ORNL-TM-3832.txt

 

ORNL-TM-3832

 

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DESIGN STUDIES OF A "

MOLTEN-SALT REACTOR
DEMONSTRATION PLANT

E. S. Bettis
L. G. Alexander
H. L. Watts

 

 

 

 

 
 

 

 

 

 

 

This report was prepared as an account of work sponsored 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, expréss 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.

 

 
 

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ORNL-TM-3832

Contract No. W-T405-eng-26

Reactor Division

DESIGN STUDIES OF A MOLTEN-SALT REACTOR

DEMONSTRATION PLANT

E. S. Bettis, L. G. Alexander,

H. L. Watts

Molten-Salt Reactor Program

JUNE 1972

—— e —— -

— NOTICE

This report was prepan‘ed as an account of ‘jvork
sponsored by the Unitad States Government, Neither

their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, com-

‘| pleteness or usefulness of any information, apparatus,

_product or process disclosed, or represents that its use

 

would not infringe privately owned rights.

the United States nor the United States Atomic Energy |-
.| Commission, nor any; of their employees, nor aay of

 

 

NOTICE This document contains information of o preliminary nature
ond was prepared primarily for internal use ot the Qaok Ridge National
Laboratory. It is subject to revision or correction and therefore does

not represent o finol report.

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830

operated by

UNION CARBIDE CORPORATION

for the

U. 5. ATOMIC ENERGY COMMISSION

BISTRIBUTION GF THIS DOCUMENT 1S UNL

AR AP TR TR e+ 21 o,

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CONTENTS

 

Page

List of Figures =e-ec-ecceccec=a- - o e B e e iv
List of Tables =~eemww-v-wncne=- crmmme——— - o e v
ADSETBCE m—mmcmcccmemcmmeememmeemeesesemmesmee—s—sesseme-meeee-——e 1
SUMMBYY ==meee e oo nm e e e e e o o e o o o e e e e e e 1
Introduction e e e e e e 3
General Description -—-----eccccccmcccrcccceccrmrrmrmc e e e 5
Primary Systell e~eeeecccecmcccccmcccccacccccmcncccccccccenseeenan- 10
Reactor ~--ecececcccccccnaccccecnaaaax - 2 40 e o i e 10
Primary Heat Exchangery ---e-ewcwcceccecorrorcccccacccccccena- 26
Primary Pumps =-mee-c-cer-cccccccccccccccscncssrcmmmanen e 32
Secondary Circuits ==-eeesccccccccccnucaccrcerccorarcrcrccmecnaaea 33
Tertiary Salt Circuit e-ccce=c-- - o o o e o o e e e e 35
Steam SysStel mec-ceccemeccccccccccsscnsnsser e e — e — e e e —— e 37
Reactor Building ~==-==- e EscmcsRsceceRREre e, — e - ———————————— 38
Cell Heating and Cooling -=--w=eveccmccecreccrcrcacecsmcrccncaecax 39
Drain Tank System ewe-cmeececccccccccccccerrcrrccmcccccrerr e e ee e 45
Off-Gas System -------- e ——————————— .---_ ..................... 57
Control Rods =emmmros—emcessecosemeoscsosoeoe ;-----_----------;- 60
Instrumentation =---===--= S S ——— 61
Maintenance —eomemmmiem—- --------f;-;-;-----_-f----, _____ ——————— 62
Performance ------~ssceev-- rermecssrmessssscsmmseeeee e —————————— 62

 
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6
Fig. 7.
8
9

Fig. 1.

Fig. 2.
Fig. 3
Fig. h;
Fig.

Fig.

Fig.

Fig.

Fig. 10.
Fig. 11.
Fig. 12.
Fig. 13.
Fig. 1k,
Fig. 15.

Fig. 16.

Fig. 18.
Fig. 19.

Hgo 20.

Fig. 2l..

Figo 220

Fig. 23.

Fig. 2k.

