concentration. The shotgun pattern and

95Nb in Figure 5.9 is

also the disappearance and reappearance of
apparently related in some way to the fuel salt fluctuating between the
oxidized and reduced state.
3. The sample capsules shdwing the highest ratio of measured to
theoretical concentration are mostly empty. The circled cluster of
samples labeled as Sample Grouping No. 1 are representative of this end
of the curve. They are all double-wall freeze valve samples. They con-
tain 2-5 grams of salt but if they were full they would hold 1k-15 grams.
I Conversely, the samples showing the lowest measured to theoretical
concentration ratio are full. The circled cluster lebeled Sample Grouping
No. 2 is uéed to represent this end of tfie curve. This group represents

235U runs. In taking

the only freeze valve sample taken during the
samples, these capsules were filled from 75% to 90% of capacity.

5. The samples are internally consistent; i.e., the concentration
of each noble metal isotopes is practically always either high or low in
any individual semple. This is illustrated by the circled clusters of
date points referred to as Sample Grouping No. 1 and Sample Grouping No.
2. FEach grouping on each figure encompasses the same samples. Notice
how consistently tight each grouping is. Sample Grouping No. 1 repre-
sents the high and of thé curve and Sample Grouping No. 2 represents the
lower end of the curve. |

6. One might make the observation that the curves seem to be

- approaching a low value of the measured to theoretical concentration ratio

at high sample weights. Particularly the shorter lived isotopes
(99Mo end 132

concentration = theoretical COncentration). This observation is not

Te) almost seem to be approaching a value of unity (measured

warranted. Recall that the theoretical concentration is computed for

the case that noble metals 4o not deposit on liquid-gas interfaces.

“We have hypothesized in previous sectionsland later in this section that

they must deposit on these interfaces. If this depository is included

in the calculation then the computed concentration will be reduced and
the measured to theoretical concentration ratio will be increased
accordingly. The uncertainties associated with this term are so great

(particularly the sticking fraction) that we cannot go into any detailed

 
 

56

.analysis of it. Let us only say that if we did include this term in

the calculatlon then the datsa p01nts in Figures 5.4 - 5.8 could rise more
or less uniformly by as much as a factor of 5 during the 35U operation
(stick fraction to bubbles = 1.0) and a good deal more for the 233y
operation. |

How can we explain these curves in a manner consistent with previous
observations and conclusions on noble metal migration? We have already

hypothesized that noble metals deposit on liguid-gas interfaces (circula-

ting bubbles), and data from the heat exchanger and core surveillance

samples seem to confirm this. Many qualitative observations discussed
in section 3 also confirm this hypothesis. Presumably the bubbles would

carry the noble metal atoms or small clusters of them to the pump where

they would collect on the large amount of liquid-gas interface available

in this region. The small clusters could then coalesce into larger

clusters and eventually form a scum floating on the salt. Let us hypothe-
size that these noble metal clusters have properties similar to insoluble
surface active agents and that the pump bowl is behaving in a manner

similar to a froth flotation chamber. This suggestion has been made
(31, 32)

before. It has been suggested in Section 3.3 of this report that

noble corrosion products and fission products accumulating in the pump

bowl and displaying the properties of insoluble surface active agents

235 233

are responsible for the differences between the U and U operating

characteristics of the MSRE. As a matter of fact, flotation of reduced
Fe, Ni and Cr from fuel salt has been demonstrated in a test tube scale
(32)

laboratory experiment. Froth flotation is a common unit operation

in the ore processing industries. The fundamental requirements for a
flotation process to work are:(33)
1. The solid phase must be insoluble in the llquld phase.

2. The gas-liquid-solid contact angle must be greater than zero

(the solid only partially wet by the liquid).

3. The solid partlcles must be small enough so that their sheer

mass will not sink them below the surface (gravity forces < surface

tension forces).

i
 

 

)

L4

 

o7

The noble metals (and noble corrosion products from the chemical re-
processing) satisfy these conditions. They are insoluble in fuel salt.
Contact angles are reported in the range of 90-105°.(17) They are mostly
unwet and in & range for good flotation. Particle diameters of materials
obtained from the gas phase over salt samples'have been estimated by various
techniques. These diameters, thought to be indicative of noble metal 7
particle diameters, range from a few angstroms to a few microns.(3h’35’36) )
The lower end of the range possibly represents the diameter of individual
particles where the upper end is associated with flocks of particles. The
uncertainty of effective particle dismeter is unimportant since the entire
renge falls in a region of good flotation. As noted in Section 3.3, the
property of surfactants that is significant to this analysis is that they
enhence bubble stability and also result in smaller bubbles being generated
in the pfimp bowl. It would not seem unreasonable then, that small bubbles
are present in the salt sampling pool either as a feirly stable froth
floating on the salt or as very small bubbles drifting with the salt or
both. These bubbles would be high in noble metal content (on their sfirface).
When a freeze valve capsule thaws and draws its sample, it would be getting
a greater or lesser amount of gas with the salt, depending on the situation
in the salt pool at thatitime.‘_If it took in a large amount of gas, the
capsule would be mostly empty but have a high noble metal content (as for
instence Sample Grouping No. 1 on Figures 5.1 - 5.8)./ If it took in a small

amount of gas, it would be full and relatively low in noble metal content

(as Sample Grouping No. 2 on the samefiguresfl This then is the suggested

explanation for the curves in Figures 5.4 - 5i8. Note that if this hypothe-

sis is true, then even the least contaminated samples, represented by

_Sam@ie Groupihg*No. 2, are still not representative of the fuel salt.

- One might expect to be able.to'correiate the cofidition-df the salt

- sample (high or low noble metal concentration) with some other measurable

*'operating varisbles of the reactor, for instance,\overflcw rate, cir-

culating void fraction, etec. An attempt was made to do this, however,‘the

MSRE being an experimental reactor, was subjected to a great many per-

tuibations-(pcwer level changes, reductent and oxidant additions, pump
speed changes, draining and flushing the fuel loop, cover gas changes, ete),
most of which would be expected to effect the condition of the sample, and

we were uneble to extract any obvious correlations.

