ORNL-2183.txt

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- Report Number: fifii-alag,

Contract No. W-7405-eng-26
CHEMICAL TECHNOLOGY DIVISION
Chemical Development Section B

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DISSOLUTION OF URANIUM-ZIR§ONTUM FUEL Eégmms
IN FUSED Nefozrs, 4

 

R

R. G. Wymer

Technician: J. Fle.nd

5 o
'»r '
=

~ DATE ISSUED

JAN 2 4 1°R7

 
   

o = ~ TOC LR
A Division of
UNION CARBIDE AND CARBON CORPORATION

AATIN MARIETTA ENERGY SYSTEMS LIBRARIES

e T

3 4456 035034k b

 

 

 

 

 

 

 

 

 

 

 

 

 

 
W ~ii- W

Chemistry-Separation Processes
for Plutonium and Uranium

M-3679 (18th ed.)

INTERNAL DISTRIBUTION

 

  
  
 
 
   
  
 
 
 

   
    
     
      

    
  

1. C. E. Center L6. y. E. Blanco
2. Biology Library LW G. E. Boyd
3. Health Physics Library 0. W. E. Unger
L-5. Central Research Library;i%éli k9. R. R. Dickison
6 g Reactor Experimental ; 50. A. T. Gresky
igineering Library 51. E. D. Arnold
7-11. L3goratory Records Department 52. C. E. Guthrie
12. Lab®gatory Records, ORNL R.C. 53. J. W. Ullmann
13. A. MN\geinberg 54. K. B. Brown
14, L. B. Mget (K-25) 55. K. 0. Johnsson
15. J. P. MuNgay (Y-12) 56. H. M. McLeod
16. J. A. Swargut 57. J. C. Bresee
17. E. H. Taylo 58. W. H. Carr
. 18. E. D. Shipley 59. G. I. Cathers
19-20. F. L. Culler 60. W. E. Clark
21. M. L. Nelson 61. 0. C. Dean
22, W. H. Jordan 62. L. M. Ferris
23. C. P. Keim 63. J. R. Flanary
2k, J. H. Frye, Jr. 64. I. R. Higgins
25. 8. C. Lind 65. J. F. Land
26. A. H. Snell 66. W. H. Lewis
27. A. Hollaender 67. R. E. Leuze
28. K. Z. Morgan 68. J. T. Long
29, M. T. Kelley 69. J. P. McBride
30. T. A. Lincoln 70. J. A. McLaren
31. R. S. Livinggebn 1. R. A. McNees
32. A. S. House der 2. R. P. Milford
33. C. S. Harr 73. R. H. Rainey
3k, C. E. Wingfrs Th. J. T. Roberts
35. D. W. Cajwell 75. J. E. Savolainen
36. E. M. g 76. S. H. Stainker
37. W. K. j¥ster X7. W. W. Weinrich (consultant)
38. F. R.#Bruce & M. D. Peterson (consultant)
39. D. Ferguson 79N\D. L. Katz (consultant)
40. R. #. Lindauer 80. §. T. Seaborg (consultant)
L1, H4F. Goeller 81. Benedict {consultant)
L2, i D. Cowen 82. C.\E. Larson (consultant)
43. M. A. Charpie 83. ORNN - Y-12 Technical Library,
LY &J. A. Lane Docugnt Reference Section
LSEM. J. Skinner

8L. Division :of Research and Development, AEC, ORO
85-363. Given distribution as shown in M-3679 under Chemij
Processes for Plutonium and Uranium category

 

 
~iii-
CONTENTS

Page

1.0 ABSTRACT 1
2.0 INTRODUCTION 1
3.0 EXPERIMENTAL WORK 2
4.0 RESULTS 7
5.0 DISSOLUTION MECHANISM 17
6.0 REFERENCES 21
7.0 APPENDIX 22

 

 
 

 

 
1.0 ABSTRACT

Further experiments confirmed previous indi-
cations that NaF-ZrF, is a satisfactory dissolvent
for zirconium-uranium fuel elements., A very preli-
minary dissolution flowsheet, as a basis for a fused
salt--fluoride volatility process, is presented.