 

LIST OF FIGURES
Simplified Flowsheet for 300-Mi(e) Molten-Salt
Demonstration Reactor ~==-mweccna-- —————— - o o o o
Reaétor Vessel Elevation =seeccrcrmccmmmcmccmcmmcacnamce0-
Reactor Vessel PLAN =ommmemmmmc-——————————————————————
Axial Reflector Mounting =-----s=-----==mmou-mun e
Reflector Attachment -s---cemmeccccmammccmcmcmccocaccmana
Reactor Core and Reflector Plam —-s-=-mw--mco-cmmeooooooe
Core‘Cell Plan ==weee m-—————— e eemeem—m— e e ———————
Core Peripheral Cell Plan m=-m=--=--ccecmomoccaomocooonoao
Bottom Graphite Grid ------r--ccomomcmmcmoom oo
Graphite Collar - mmmemer—e——oee .- e o o om0 o e
Co:e Orificing end Tie Plates --------------- —mmmm—————
Control Rod Cell —cemmmmeccescmccmccccamemocammaaaacacan
Primery Heat Exchanger --=---- meme— e —————— ———— -
Primary Heat Exchanger Head Closure Detail «=wmcecceaca--
Reactor Building Flevation eemrmumcwccvccvccmwncccconnnaan
Reactor Building Plag, --------- o o e
Reactor Ce11 and Heat Exéhanger‘Cell Elevafion ---------
Drain Tank Cell‘Elevationv----;---.a;--.-; ...... e
PrimaryHeat'EXchangerSupport.-Q-;-----;-;-;----; ......
Drain Tank Plafi.—--»--g—;----;---5-_---.--_-----__ ______
Drain Tank_Elevafion_—---;w--a-w --------- -——— m————
DraifiTafik Heat_sink---;-gé----------------------------
Dréin Valve ;--------;------;-------,__--_-u---w--;-_--_

Drain Tank Jet Pump Assembly =weeememmcccccocccmcncnaea-

 

12
13
15
17
19

20

43
LL

L8

52

53

55

 
 

Fig: 25.

Fig. 26.

Table 1.
Teble 2.

Table 3.

Tableh.:_
Table 5.

Table 6 '“"‘ ‘

Table 7.

Table 8.

Table 9.

 

| | Page
Processifig-afid Stbrage’TanksPlan_#é——a ------------- mmm—= 58
Processing snd Storage Tenk Elevation —------mm--------= 59
LIST OF TABLES
Axial Reflector'Hydfaulicfinat&;ff--*"'? """"""" ----- 16
AMSDBIPrimary‘Heat Exchanger Design Date --e=e-=-mee--e--- 28
'uPh&sicalPrbperties of the Fuel and Coolamt . =
Salts Used in the MSDR ==-e-=--ecc—amcca-—- mmmmcecmeeem== 29
MSDR Secondary Hest Exchanger Design Date -c-ommesmmmene 3%
Stean Generating Dabte -----smmmmmmemmmmmemcmmmmmmemamin 37
‘Hoater Design Dats for MSDR Contaimment Cells —m-m-nmen-= 46
 ée1i Cooling System e meem e LT
Dump Tenk Heat Sink Data cmmmcmemmemmmemmememmmmmmmsen 50
“,Lifetimé Averaged Performance of & 750-MW(t) | ‘. | |
 Molten-Salt Demonstration Reactor =—------c--escecacaaa- 63
 

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| DESIGN STUDIES OF A MDIEEN-SALT REACTOR
DEMDNSTRATION PLANT

 

E. S. Bettis, L. G. Alexander,
H. L. Watts '

ABSTRACT

The MSDR, a 350-MW(e) Molten-Salt Reactor Demonstration
Reactor, is based on technology much of which was demon-
strated by the MSRE. The cylindrical vessel (26 ft diam by

26 ft high) houses a matrix of graphite slabs forming salt
passages having a volume fraction in the core of 10%. The
flow of fuel salt is distributed so that the temperature
rise along any path is the same — from 1050 to 1250°F., 1In
the primary exchanger, heat is transferred to barren carrier
salt (in et 900°F, out at 1100°F). 1In the secondary ex-
changer, heat is transferred to a stream of Hitec salt (in
at 800°F, out at 1000°F). The Hitec oxidizes tritium to
tritiated water which is removed and disposed of. The
Hitec generates steam at 900°F, 2400 psi in a boiler, super-
heater, and reheater. Electricity is produced at an overall
efficiency of 36.6%. Soluble fission products are removed
by discarding the carrier salt every 8 years after recovery
of the uranium by fluorination. Volatile fission products
are removed by sparging the fuel salt with helium bubbles
in the reactor primary system. The fuel cycle cost was
estimated to 0.7 mill/kWhr for inventory, 0.3 mill/kWhr
for replacement and 0.1 mill/kWhr for processing, giving
a total of 1.1 mllls/kWhr

 

- The" purpose of this study was to describe & semi- commerC1al-sca1e
molten-salt reactor and power plant that would be based on the technology,
mich of whlch was demonstrated by the Molten-Salt Reactor Experlment.