 
 

58

The interal consistency of the samples allows us to carry the analysis
one step further. As noted in Observation No. 5, the concentrations of all
noble metal isotopes are usually consistent in an individual sample (either
high or low). This is illustrated by Sample.Grouping No. 1 and No. 2 in
Figures 5.4 - 5.8, Each grouping-on each figure contains the~samé samples,
and each grouping is rather tight. The geometric mean of each group was
determined and plotted againét the noble metal half life. The resulting
correlation is shown in Figure 5.10 and represents a plot similar to that
used in enalysis of noble metals on the heat exchanger and core surveil-
lance samples. Recall that the equivalent plots for the heat exchanger and
core surveillance sample yielded a line with a slope of zero, whereas the
slopes of the lines in Figure 5.10 arelQuite high. If the analytical model
is correct and the samples are truly representative of the fuel salt, then
the slope of this line should also be zero. We have shown hofiever that the
salt samples are not representative of the salt, but rather are more
representative of the noble metals accumulated on the liquid-gas interfaces
in the pump bowl. It is not surprising then, that the slope is not zero. .
In Section 5.6 of this report we will attach some physical significance to
the slope of these lines.

Figure 5.11 is the same kind of plot as Figure 5.10, but instead of
picking grouped data points, we averaged geometrically all the freeze valve
and double walled freeze valve capsule data for each MSRE run. Beside each
data point in parenthesis is the number of samples included in the aversge.
Note that the curves move up the ordinate with run number. This observation
will be made more drematic when we compare it to the same kind of plot for
the gas phase samples. The gas samples do the reverse, that is, decrease
with run number. We will discuss this observation at that time.

Conclusion. The emphasis in this section from examination of fuel
salt samples, concerns the attachment of noble metals and reduced corrosion
products to liquid-gas interfaces, and how they epparently accumulate in the
MSRE pump bowl and enhance the frothing and bubble making capacity of
this region. This is consistent‘with conclusions reached from examination

of data from the primary heat exchanger and core surveillance samples.
 

 

 

*

L]

*

»}

L

 

29

ORNL-DWG 73-1549
105

DATA POINTS REPRESENT -GEOMETRIC AVERAGE OF CIRCLED GROUPS
ON FIGURES 5.4 - 5.8 '

104 O SAMPLE GROUPING NO. 1
® SAMPLE GROUPING NO. 2

103

102

MEASURED CONCENTRATION IN SALT (dpm/gm-salt)
THEORETICAL CONCENTRATION IN SALT (dpm/gm-salt)

 

10

1
o1+ 5 20 103 5 2 102 5 2 . 10

S

 

NOBLE METAL HALFLIFE {hr)

FIGURE 5.10. COMPARISON OF MEASURED TO THEORETICAL CONCENTRATION
IN FUEL SALT TO THE NOBLE METAL HALF LIFE BY GROUPS
 

 

 

 

 

60

ORNL-DWG 70-15022

10%
DATA POINTS REPRESENT GEOMETRIC AVERAGE OF CONCENTRATION RATI
FROM ALL SALT SAMPLES TAKEN DURING INDICATED RUN

5
- 129m

(2}
(2

(5

103

102

 

MEASURED CONCENTRATION IN SALT (dpm/gm-salt)
THEQRETICAL CONCENTRATION IN SALT (dpm/gm-salt)

10

 

10 2 5 102 2 5 103 2 5 104
NOBLE METAL HALF LIFE (hr)

FIGURE 5.11. COMPARISON OF MEASURED TO THEORETICAL CONCENTRATION
| IN FUEL SALT TO NOBLE METAL HALF L1FE BY RUN NUMBER
 

&

»

*

o

 

61

5.5 Comparison of Gas Samples with Cs

 

The fact that the noble metals are found in large quantities in the
gas samples from the pump bowl may be surprising. They have a vanishingly
small vapor pressure so that certainly isn't the mechanism. In Section 5.2
it was noted that noble metal atoms or very small clusters of them can be
spontaneously expelled from the surface of quiescent fuel salt. This in-
formation was determined in a carefully controlled laboratory test. Under
conditions that exist in the pump bowl, one may feasonably expect the bulk
of these single atoms and very small clusters to be trapped and coalesce
with the larger amounts of noble metal fission products (and noble cor-
rosion products from the chemical reprocessing) in the form of scum and
deposits on the bubbles, rather than to break through and migrate to the
off-gas system themselves. A more likely mechanism, which is consistent
with the conclusions reached so far in this report, is that most of the
noble metals measured in the gas samples are carried by a salt mist that is
high in noble metal content. The impingement of the Jets from the spray
ring generates large quantities of bubbles. These bubbles then work their
way back to the surface and become froth. We have theorized that this
froth is high in noble metal content. A well accepted observation is that
pure liquids will not foam. If there is foam present in the pump bowl,
it is surely heavy in noble metal fission products and reduced corrosion
products. Presumably the pump bowl reeches a steady state foaming condi-
tion in which bubbles burst as fast as they are formed. .They burst spon-
taneously or because of mechanical action of the spray. At any rate this

process is accompanied by the(ge?eration of a salt mlst whlch is highly
37

'concentrated in noble metals. Presum&bly this mist will drift into the
- g8s phase of the sampling volume and influence the gas samples. Small

"amounts of salt-seeking fission products are elwqys found in gas phase

samples indicating fuel salt is present., This mist wduld also migrate
into the off-gas system Where noble metals have been found in signiflcant
concentratlons. Thls then is the suggested mechanlsm for the appearance

of noble metals in gas phase samples,

 
 

 

 

62

The analytical model developed in Section L does not contain a term
for the noble metal concentration in the gas phases. There is no funde-

mental basis for using concentration units. The best we can do is to com-

- pare the measured noble metal concentration in the gas sample to the

theoretical concentration in the fuel salt, at the time of sampling. The
theoretical concentration in salt then becomes & normalizing factor: The
same conditions apply to the calculated concentration és before. The
principle beihg that the theoretidal concentration is computed for the case
that noble metals do not migrate to liquid-gas interfaces. Figure 5.12 |
shows the results of this calculation; The ordinate normalizes the total
noble metal count in the gas sample to the theoretical concentration in
one grem of salt. The abscissa normalizes the total measured dmount of
salt seeking fission products in the Sample to the theoretical amounf
in one gram of salt. Since the salt seekers are uniformly distributed in
the fuel salt, the abscissa is measured in the amount of salt in the sample.
In order to decrease the scatter and the total number of data points, the
ordinate value of each data point represents the geometric mean of the
five noble metals isotopes indicated in the plot and the abscissa value
represents the geometric mean of four salt-seeking fission product |
isotopes indicated. There does seem to be a correlatioh between the
amount of noble metal fission products in a sample and the amount of salt-
weeking fission products. Although the scatter is high, the noble metal
content of a sample is in proportion to the salt content. This is con-
sistent with the idea that the fuel salt is the carrier of noble metals.
Figure 5.13 is a plot of gas sample data equivalent to Figure 5.11
for salt samples. All the reported data from routine gas samples taken

233U run were geometrically averaged and plotted against the

during each
noble metal half life. The number in parenthesis beside each data point
indicates the number of samples used in the geometric average. Note that
the curves move down the ordinate with time as indicated by the rufi number.
As noted before this behavior is particularly striking when these curves
are compared with the curves in Figure 5.11 for salt samples. The salt
semple curves move up the ordinate with time, but less uniformly than the
gas sample curves., This behavior is not understood but probably repre-

sents a change in frothing characteristics in the pump bowl with time.
 