2.0 INTRODUCTION

Dissolution in fused salt media of heterogeneous reactor fuels
containing enriched uranium is economically attractive as a means of
converting the uranium to a pure, easily recoverable form. Fluoride
media are currently receiving attention, and STR fuel elements, whose
principal constituent is zirconium, are particularly amenable to
dissolution in fused salt media containing Zth. Previous studies
have shown the feasibility of fused salt processing. The experiments
reported here were carried out in order to obtain additional disso-

lution data needed for design of & tentative STR processing flowsheet.

1-5

The results presented are very preliminary, and are based on only a

few laboratory-scale experiments.

The author acknowledges the assistance received during the studies
from G. R. Wilson and his group, of the Analytical Chemistry Division,
who performed the many new and difficult chemical analyses which made
this work possible, and of the late G. E. Klein of the Solid State

Division, who made the x-ray analyses.

 
 

3.0 EXPERIMENTAL WORK

The equipment used in these studies (Fig. 1) consisted of a
nickel dissolver vessel, an HF cold trap and wet test meter for
measuring H2 evolved, a voltage supply recorder for the metering
equipment, and HF and N2 gas metering equipment. 1In general, dis-
solution rates were determined both by H2 evolution (as measured
by the wet test meter) and by direct weighing of the specimens

before and after their partial dissolution.

The initial dissolutions were made in a tubular dissolver (Fig.
2) in which the entire melt was agitated by the flowing gas. Repro-
ducibility of date was very poor, and examination of the specimens
after a dissolution run showed extreme inhomogeneity of attack.,
Attack was severe where the HF gas pessed over the surface of the
specimens, but was mild on the remainder of the surfaces. For later
experiments, the dissolver was modified so that the entire specimen
wes contacted with HF gas as long as any gas was flowing to ensure
greater dissolution uniformity. The modified dissolver (Fig. 3) was
actually a 1lift pump, in which the gas, and therefore the melt
agitation, was localized to a small region in the vicinity of the
metal specimens. The fused salt was pumped up over the STR specimen
by the pumping action of the HF gas, so that attack on the specimens
was much more uniform. 1In spite of this, reproducibility of disso-
lution rate data was still poor. All dissolutions were made with the
HF gas at atmospheric pressure, and 100 ml of fused salt was used in

all runs.

In the lift-pump dissolver, effects due to HF alone cannot be
differentiated from those due to HF dissolved in the fused salt.
Studies on the mechanism of the reaction were therefore made in a U-

tube dissolver (Fig. 4). 1In this dissolver an STR specimen can be

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. 7 PHOTO 17441
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Fig. 1. Fused Salt Dissolution Equipment.
 

4 -in. TUBING

THERMOCOUPLE
WELL

 

 

UNCLASSIFIED

Y= in. SWAGELO

ORNL-LR-DWG 8037-A

K FITTING FOR

o REPLACEMENT OF INLET LINE

f-in. SWAGELOK FITTING ON
- FILL PORT

Y%, ~in. TUBING WITH
FLARE FITTING\‘

—

 

 

 

—————

rree——

 

N

 

 

//

 

 

 

 

 

9-in. LENGTH OF 2-in—
/ DIA TUBING

HF INLET TUBE , %4 —in.

Fig. 2. Schematic Diagram of Reactor Used in Initial Dissolution Experiments.

 
HF Outlet o ~—

Fused Salt

HF Inlet

$

 

 

 

.

 

 

 

 

 

o

ORNL-LR-DWG. 15103

STR Specimen

Level of
Fused Salt

Ni Wire Specimen
Support

Fig. 3. Schematic Drawing of HF Lift Pump Dissolver. The HF gas follows the
paths of the solid arrows and the fused salt, the broken arrows.

 
 

UNCLASSIFIED
PHOTO 17442

THERMOCOUPLE WELL
A-""f-r

 

 

HF EXIT
TUBE

 

 

 

HF INLET
TUBE

     
 

APPROXIMATE LEVEL
OF SALT IN DISSOLVER

 

SIEVE PLATE SIEVE PLATE

 

 

Fig. 4. U-Tube Dissolver.

 
placed in each of the arms, A and B, of the U tube attached to the
bottom of the dissolver. The specimens rest on sieve plates. When
HF gas is passed in through the inlet tube, it goes up through arm
A, over the specimen on the sieve plate, rises through the salt in
the main body of the dissolver, and passes out the HF exit tube.
This sets up a salt-pumping action in arm A, which causes circu-
lation of salt around the loop. At the same time, the fused salt
becomes laden with dissolved HF. The specimen in arm B is thus
exposed to salt containing dissolved HF but not to HF gas. The
small diameter of arm A provides uniform gas coverage and disso-
lution, much as that achieved in the lift-pump dissolver. The
auxiliary N2 drying and deoxygenating equipment attached to the U
tube for the experiments in which HF was replaced by nitrogen is

shown in Fig. 5.