 

&

»

  

MEASURED AMOUNT OF NOBLE METALS IN GAS SAMPLE (dpm/sample
THEORETICAL CONCENTRATION OF NOBLE METALS PER GRAM OF SALT (dpm/gm-

 

63

ORNL-DWG 70-15018
1.4 X 104

  

10%
® 18-15 - FREEZE VALVE CAPSULE _ .02
O 18-14 - DOUBLE-WALL FREEZE VALVE CAPSULE 18-21
5 SAMPLE NUMBER TAKEN DURING

THIS RUN NUMBER

103

 

2
fim -28
102
19-410 _

ORDINATE - EACH DATA POINT IS GEOMETRIC

5 AVERAGE OF RATIO FOR 103Ry,
. | 106Ry, 99Mo, 129mTe AND 132Te

20-270 ABSCISSA - EACH DATA POINT IS GEOMETRIC

' 91
19-770 _ . AVERAGE OF RATIO FOR 51,
) . zr’ lklce’ lhfice
20-90 O19- 019-20 | 0

1e-e50 oz | P19
| ' 019-78
5 X 1076=0 19-38 i |
N ' 20-12 | .
10-5 -2 5 . 10°% 2 .5 107 2 5 10-2

MEASURED AMOUNT OF SALT SOLUBLE FISSION PRODUCTS IN GAS SAMPLE (dpm/sample
THEORETICAL AMOUNT OF SALT SOLUBLE FISSION PRODUCTS PER GRAM OF SALT (dpm/gnm-saTt

FIGURE 5.7 1.2. COMPARISON ‘OF THE AVERAGE AMOUNT OF NOBLE METALS
IN GAS SAMPLES TO THE AVERAGE AMOUNT OF SOLUBLE FISSION PRODUCTS
IN GAS SAMPLES

 

 
 

 

6L

.ORNL-DWG 7G-14019

105
DATA POINTS REPRESENT GEOMETRIC AVERAGES OF INDICATED RATIO
5 OF ALL GAS SAMPLES TAKEN DURING INDICATED MSRE RUN
O RUN NO. 17
RUN NO. 18
A RUN NO. 19
2 A RUN NO, 20

 

MEASURED AMOUNT IN GAS SAMPLE (dpm/sample)

 

10 2 5 102 2 5 108 2 5 104
NOBLE METAL HALF LIFE {hr) -

FIGURE 5.13. COMPARISON TO THE MEASURED (GAS SAMPLE)
TO THEORETICAL (IN SALT) CONCENTRATION TO NOBLE METAL HALF LIFE
| BY MSRE RUN NUMBER
 

 

e 15 it i e i i i

$.

»

iy

 

65

Recall that et the start of 233

corrosion products (which act as noble metals) suddenly appeared in the

U operation, a large amount of reduced

reactor system. Over a period of time they would eventually work their
way out of the reactor system and into the off-gas system or possibly
the overflow tank or dump tank. At any rate the foaming characteristiecs
of the pump bowl might be expected to change as the amount decreased.
Conclusions. The only conclusion that cean be reached from analysis
of the gas samples is that the noble metals in the gas samples owe their
existence to a salt mist generated by bursting bubbles that are high in
noble metal content. This confirms previous cqnclusions that noble metals

adhere to liquid-gas interfaces.
5.6 Time Constant for Noble Metals on Interfaces

We have concluded that noble metals deposit on liquid-gas interfaces

and remain there with a high degree of stability. This conclusion was

reached from qualitative observations of the reactor behavior, and from

noble metal migration analysis in this report. In fact, it appears that
the measured noble metal content of both, the fuel salt and gas phase
samples, is primarily a measure of the concentration associated with these
interfaces. This was the rationalizatiofi to explain the slopes of the
lines in Figures 5.10, 5.11 (salt samples) and 5.13 (gas samples) being
other than zero. If we could write an equation for the amount of noble
metals associated with the liquid-gas interface, we could compare the
measured concentrations from fuel salt_a@d gas phase samples to this
quantity'andrpoSsibiY“make a'significaht‘observatioh. An equation to do
this was derived in Sect. 4.2.10 (Eq. (27)).This e@uatioh is the result of

a rate balance arqund_the entire gas phése of the fuel loop where the gas

. phase is defined to include the'liquid«gas'interfaces._ It considers the

circuiating_bubbles and the gas phase of the pfimp»bqwl and all their
respective interfaces to be weli mixed. _Thefe/is no fundamentel basis

for using cohqehtration unitsrin the gas phase, so we will compute the
tgtal smount of noble metélsjin this\fiikture. The sink terms for noble
metals in the.gés phase ‘involve deéay'and an afbitrary rate constant‘which
is a measure of the rate at fihichrthey migrate out of the gas phase as

defined above. In application we will evaluate the undefined rate

 
 

 

66

constant by forecing the slope of curves equivalent to those referred to

gbove to equal zero. Then we will see if any reasoneble physical sig-
nificance can be assigned to these rate constants. For this analysis
we will show only the gas sample data from Run 19. These data will be

used, first, because they represent the largest number of samples (15)

taken during a single run, second - because the data points are tightly

grouped around the line (Figure 5.13) and third - because the slope of
the curve (Fig. 5.13) is intermediate between the slopes of all the
curves in Figures 5.10, 5.11 and 5.13.

The results of the calculation appear in Figure 5,14, where the

- measured noble metal in the gas sample (dpm/sample) normalized to the

totél amount theoretically associated with the liquid-gas interface and

gas vhase phase (dpm) is plotted against the noble metal half life for

- three different values of the rate constant. Each data point is the

geometric average of the indicated ratio for all gas samples taken during
Run 19. The sbsolute value of the ordinate is of little or no signifi—
cence in this analysis since we are only looking at the slopes of the
lines. It is apparent that no single rate constant will flatten out

the entire curve. How then can we rationalize these curves? Note

that the curve between the 1O3Ru and 129mTe points and the 132 99

Te and ““"Mo
points has a zero slope when the rate constant of noble metals in the
pump bowl is about 0.01 hrs-:L (equivalent half life approximately 3 days).
If the horizontal line is extended'whan one would observe that méfe

106_Ru was measured than this analysis would indicate. If a mechanism

‘were present whereby noble metals couid be stored out of the reactor

system for a few hundred days and then introduced into the reactor

system, the measured 106Ru concentration would be high. In a few hundred

' days the shorter 1ivedlisotopes would have decayed away; therefore, the

slope of the right hand slide of the curve would not be significantly
affected., ILikely suspects for this mechanism would be the overflow tank
and the dump tank. The other groups of gés and fuel salt samples have
been anelyzed in this framework but the results will not be presented. Let
me summgrizerby séying that they are more or less consistent. That is, the
curves in Figures 5.10, 5.11 and 5.13 can generally be reduced to a line

with slope zero if a half life in the pump bowl of a few days is imposed

O
 

 

 

 

67

ORNL-DWG 70-15020
10-6

A FROM GAS SAMPLES OF RUN 19 (FUEL - 233y)

10-7

   

» EQUIVALENT RATE
HALF LIFE CONSTANT
da (hr-1)

1081 2.9 0.0

29 6.001
5| 29 . o0.0001

MEASURED AMOUNT IN GAS SAMPLE {dpm/sample)
THEORETICAL TOTAL AMOUNT IN GAS PHASE INCLUDING INTERFACE (dpm}

10-2

 

1o-10 - ' ——
10 2 5 102 2 5 0 2 5 10

NOBLE METAL HALF LIFE (hr)

 FIGURE 5.14. COMPARISON OF MEASURED AMOUNT IN GAS SAMPLE
~ TO TOTAL THEORETICAL AMOUNT IN GAS PHASE INCLUDING INTERFACE
WITH PARAMETERS OF AN ARBITRARY RATE CONSTANT FOR REMOVAL FROM SYSTEM

 
 

 

 

68

on them plus a long decay time of a few hundred to several hundréd days

after which they are re-introduced into the reactor system. The few
days half life may be associated with the time required for the spray
ring to splash half the noble metals into the off-gas system as a

mist. The few hundred to sevefal hundred day decay time may be associa-

ted with the overflow tank or with the fregquency of draining and refilling
fuel salt in the fuel loop. :

Conclusion; The slopes of the curves in Figures 5.10, 5.11 and 5.13
can be explained in terms of reasonsble physical phenomena in the MSRE.
The physical phenomena have as‘théir foundation the thesis that noble

metals migrate and adhere to liquid-gas interfaces.

5.7 Miscellaneous Noble Metal Observations |

5.7T-1 Laminar Flow Core Surveillance Sample
Following Run 18, the regular core surveillance specimen fixutre

" (Fig. 2.4) was removed and a special test fixture was installed. Since

this was to be the last chance to expose material to an operating molten-
salt reactor core, a large number of special experiments was incorporated
in this assembly, meny of which were not directly related to fission
product deposition. The entire assembly is described in detail in

Ref. 38. One of the elements that was devoted to fission product deposi-
tion is illustrated in Figure 5.15. It was designed to generate laminar
flow in a thin annular flow region, the outside surface being graphite
and the inside surface being Hastelloy-N. Different surface roughnesses
were also incorporated as shdwn in the figure. The intent of this test
was twofold. First, since the graphite and Hastelloy-N annular surfaces

were exposed to essentially identical fluid dynamic conditions, any

‘difference in noble metal deposition would be a direct measurement of the

sticking fraction of one surface relative to the other, and second, to
note sny effects of surface roughness. This sample fixture was removed
after Run 20 when the MSRE was permanently shut down, and the noble metal

deposition was measured.

F
 

~

 

69

ORNL-DWG 71-1883

fe——— 5 M5 ——tpatema——— 125 rms INSIDE GRAPHITE SURFACE ROUGHNESS

 

 

- 5 rms ————wf=e——— 25 rms ——=={=— [|25 rms*{ OUTSIDE GRAPHITE SURFACE ROUGHNESS

2 // //////////V///////////,l////////5

 
  

 

    

 

 

// GRAPHITE O,fiTER ,'BOE'W’///////////Z(

\II'IG in. THICK ANNULAR FLOW PASSAGE

APPROXIMATELY TO SCALE ~

FIGURE 5.15. SCHEMATIC OF THE LAMINAR FLOW CORE SURVEILLANCE SAMPLE

 
 

 

 

T0

In determining the measured amount of noble metals on these surfaces,
the Hastelloy-N core was sectioned at the surface roughness lines of
demafkation. Two fission product determinations were therefore made,
one for each end. Similarly, the graphite oufier beody was cut into three
sections, one for each surface roughness on the outside, Note that the
inside has two surface roughnesses. Theoretical amounts of noblé metals
were computed and compared to the measured amounts in Figure 5.16. Again
the computed amount is'for the case where noble metals do not deposit on
liquid-gas interfaces. The estimated salt velocity through'the annulus
is 0.7 ft/sec. This will yield a Reynolds number of 206 and a mean mass
transfer coefficient throughout the annulus of about 0.23 ft/hr. The

data points on Figure 5.16 represent the geometric averages of all determ-.

inations on' the Hastelloy~N and graphite annular surfaces. The wings
indicate the maximum and minimum data points. There was considerable
scatter in the data. It was not possible to distinguish fluid dynamic
entrance effects and surface roughness effects from the data. For laminar
or marginally turbulent flow, the surface roughness effects should have
been minimal. Date from the outside surface of the graphite body indi-
cated this to be true. The outside fluid dynamic conditions are not

known but the flow should have been either laminar or just barely
turbulent. The curves in Figure 5.16 are generally consistent in magni-
tude of the ordinate with those from the primary heat exchanger (Fig. 5.1)
and the Hastelloy-N and graphite core surveillance specimens (Figures

5.3 and 5.2, respectively). The principal observation is that the sticking
fraction to graphite is apparently less than the sticking fraection to
Hastelloy-N. Because of scatter in the data, it is difficult to draw

a firm value, but something in the range of 0.1 - 0.6 would be in order.