In order to avoid corrosion, insulation deterioration, and other
problems associated with very high temperature processing of fluoride
mixtures, 55000 was set as the maximum melting point of the salt.
This restricts the Zth content of the fluoride melt to the range
38-57 mole % (Fig. 6).

4,0 RESULTS

The dissolution rate of the simulated STR fuel element pieces
was affected by the HF flow rate, temperature, and melt composition.
Because of considerable scatter in the data, the results may be
considered only as indicative of a trend. The nature of the metal
did not affect the rate of dissolution, but did affect the type of
attack by the reagent.

Effect of HF Flow Rate. At 7OOOC the dissolution rate increased

 

 
 

VALVE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

UNCLASSIFIED
ORNL-LR-DWG. 15104

 

 

—— TO DISSOLVER

 

 

 

 

 

 

 

 

 

 

 

 

|
| ( r'-_-tfi_-,";—'. TUBE FURNACE
q-o—.
| N _-g_—_ COPPER TURNINGS AT 550°C
—T o/
| | -
Cc:SO4 H2$O4

Fig. 5. Auxiliary N, Drying and Deoxygenating Equipment.
TEMPERATURE (°C)

{000

800

600

400

ORNL-LR- .

 

 

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NagZrF,

 

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10 20 30

40 50 60 70

2r F4(mole %)

Fig. 6. The System NaF-ZrF4.

80

90

{00

-.6-

 
- 10 -

with increasing HF flow rate (Fig. 7). There did not appear to be
any effect at 600°C, but at 800°C the rate increased and then de=-
creased. In the preliminary studies in the ordinary dissolver, the
rate at 600°C increased with increasing HF flow rate (Table 1).

Comparison of results in the two dissolvers indicates a definite

dependence of dissolution rate on HF flow rate, even at 6OOOC, up to
a8 maximum value. It is assumed that this maximum value is reached
at or before an HF flow rate of 25 mg/min in the lift-pump dissolver,
where the excellent agitation leads to & high HF utilization

efficiency.

Table 1, Dissolution Rates in Ordinary Dissolver

Conditions: 600°C; NaF/ZrF, = 1/1; atmospheric pressure;
100 ml of melt

 

 

HF Flow Rate Dissolution Rate
(mg/min) (mg/min/cm®)
25 0.58
40 0.50
50 0.7k
83 1.1

 

 

The efficiency of HF utilization decreased rapidly with increasing
flow rate (Fig. 8). This factor affects the magnitude of the HF re-

cycle problem on a plant scale.

Effect of Temperature. At all HF flow rates tested, the disso-
lution rate increased with increasing temperature (Fig. 9). This

confirmed observations of other workers.l’2

Effect of Melt Composition. In the range 38 to 57 mole % ZrF),

 
2)

Dissolution Rate (mg/min/cm

co

O~

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-11-

ORNL-LR- . 15100

 

 

 

 

 

o
© 800°C
©
O 600°C
— ® 700°C :/
O
B O
g
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O
| O
—_— O
7% o
o O
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20 40 60 80 100

HF Flow Rate (mg/min)

Fig. 7. Dissolution Rate of STR Specimens in 50-50 mole % NaF-ZrF, as a
Function of HF Flow Rate,

 

 

 

 

80
o 600°C
—_ -0 e 700°C
R o
~ 60 Do © 800°C
0
= ® ©
D 40 L 8 0
LIL O\
i o
o —
20 l | | ] | ] cl)
20 40 60 80 100

HF Flow Rate (mg/min)

Fig. 8. Utilization of HF as a Function of Flow Rate
in Dissolutionof STR Specimens in 50-50 mole % NaF-ZrF,.

 
 

 

1.0

0.8

Log of Rate
e
o

0.4

0.2

-12-

)

ORNL-LR-DWG. 15099

 

o 25 mg HF/min
- e e 47 mg HF /min
3 © 90 mg HF/min

 

 

 

80:.')°C | 7(?0°C | 60|0°é

 

9 10 1 12
10% frox

Fig. 9. Dissolution Rate of STR Specimens in 50-50 mole % NaoF-ZrF ,
as a Function of Temperature.
- 13 -

the dissolution rate was considerably higher with the lower per-

centages of ZrFu:

 

 

NaF/Zth mole ratio Dissolution Rate (mg/min/cm?)
0.62/0.38 7.9
9.6 avg., = 8.3
7-3
0.43/0.57 1.0

0.8 8&ve. =0.9

The dissolution rate in 50-50 mole % LiF- ~ZrF) , which is about half
as viscous as the comparable NaF-Zth, was only 2,9 mg/min/cm . The
dissolutions were made at 600° C, with an HF flow rate of 47 mg/min,

at atmospheric pressure, in 100 ml of melt,

Effect of Nature of Metal. There was no significant difference
between the dissolution rates of STR specimens, zircaloy-2, and
crystal bar zirconium. There was a difference in the nature of the
attack, With the STR specimens the attack on the core was greater
then on the cladding (Fig. 10). With the zircaloy-2 specimens the
attack along the upper edge was very regular (Fig. 11). With
crystal bar zirconium there was extreme hydride formation, as indi-

 cated by myriad black areas suffusing the specimen (Fig. 12).

Nature of Product and Residue. After typical dissolution
experiments, when the molten salt was poured onto a stainless steel
pan it froze to a chalk-white solid. Invariably black particles 1
to 2 mm in diameter were distributed throughout the solid., X-ray
analysis of the particles, which were picked out of the bulk of the
salt, showed lines which could be taken as evidence for uranium
metal, ZrOFE, UOE’ ZrO2
stable compound whose composition lay between NaZrF5 and Zth. It

, and UFH' There were also lines for a meta-

may be conjectured that the uranium metal was deposited from a
fluoride of uranium in the melt by reduction by the more electro-
positive zirconium metal. The uranium would have been introduced
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Fig. 10. STR Fuel Plate. Bright field; 100X. (a) As received. (b) After

dissolution treatment. (Confidential with caption)

 

 

 

 

 

 

 

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PHOTO Y-19702
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PHOTO Y-19704
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PHOTO Y-19706

 

   

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- o Lo e OIS e 5 =gy P
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(b) After

ived

(a) As rece

100X,

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Bright field
dissolution treatment. (Confidential with caption)

-2

ircaloy

Z

M

ig.

F
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UNCLASSIFIED]
PHOTO Y-19708
w
l—m—
- =
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0.03
UNCLASSIFIED
PHOTO Y-19710
0
b= L=
x
2
=
1 0.02
2 0.03

 

 

 

 

Fig. 12. Crystal Bar Zirconium. Bright field; 100X. (a) As received.
(b) After dissolution treatment. (Confidential with caption)

 
 

- 17 -

into the melt in the first place by dissolution out of that fraction
of the core which was exposed along the edges of the STR specimens.

In general, the partially dissolved STR specimens were covered
with a black adherent layer. When some of this layer was scfaped
off and analyzed by x-ray diffraction, lines attributable to UO2 and
zirconium metal were obtained.

2.0 DISSOLUTION MECHANISM

Little work has been reported so far that gives much insight
into the details of the mechanism of dissolution. Results of a
limited number of experiments designed to clarify this point indi-
cated that the presence of HF is not necessary for dissolution.

The slight attack of STR fuel elements by HF gas alone is attri-
buted to the presence of a protective coating of Zth, which dissolves
in the presence of fused salt and exposes more bare metal to the HF
gas. However, in experiments in the U-tube dissolver (Fig. 5), there
wes no significant difference between the dissolution rate of STR
specimens in fused salt containing dissolved HF with or without the
flowing HF gas (Table 2)., In the complete absence of HF, i.e., when
the flowing HF gas was replaced by dry oxygen-free nitrogen, the
rate of dissolution was still quite satisfactory (Table 3). Possible
mechanisms in keeping with these results are pyrosol formation, as
has been suggested6 for dissolution of titanium in chloride melts,

end reactions of the type

2r° + 3Zrh
ZrO + Zrh

+ e ZI‘3+ + 3Zr3+
+ N S Zr2+ + Zr2+

in which HF is not required for the primary disintegration step.
 