5.7.2 Noble Metal Distribution in the MSRE
It would be informative to determine the noble metal distribution

in the reactor fuel system. The results of this inventory calculation

are shown in Table 5.1.

h

 
MEASURED AMOUNT ON SPECIMEN (dpm/cm?)

 

T1

] ORNL-DWG 71-1881

0
THEORETICAL AMOUNT COMPUTED FOR CASE WHERE NOBLE METALS DO NOT
DEPOSIT ON LIQUID-GAS INTERFACE

5.0 O HASTELLOY-N SURFACE

® GRAPHITE SURFACE

2.0

1.0

0.5

0.2

HASTELLOY-N

o
.
—

0.05

 

THEORETICAL AMOUNT ON SPECIMEN (dpm/cm2)

0.02

 

0.01
, 0.0016 . 0.006 ‘
10 2 5 102 2 5 103 - 2 5 10 2 5 10°

_ NOBLE METAL HALFLIFE (hr)

FIGURE 5.16., COMPAR!S_ON OF THE MEASURED TO THEORETICAL AMOUNTS OF NOBLE
METALS ON THE LAMINAR FLOW CORE SURVE!LLANCE SPECIMENS

 
 

 

between the

T2

Table 5.1

Noble Metal Distribution in the MSRE¥

 

 

During 235U Runs During 233U Runs

Noble Metals on Heat Exchanger Lo% 6%
Surfaces
Noble Metals on Other Hastelloy-N 50 8
Burfaces in Fuel Loop .
Noble Metals on Graphite Surfaces 1 0.k
in Core
Noble Metals in Pump Bowl, Overflow 9 : 86
Tank, Off-Gas System, etc. (by
Difference) —_—

100% 100%

¥Reported as percent of total inventory of ndble metals in the reactor
system as time measurements were made.

In determining this distribution, the measured amounts of noble metals
on the heat exchanger were the same values used to determine the ordinate
of Figure 5.1. The percent of inventory of noble metals on other
Hastelloy-N surfaces in table, was determined by multiplying the percent
of inventory on the heat exchanger by the appropriate ratio of products
of mass transfer coefficients and surface area as tabulated in Table
B-1. The measured amounts of noble.metals on the graphite surfaces were

the same values used to determine the ordinate of Figure 5.2. Specifical—

1y, the data were obtained from the core surveillance samples removed

after run 1k (235U) and run 18 (233U). The measured concentrations from
the core surveillance samples were then multiplied by a ratio of mass
transfer coefficients (0.063/0.25) to convert them to concentrations on
the bulk graphite (see Appendix B). The difference between 100 percent
énd all the above sinks was assumed to have deposited on bubbles and is
therefore distributed in an unknown manner between the pump bowl, over-
flow tank, off-gas system and dump tank. The difference in distribution
235U and 233

U runs is quite dramatic, but may not be as
large or as significant as it appears. Recall from Section 3.2 where the

A ]
 

 

e b e

 

¥

was used for noble metal deposition'following run 14 (

 

T3

gamma spectrometry of the heat exchanger is discussed, it was pointed out
that the lower measured values were used for noble metal deposition on
233U*bu‘t more of an average value

23511) . If the

the heat exchanger following run 19

measured values following run 14 were weighted lower, then the percent of
inventory on the heat'éxchangér and other Hastelloy-N surfaces in

Teble 5.1 for the 235
pump bowl would be higher.

U runs would be less, and the noble metals in the

6. CONCLUSIONS AND RECOMMENDATIONS
- Conclusions

1. Noble metal fission products migrate from the fuel salt, where
they are born, to their various depositories in accordance with the laws
of mass transfer theory. ,. | |

2. Each‘noble metal isofope migrates as a function of its own con-
centration in fuel salt, and is nét influenced‘byrother elemental species

of noble metals, or even isotopic species of the same element. There are

" however second order effects(prdb&bly chemical) which must be considered,

for instance, a chemical interaction between Nb and graphite had to be

95

resorted to, in order to explaln the high ““Nb concentration found on the
MSRE core surveillance samples.,

3. The liquid-gas interface (circulating bubbles and the free sur-
face in the pump bowl) apparently present a stable surface for noble metal
depositibn, however; the effective‘sticking_fraétion to this interface is
considerably less than unity. This observétion shbuld not be regarded as
ab501ute1y'conclusive, rather7it'is primarily a deduced-phenomenon used
to explain many of the results in this analysis and many of the qualita-
tive observations from the MSRE.

4. Assoc1ated with the last conc1u51on is the idea that noble metal
fission products and also reduced corrosion products attached to a liquid-
gas interface have mény of the properties of an insoluble surface active
material. Again, this idea should not be regafded as conclusive. It was,
however, used to suggest an explanation for the dramatic differences in

235 233

MSRE operating characterlstlcs between the® U runs.

 
 

 

 

Th

5.. The sticking fraction of noble metal fission products to graphite

is apparently less than the sticking fraction to Hastelloy-N.

Recommendations

 

The fundamental questions are - how does one extrapolate these results

from the MSRE to a large molten salt reactor in a quantitative manner, and

- can one use noble metal migration to his advantage in the design of a

(39)

| reactor? The conceptual design of a single fluid 1000 MW(e) MSBR calls

l35Xe. In addition, any molten

for circulating helium bubbles to remofie
salt circulating pump would probably have a free liquid surface associated
with it to eliminate the need for a packing gland. There may be other

liquid-gas interfaces associated with a large reactor. Presumably then,

‘noble metals would migrate to these surfaces. The amounts would be extreme-

ly difficult to estimate because of uncertainties in the sticking fraction
and other parameters. Then there is the question of what happens to noble

metals after they reach these interfaces. If the pump bowl is rather

'quiescent, noble metals would accumulate and develop into a heat source.

On the other hand, noble metals associated with eirculating bubbles may
migrate by some mechanism into the off-gas system and settle out on |
various components there. Noble metal sinks associated with liquid-gas
interfaces cannot be handled quantitatively at this time.

It is recommended then, that noble metal migration be studied in a
circulating salt loop. A special loop may not have to be built, but
rather a loop built for some other purpose could be used. Noble metals
(probably their precursors) could be added to the salt in tracer amounts
and the following studies made. | |

1. Deposition on solid surfaces and correlation of results with
méss transfer theory. |

2. Deposition on liquid-gas interfaces and correlation of results

with mass transfer theory.

3. Stabilizing effects of noble metals (fission products and cor-
rosion products) on bubble and foam interfaces.
L. Effects of the noble metal chemical species.

5. Effects of the chemical state of the salt.
 

 

Ly

5

6. Various aspects of circulating bubble mechanics when noble metals
are attached to their surfaces, such as

a. Interaction of bubbles with solid surfaces.

b. Bubble dissolution - This probaebly results in a
small packet of noble metals associated again with
the salt. |

c. Bubble renucleation - The small packet from sbove will
then probebly serve as an excellent bubble renucleation
site.

7. Prototype testing of bubble sensitive components proposed for the
next generstion molten salt reactor to estimate the deposition and further
migration of noble metals in these areas.

These studies would be accompiished with tracer'quantities of noble
metals. The behavior of the next generation reactor would undoubtedly be

a function of the gross amounts of noble metals and reduced corrosion

'products present, and we couldn't hope to properly simulate this in a cir-

culating loop. Therefore, the results of the proposed study would not
answer all the questions pertainiug to noble metal migration in the
reactor, but it would cérfainly lay a gdod foundation. The complete story
of noble metal migration would ultimately have to come from the operation

of the reactor itself.

 

 
 

 

lo.

11.

12.

13.

1k,

15,

16.

76

T. REFERENCES

R. C. Robertson, MSRE Design and Operations Report: Part 1, Déscrip—
tion of Reactor Design, USAEC Report ORNL-TM-T28, ORNL, January 1965,

. Nuclear Applications and Technology, V. 8, N. 2, February 1970.

Molten-Salt Reactor Program Semiannual Progress Report for Period

Ending July 31, 1964, USAEC Report ORNL-3708.

. dJd. R. Engel et.al., Spray, Mist, Bubbles and Foam in the Molten

Salt Reactor Experiment, USAEC Report ORNL-TM-3027, ORNL, June 1970.

.J. C. Robinson and D. N. Fry, Determination of the Void Fraction of

the MSRE Using Small Induced Pressure Perturbations, USEAC Report
ORNL-TM-2318, ORNL, Feb. 6, 1969.

Molten-Salt Reactor Program Semiannual Progress Report for Perlod

‘Ending February 28, 1970, ORNL-4548, p 115.

Molten-Salt Reactor Program Semiannuel Progress Report for Period
Ending August 31, 1966, ORNL-L4037, p 181.

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending August 31, 1968, ORNL-434Lk, pp 1u4L-1Ls5, =

R. J. Kedl, A Model for Computing the Migration of Very Short Lived
Noble Gases into MSRE Graphite, USAEC Report ORNL-TM-1810, ORNL
July 196T7.

R. Blumberg et. al., Measurements of Fission Product Depdsition in
the MSRE with Ge(Li) Gamma Ray Spectroscopy, ORNL Report ORNL-CF-

- 68-11-20, November 19, 1968, Internal Distribution Only.

A, Houtzeel and F. F. Dyer, A Study of Fission Products in the Molten-
Salt Reactor Experiment by Gamma Spectrometry, USAEC Report
ORNL-TM-3151, ORNL, August 1972.

R. B. Lindauer, Processing of the MSRE Flush and Fuel Salts, USAEC
Report ORNL-TM-2578, ORNL, August 1969.

R. E. Thoma, Chemical Aspects of MSRE Operations, USAEC Report
ORNL-4658, ORNL, to be published.

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending February 28, 1969, ORNL-4396, pp 139.

Ibid., pp 95-97.

Molten-Salt Reactor Program Semiannual Progress Report for Period f
Ending August 31, 1969, ORNL-L449, p T73. $
1T7.

18.

19.

20.

21.

22.
23.
2k,
25.
26.
27.

28.

29.

30 ,

31 .

32,

33.

 

17

Molten-Salt Reactor Program Semiannuel Progress Report for Period
Ending January 31, 1964, ORNL-3626, pp 132-137.

"T. S. Kress and F. H. Neill, Calculating Convective Transport and

Deposition of Fission Products, USAEC Report ORNL-TM-2218, ORNL,
September 1968

M. N. Ozisik, "A Heat/Mass Analogy for Fission Product Deposition from
Gas Streams," Nuclear Science and Engineering, 19, 164-171, 196k,

 

G. E. Raines et. al.z Experimental and Theoretical Studies of Fission-
Product Deposition in Flow1ng Helium, USAEC Report BMI-1688, BMI,

196k,

F. H Neill, D. L. Grey and T. S. Kress, Iodine Transport and
Deposition in a High-Temperature Helium Loop, USAEC Report
ORNL-TM;1386 ORNL, June 1966 .

Molten—Salt Reactor Program Semiannual Progress Report for Period
Ending February 29, 1968, ORNL-4254, pp 100-108..

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending August 31, 1970, ORNL—h622 pp TO-T1.

Molten—Salt Reactor Program Semiannual Progress Report for Period
Ending August 31, 1969, ORNL-Likg, pp 8-10.

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending February 28, 1970, ORNI-L5L8, pp 6-T.

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending August 31, 1966, ORNL-L4037.

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending August 31 1967, 0RNL—h191.'

:Molten-Salt Reactor PrOgram Semlannual Progress Report for Period

Ending August 31, 1968, ORNL-h3hh

Molten-Salt Reactor Program Semlannual Progress Report for period
Ending February 28, 1970, ORNL—hShB | -

S.. 8. Klrslis, Personnel Communicatlon Reactor Chemistry Division,
Oak Ridge National Laboratory, December 1970. :

Molten-Salt. Reactor Program Semiannual Progress Report for Period

Ending February 28, 1967, ORNL-h119, P ‘131,

Molten-Salt Reactor Program Semiannual Progress Report for Period

Ending February 29, 1968, ORNL-L425hk, pp 125-128.

E. J. Roberts et. al., "Solids Concentration," Chemical Engineering,
June 29, 1970.

 

 
 

 

3k.

35,

36.

37.

38,

39.

Lo.

by,

“Molten-Salt Reactor Program Semiannual Progress Report for Perlod

T8
\&/
Endlng February 29, 1968, ORNL-4254, pp 100-113. »

Molten-Salt Reactor Program Semiannual Progress Report for Period
Ending August 31, 1968, ORNL-4344, pp 150-151. | -

Molten~Salt Reactor Program Semiannual Progress Report for Périod
Ending February 28, 1969, ORNL-h396, pp 148-150.