- 18 -

Table 2, Dissolution Rates of STR Specimens in Fused Salt
Containing Dissolved HF, with and without HF Gas

 

 

 

 

 

 

 

 

 

Conditions: 600°C; NaF/Zth = 1/1; atmospheric pressure
HF Flow Rete Volume of Melt Dissolution Rate (mg/min/cm?)
(mg/min) (ml) In Salt In Salt + Gas
25 75 2.2 2.6
56 75 3.0 2.8
L7 100 5.1 4.2
Table 3., Dissolution of STR Specimens in Fused Salt
in Complete Absence of HF
Conditions: 600°C; NaF/zrF, = 1/1; melts swept for 2 hr

with dry, oxygen-free nitrogen before specimens

lowered into it; 100 ml of melt

 

Duration of Run

N, Flow Rate

Dissolution Rate

 

(min) (arbitrary units) (mg/min/em®)
b2 12 1.4
50 _—- 1.1
120 Lo 0.4

 

 

 

Reducing Normelity of Melts.

Since STR specimens dissolve in

the absence of known oxidizing agents, the resulting melts should be

reducing in nature.

The reducing normality of a 50-50 mole % NaF-

ZrF) melt was found to be of the order of 1073 meq/g (Table 4), In

these experiments an ordinary dissolution was first performed, using

HF gas in the usual way,.

The melt was cooled under dried nitrogen

until it solidified, and was then transferred to a dry box where an
 

- 19 -

approximately 50-g sample was dug out. This sample was dissolved in
300 ml of solution* which was 1 M in Ale(SOh)3, 1.5 M in H,50), and
0.017 N in KéCr207, and the amount of a standard reductant required
for a back titration was compared with that required for a blank. In
a second experiment, nitrogen was used instead of HF. In the third
run nitrogen ges replaced the HF, but no STR specimen was used, so
that any reducing normality present came from the melt itself, from
dissolver vessel corrosion products, or from the nitrogen supply.
Run 4 was a check made because a new supply of Zth salt for melts
was obtained, and it was important to determine that no reducing
impurity was introduced from this source. Run 5 was made with Né
and no STR specimen, exactly as in runs 3 and 4, except that 1 wt %
N:i.F2 was added to see what effect the nickel salt would have on the
reducing normality., Run 6 was a check for reducing impurities in

the HF gas used.

There appeared to be an equilibrium between the oxidized and
reducing forms of nickel, When 1 wt % NiF,
melt, which contained 0.05 wt % NiF, initially, the reducing
normality was doubled. The implication is that the nickel vessel
wall enters into an equilibrium of the type Ni + NiF2 = 2NiF,.
At 600°C the solubility of NiF, in a 50-50 mole % NaF/ZrF) melt is
~0.4%. Therefore the NiF concentration in the postulated equili-
brium is fixed when the NiF2 is present in concentrations above
1 wt %, and vessel corrosion by NiF, is inhibited.

was added to a typical

It is apparent that the zirconium which dissolves in the

absence of HF gas is not a factor contributing to the reducing

 

* This solution must be heated to completely dissolve the (soh) ,
which reprecipitates on cooling. However, after the melt safiple his
been dissolved, sufficient complexing of eluminum ion with fluoride
ion occurs that cooling without precipitation is possible. Auxiliary
experiments showed such a solution to be an efficacious, rapid dis-
solvent for the melts, which are ordinarily very difficultly soluble.
 

- 20 -

normality. It may be supposed that the zirconium dissolves by pyro-
sol formation, but that the resulting dispersed metal is not oxidized

by the dichromate-containing solution, and so is not measured in this

 

 

 

w&y.
Table 4, Reducing Normalities of Various Melts
Conditions: 6OOOC; NaF/Zth = 1/1; 100 ml of melt; atmos-
pheric pressure
NiF, Run | Dissolution | Reducing Normality
STR Specimen | in Melt | Time Rate o of Melt
Run | Gas in Melt (wt %) | (br) | (mg/min/cm®) (meqa/g)
1 |HF Yes ~0.05 | 1 4.0 6.0 x 1073
2 | N, Yes ~0.05 | 2 0.36 6.7 x 1073
3 |1, No ~0.05 | =-- - 6.8 x 1073
Lo | n, No ~0.05 | 2 -—- 6.1 x 1073
5% | m, No 1 2 --- 12.2 x 1073
6° | uF No 1 1 ——— 8.6 x 1073

 

 

 

 

 

 

 

%1 wt ¢ NiF, was added to the melt.