H. W. Kohn Bubbles, Drops and Entrainment of Molten Salts, USAEC
Report ORNL—TM-23T3 ORNL, December 1968.

C. H. Gabbard, Design and Construction of Core Irradiation-Specimen
Array for MSRE Runs 19 and 20, USAEC Report ORNL-TM—EThS ORNL,
December 22, 1969.

Molten-Salt Reactor Program Staff, Single-Fluid Molten-Salt Breeder
Reactor Design Study, USAEC Report ORNL-L4541, ORNL (to be published).

R. B. Bird et. al., Transport Phenomena, John Wiley and Sbns, Ine.,
1960

F. N. Peebles, Removal of Xenon-135 from Clrculatlng Fuel Salt of
the MSBR by Mass Transfer to Helium Bubbles, USAEC Report
ORNL-TM-2245, ORNL, July 23, 1968. *
 

 

 

(hrs-1)

T9

APPENDIX A

NOMENCLATURE

Concentration of noble metal isotope in fuel salt (atoms/vol)
Value of CS at time zero

Concentration of noble metal isotope in fuel salt at liquid-
gas or liquid-solid interface (atoms/vol)

Concentration of the soluble precursor of the noble metal in
fuel salt (atoms/vol)

Value 6f Cps at time zero

Concentration of noble metal isotope on solids surfaces
(atoms/area)

Value of Cm at time zero

Total amount of noble metal isotope in gas phase of MSRE fuel
loop including liquid-gas interfaces (atoms)

Value of I at time zero

Volume of fuel salt in fuel loop

Time
Reactor power level (MW)

Cumulative fission yield of soluble precursor of noble metal
isotope (fraction) ’ '

Direct fission yield of noble metal isotope (fraction)

Decay constant of soluble precursor of noble metal isotope

Decay constant of noble metal isotope (hrs-l)

Mass transfer coefficient to graphite in MSRE core (ft/hr)

Mass transfer coefficient to primary heat exchanger (ft/hr)

Mass transfer coefficient to circulating bubbles (ft/hr)
Mass tfansfer coefficient to g0lid surface of interest (ft/hr)

Surface area of graphite exposed to fuel salt (ft2)

 
 

 

‘he

bub

hesat

mass

 

80

Total surface area of Hastelloy-N in heat exchanger exposed

to fuel salt (£t2)

- Surface area of circulating bubbles (fte)

Theoretical rate constant for migration of noble metal
isotope from fuel salt to all surfaces exposed to fuel salt-
see text (nrs=1)

Generalized rate constant for transport of noble metal isotope
from gas phase of fuel loop (including liquid-gas interfaces)
to off-gas system (hrs-1)

' Heat transfer coefficient (Btu/hr-ft2-°F)

Mass transfer coefficient (ft/hr)
Equivalent diameter (ft) ‘
Thermal conductivity (Btu/hr-ft2-°F)
Diffusion coefficient (£t°/hr)

Heat capacity (Btu/1b-°F)

Viscosity (1b/ft-hr)

Density (lb/f't3)

Velocity (ft/sec)

 
 

ok

 

81

APPENDIX B

EVALUATION OF MIGRATION PARAMETERS FOR THE MSRE

Mass Transfer Parameters to Fuel Loop Surfaces

It has been hypothesized in this report that noble metals migrate
in the MSRE in accordance with mass transfer theory. The physical and
chémical basis of this hypotehsis are presented in Section 4.1. This
being the case, conventional mass transfer coefficients should control the
migration from bulk salt to the various surfaces. These coefficients will
be evaluated by the heat transfer-mass transfer analogy. For an excellent
derivation of this analogy and assumptions involved, see Reference LO,
Chapter 21. The essentials of the analogy\state that mass transfer coef-

ficients may be éomputed from conventional correlations for heat transfer

coefficients after the following substitutions have been made.

Heat Transfer | Mass Transfer
heat mase
For Nu = 1_1____1_‘:__&1_ substitute Nu =°° = *_1_5_2
For ' Pr = £ substitute Sc = -
| k. T pD
For | Re = 2LV . substitute Re = &Y

u M
The analogy holds true for any flow geometry and for laminar or turbulent
flow. By way of example we may apply these substitutions to.the Dittus-

Boelter equation for turbulent flow as follows:

0.8 Cu 0.k

heat '(R%X) :(_E_Q

iR

For heat transfer h = 0,023

4

| ., mass omm D pdv.o'ay 0.
"For mass transfer h = 0.023 = (~) (E“J
- o d " u pD o |
 The MSRE fuel salt loop was divided into major regions. The mass
transfer coefficient for each of these regions was estimated using
accepted heat transfer coefficient correlations and the analogy s&bove.

Results are listed in Table B-1.

 
 

 

 

 

82

Table B-1

 

Mass Transfer Parameters for MSRE Fuel Loop

 

 

Mass Transfer¥* Surface
Coefficient Area Product
(ft/hr) (££2) (£t3/br)
Heat Exchanger (Shell side) 0.55 315 173
Fuel Loop Piping, Core and Pump 1.23 71 87
Volute
Core Wall Cooling Annulus : 0.51 15k 78
Core Graphite (Exposed to salt) 0.063 1465 92
Miscellaneous {Pump Impeller, Core | 43
Support Grid, etc.) 10 Percent ;
of Summetion of Above Product
 Summation of Products. LT3

¥Physical properties from'Table 2.1. Diffusion coefficient of noble metals
in fuel salt estimeted to be 5.1 x 10-5 ft2/hr with no distinction between
chemical species. ‘ '

Mass Transfer Parameters to Circulating Bubbles

 

In Section 3.3 it was noted that the void volume of bubbles cir-

culating with the fuel salt was estimated to be in the range of 0.02 per-

cent to 0.0L45 percent during the 235

U operation and in the range of 0.5
percent to 0.6 percent during the 233U operation. The bubble diameter is
not known, but estimates from various indirect observations would put the
diameter somewhere between 0.601 and 0.010 in. We will use a mean value
of 0.005 in. A program is underway at Osk Ridge Natiofial Laboratory to
determine the mass transfer coefficient to bubbles circulating with a
turbulently flowing fluid.(hl)

but initial information indicates a mass transfer coefficient to circulating

This program has not yet been completed,

bubbles of asbout 5.0 ft/hr is a reasonable value. Using mean values of

_the above ranges of void fractions and a fuel salt volume in the fuel loop

of T0.5 ft3, we compute the mass transfer parameters in Table B-2.

 
 

 

83

Table B-2

Mass Transfer Parameters to Circulating Bubbles

 

 

Mass Transfer Surface
Coefficient Area Product
| (ft/hr) (£t2) (£t3/nr)
During 2>°U Runs 5.0 3Ls5 1725
During 25U Runs 5.0 5581 27,900

 

Mass Transfer Parameters to the Core Surveillance Specimens

The complexity of the fluid dynamic conditions in this region has been

discussed briefly in Section 3.2. As the cross sectional view in Figure

"2.4 shows, the rectangular graphite specimens are packed together to form

a rather complicated fluid dyanmic arrangement featuring both inside and
outside corners. Adjaéent to each major graphite surface and almost
touching it are the Hastelloy-N tensile specimens and dosimeter tube.

The tensile specimens are particularly difficult to analyze because they
form a large number of fluid entrance and exit regions for themselves and
the adjacent graphite. This effect would be expected to increase the mass
transfer coefficient. On the other hand,lthe close proximity of the wide
part of the tensile specmens to the graphite (about 0.040 in.) might be
expected to locally stagnate the fuel salt. This effect would tend to
decrease the mass transfer éoefficient-in the affected region. Finally,
the entire sample assembly is enclosed in a perforated basket. The

intent of the basket is to allow for cross flow, but'it also contributes

to the uncertainty of fuel salt velocity past the sample. The perforations

also generate an unknown amount of turbulence in the salt. All these fluid

dynamic complications were a necessary result of inéorporating a large

_number of different kinds of sampleSf(bothlgraphite and metal) for differ-

ent purposes into a rather confined volume. I should aiso point out that
when the sample station was built, the necessity of being able to accurate-

ly estimate mass transfer coefficients.was-nof fully realized. The salt

-fielocity in the sample station has been estimated to be about 2 ft/sec,

partly by indirect measurements and partly by estimate. The uncertdinty
could be as much as 25 percent. The equivalent diameter of the annular

region between the samples and perforated basket, and based on all sample

 
 

[ atems e e e et e e e % a3 e e n ¢ m . e

8l

components exposed to salt is about 0.62 in. The Reynolds Number then,
based on these values, is about 3000. This implies that the flow is only
marginally turbulent. This being the case, one might expect that the
confining nature of the inside corners formed by the graphite, and the zones

between the Hastelloy-N samples and graphite surfaces would tend to make

 the flow laminar. Using the usual relationships for heat transfer coef-

ficients and applying the heat-mass transfer analogy, we would compute an
overall mass transfer coefficient of 0.31 ft/hr for turbulent flow and
0.077 ft/hr for laminar flow in this region. In this analysis then, we
will pick an intermediate value of 0.25 £t /hr for the mass transfer coef-
ficient to both the graphite and Hastelloy-N specimens. The uncertainty
in this nfimbér is high, probably more so for the Hastelloy-N than for the
graphite.

Noble Metal Fission Product Parameters

 

‘Table B-3 lists the noble metal (and precursor) yield and half life
quantities during the 233U runs, and Table B-L4 1lists those parameters

 

 

 

 

during the 235U runs, used in this analysis.
Table B-3
Noble Metal Fission Product Parameters (233U Runs )
Cumulative Yield¥ Direct Yield
Noble of Precursor of Noble Metal Half Life Half Life
Metal A % of Precursor of Noble Metal
Mo 4.89 0.0 2.4 min 66.5 hrs
103Ry 2.00 0.0 1.2 min ~ 39.7 days
105gy 0.706 0.0 9 min 4.45 hrs
106Ry 0.438 0.0 <1 min 1.01 yrs
125gy 0.0839 0.0 G days 2.0 yrs
127gp = 0.58 0.0 1.9 hrs 91 hrs
129mpe 0.71 0.0 4.6 nrs - 37 days
131mqe 0.441 0.0 23 min | 30 hrs
132mpg b.h3 0.0 2.1 min - 7T hrs
25Nb 6.00 0.0 65 days 35 days
¥Based on the following Fission Distribution -
Component 233U 235y 239py

Percent of Fissions 93.2 2.3 I

 
 

 

85
Table B-L

Noble Metal Fission Product Parameters (235U Runs )

Cumulative Yield Direct Yield

 

Noble of Precursor  of Noble Metal Half Life Half Life
Metal % % of Precursor of Noble Metal
39Mo ' 6.06 , 0.0 2.4 min 66.5 hrs
103gy 3.00 . 0.0 1.2 min 39.7 days
106Ry 0.39 0.0 <1 min. 1.01 yrs
1291mpg 0.71 0.0 4.6 hrs 37 days
1327e L. 71 0.0 2.1 min 7T hrs

35Nb 6.22 - 0.0 65 days 35 days

 

Time Constant for Noble Metals in Fuel Salt

In Section 4.2 we derived an expression for C° (the noble metal con-
centration in fuel salt) where the fuel loop is considered to be a well
stirred pot. For this assumption to be adequate, the reside: ;e time
of noble metals in fuel salt must be greater than the circuit time of salt

around the fuel loop (25 seconds). The rate constant is defined as

X = ) + %_(hgr& A8Y8 | hhe Ahe + (hA)rest of loop
+ hbub Apub)

With the parameters listed in Tables B-1 and B-2, we can compute the time
constants involved and they are iisted in Table B-5. The value of X in

all cases is negligible so the value of X is the same for all noble metal
isotopes. In.the case of fuel salt with no bubbles and fuel salt during
the 235 V

apparently not adequate during the 233U runs. However, valves in this

U runs, the well stirred pot assumpfiion_is adequate. It is

table assume the sticking fraction of noble metals to bubbles of unity.
In Section 5.2 we deduced that the effective sticking fraction to bubbles
is considerably less than unity and =a value‘of 0.1 - 0.2 is'suggested.'
If a sticking fraction to bubbles of 0.1 is aséumed, then the noble metal
half life in fuel salt during the 233U runs becomes 54 seconds. This

indicates the well stirred pot assumption is adequate in all cases.

 
 

 

 

86

Table B-5

Time Constants of Noble Metals in Fuel Salt

 

X Half Life in Salt
(hrs-1) (sec)
~ Fuel Salt with no Bubbles 6.7 370
Fuel Salt During the 235y Runs 31.2 . 80
Fuel Salt During the 233) Runs 402 ‘ 6.2

 

 
 

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87

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MOOURUOHEWERROE YSH®

 
 

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