A different source of HF gas was used,

 
- 21 -

6.0 REFERENCES

G. I. Cathers and R. E. Leuze, "A Volatilization Process for
Uranium Recovery,” Nuclear Engineering and Science Congress,
Cleveland, Ohio, December 1955.

S. I. Cohen and T, N. Jones, "Summary of Density Measurements
on Molten Fluoride Mixtures and a Correlation Useful for
Predicting Densities of Fluoride Mixtures," ORNL-1702 (May 1,

1954),

R. E. Leuze, G. I. Cathers, and C. E. Schilling, "Dissolution
of Metals in Fused Fluorides," ORNL-1877 (September 28, 1955).

G. I. Cathers and M. R. Bennett, "A Fused Salt--Fluoride
Volatility Process for Recovery and Decontamination of Uranium,"
ORNL-1885 (September 27, 1955).

"Chemical and Engineering Division Summary Report, January,
February, and March 1955," ANL-5422, p. 26,

A. W. Schlecten, M. E. Straumanis, and C. B. Gill, "Deposition
of Titanium Coatings from Pyrosols," J. Electrochem. Soc., 102,

81-5 (1955).

 
- P72 -

7.0 APPENDIX*

A tentative flowsheet for fused salt dissolutions was designed
(Fig. 13). Since no engineering studies were made, this flowsheet

is highly preliminary.

Size of Hydrofluorinator. The hydrofluorinator must be small

 

enough in diameter that the initial salt charge covers the portion
of subassembly charged. It must be large enough to accommodate the
largest subassembly in less than half its diameter if a central HF
pump is to be used (unless the assembly is put into the HF pump
tube, as at ANL). Since the assemblies can be quartered, and msy be
handled in this way, the vessel must contain at least 12 in, of salt
to cover such a portion. Subassemblies come with 5, 12, 1k, and 17
fuel plates. Since each subassembly has two side plates, there are
7, 14, 16, and 19 plates, respectively, in these subassemblies., The
edges of the plates are 6 mm thick, so the thickest subassembly is
about 0.6 x 19 = 11.k cm, or 4.5 in., thick. A 12-in.-i.d. dissolver
would thus allow enocugh room for a 3-in.-o0.d. HF pump, and a lh-in.-
i.d. tank would allow enough for a 5-in.-o0.d. HF pump, if such a

device was used.

A salt charge to the hydrofluorinator should be sufficient to
process as much as one complete 1l7~fuel-plate subassembly on the
one hand, or as little as one complete 12-plate subassembly on the
other. These are the charge limits which may be met in operation
(Table 5). Thus, the total weight of zirconium to be dissolved
ranges from 16 x 1.23 = 19.7 kg to 21 x 1.23 = 25,8 kg. These values
may be rounded off to 20-26 kg of zirconium.

 

* This appendix originally published as CF-56-5-19,

 
» *
ORNL-LR-DWG, !5] 01

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

FUEL CHARGE INITIAL SALT CHARGE
Q
(one 17-plate subassembly) 86.2 kg ZcF
1850 2k 2
~106 curies F.P.'s o e 170 g/min
p.gr.,3.16 152 kg HF
Y [ F
14-in.
DISSOLVER
600°C
f
FLUORINATOR CHARGE HF TRAP HYDROGEN
144 g/min (Diluti ith Ai
00° ilution with Air)
25_65 kgCNGF I(ZR;?C:-T]Z 13x 103 S.7.P. liters
134.4 kg ZrF4 9
184 g U
~109 curies F.P.'s
50 liters

Sp.gr.,3.20

 

 

 

*To be added one-quarter of a subassembly at a time.
Fig. 13. Tentative Flowsheet for Fused Salt Dissolution. Dissolution time: approximately 15 hr.

 

_8 Z—
 

- 24 -

Table 5. Number of Fuel Plates in a Hydrofluorinator Charge

 

 

No. of
Fuel Plates in No. of Subassemblies Total No. of
Subassembly per Batch Plates®
5 2 18
12 1 16
1k 1 18
17 1 21

 

 

 

%The end plates are twice as thick as the fuel plates; they contain
twice the smount of zirconium, and each is counted twice.

Setting 50 liters as the approximate volume desired for subse-
quent use in the fluorination, we may calculate the initial amount of
salts to be charged to the hydrofluorinator. The density of a 57
mole % ZrF,-43 mole % NaF melt at 600°C is 3.2.% Therefore, 50 liters
contains 50 x 3.2 = 160 kg of salt, or

b2 x 0.43
160 X 752557 30.4

 

 

kg NaF = 5 % 0.03 - T 3 0.190 = =°°°
L+ 987 %0.57
0.57 x 16
ke 7 - 160 X 592355 5.28 x 160 _ L2l L,
g ZrE), = 167 x 0.57 - L+ 5.288 ~ o

1+ L2 x 0.43

From this, it follows that the weight of zirconium present is 13k x
91.2/167 = 73.2 kg. Subtracting this from 26 kg, which is the most
zirconium to be dissolved in a single melt, we arrive at a value of
approximately 47 kg of zirconium, which is the least amount with which

we may start in the melt. This leads to an initial melt of 25.5 kg of

 

* Interpolated from data given in Ref. 2.

 
NaF and 47 x 169/91.2 = 86.2 kg of ZrF) . The composition is 45.9%
Zth--5H.l% NaF. At 600°C the density of such a melt is 3.16 g/cc,
so the volume, V, is (25.5 + 86.2) 3.16 = 35.3 liters. In a 12-in,-
dia hydrofluorinator this would mean a depth of 19.0 in. of salt;

in a 1k-in.-dia hydrofluorinator it would mean a depth of 14 in, of
salt. Both these values exceed the mifiimum.requirement of 12 in.

of depth. For increased capacity and freeboard space, a lh-in,.-dia

dissolver is recommended.

The following equation is useful in making calculations of the
weight of one of the salts in a system when the mole fractions and

total salt weight are known:

 

where W, = weight of salt of interest

=

= mole fraction of salt of interest

= molecular weight of salt of interest
mole fraction of second salt

= molecular weight of second salt

= total weight of combined salts

B g R

Calculation of Hydrogen Fluoride Requirements. If the fused melt is
well agitated, dissolution rates of 2-5 mg/min/cm2 may be expected.
A value of 3 mg/min/cm? has been selected for these calculations.

Consideration of the physical construction of the fuel assemblies
leads to the conclusion that a thickness of 0.65 cm must be dissolved
to produce a melt practically free of metal. (In practice, unavoidable
dissolution heterogeneities will be experienced, and additional disso-
lution time beyond that required to dissolve a thickness of 0.65 cm
will be necessary to ensure that no pieces of metal remain.) The

uranjum-containing portions of the fuel assemblies are only 0.3 cm

 
thick; therefore complete dissolution of the twice-as-thick edge
portions will ensure that the uranium-containing part has completely
dissolved.

Taking the dissolution rate of 3 mg/min/cm?, and half of 0.65 cm,
or 0.33 cm, as the maximum distance which must be penetrated (since
dissolution takes place on both sides of the 0.65-cm-thick edge), a
value of 0,33 x 6.4/0.003 x 60 = 12 hr is obtained for the time
required for complete dissolution. If 26 kg of zirconium is to be
dissolved, then 26,000 x 4/91.2 = 1140 moles of hydrogen fluoride will
be used, assuming 100% efficiency of utilization. This corresponds
to a hydrogen fluoride flow rate of 1140 x 20/12 x 60 x 43 = 0. 74
mg/min/ml. While it is not a valid procedure to attempt to compare
hydrogen fluoride flow rates on the basis of volumes of hydrogen
fluoride passed into a fixed volume of salt in a given time when
other scale-up factors are not comparable, it is the best method at
hand. On the basis of such a comparison, & hydrogen fluoride utili-
zation efficiency of about 30% may be estimated. This means, however,
that 3 x 1140 = 3420 moles of hydrogen fluoride is required, which in
turn increases the flow rate to 3 x 0.74 = 2.2 mg/min/ml, at which
rate only about 15% hydrogen fluoride utilization may be expected.
Finally, therefore, an approximate value of llhO/O.lB = 7600 moles of
hydrogen fluoride required to dissolved 26 kg of zirconium is
obtained. This means that T7600-1140 = 6460 moles of hydrogen fluoride
must be recycled in what may be a somewhat radioactive operation as
volatile fluorides (such as ZrF) , MoF, NbFé)accumulate.

In order to ensure that no metal remains, a dissolution time of
15 hr would be desirable. Therefore, a hydrogen fluoride flow rate
to the dissolver of T600 x 20/15 x 60 = 170 g/min is required, and of
this 0.85 x 170 = 1k g/min must be trapped for recycle.