The plant'was-designed to produce 350 electrioal megawatts."The nuclear

| onversion ratio is about 0 9, and the speclfic power is about 0.5 MW(e)

per kilogram flssile. : . ; .

- The "reactor consists of a cylindrlcal vessel ebout 26 ft in diameter
and about 26 ft high filled with a matrix of graphlte slabs forming flow
passages 0.142 in. thick by 9-3/8 in. wide and with a volume fraction in

 

 

 
 

the core of about lO% ‘The reflectors consist of graphlte slabs cooled
by a small flow of fael salt.; Flow through the core and reflectors is
regulated by orlflces 80 that the temperature rlse of the fuel with
minor exceptions, along any flow path is- approx1mate1y the same.

The fuel salt consists of a mlxture of the fluorides of 71i, beryl-
1ium, thorium, and uranium. (inltlally 235U) Salt leauing the reactor
at 1250°F flows through the tubeside of & shell-and-tube exchanger where
heat-ls,transferred-to_a secondaryrsalt stream composed:of barren carrier
salt. The°fue1 salt exits'at°10505F:and'is recirculated to the reactor.

Secondary salt enters the - exchanger at 900°F and- leaves -2t 1100°F.
It flows to a secondary exchanger, also shell-and-tube ~where the heat
is transferred to a tertlary salt stream, a. mixture of KNOa, Nanoa, and
NaNOz known as Hitec. The purpose of the Hitec loop is to trap tr1t1um
formed -in the reactor and which- dlffuses through the exchangers in the
direction of the steam system Tritlum is ox1d1zed by the Hitec to
trltlated water, which is readlly removed for safe dlsposal The Hitec

enters the exchanger at 800°F and leaves at lOOO°F. ;
- The heat exchangers are arranged SO that after the removal of
shield plugs, the heads may be removed and 1eaky tubes may be plugged
off by remotely manipulated equipment. ,

Heat is transferred from the Hltec tofwater in_thevsteam“generator
which consists-of a boiler, a superheater;'and.a_reheater, all shell-
and-tube types. Steam at 900°F and 2400 psi is produced which, after
being expanded through high, intermediate, and low-pressure turbines,
generates electricity with an overall efficiency of 36.6%.

In the event that there is anvinterruption in the generation of
power, the reactor is drained through a freeze valve into a drain-tank
_provided with an NaK cooling system. The NaK system dumps heat to-a
free-flow1ng water stream.by thermal convection. Hence, the system is
reliable even. when all power fails. |

| Xenon and other noble gases. are removed from the fuel stream by
sparging it with helium in a bubble generator located in a bypass loop
-from pump discharge to pump inlet. After contacting the salt to absorb
noble-gases,;the-bubbles are removed in a centrifugal gas separator.

Following & holdup of about 6 hours in the drain tank, the gases pass
 

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 through & particle trap for remofial of solids. About half the gas is

recycled to the bubble'generator.‘ The other half is routed to a cleanup
system consisting of charcoal beds where the effective holdup time is
about 90 d&ys,'alldwing for almost completé decay of radioactivity. The
effluent is recycled to pump'shaft seals and other purge points.

Removal of fission products from the fuel stream is effected by
discarding'éarriér’salt every 8 years'after fluorination to recover
uranium. The spent salt 1s stored for future recovery when a complete
molten-salt processing plant is available.

Although the main control of the reactor consiéts in adjusting‘the
concentration of fissile uranium in the fuel salt, auxiliary control is
achieved by means of 6 cruciform control rods loaded with By C and clad
with Hastelloy N. h

 

Fuel Cycle Costs

Inventory : | Mills/kWhr

 

Fissiles | - 0.62
Salt 0.07
Replacement
Fissiles 0.18
Salt ' 0.13
Total 1.0
Processing (estimated) 0.1

Total fuel cycle cost. = 1.1

 

.aj'INTRoDUCTIoN-‘

The prlmary objective of the Molten-Salt Reactor Program at ORNL is

~to develop a hlgh—performance thermal breeder reactor that utilizes a

molten salt fuel and breeds on the th0T1Uflk'33U fuel cycle. Conceptual

de31gns for such reactors have been studled for several'years. A ref-

‘erence design for a lOOO-MW(e) plant ‘end the uncertalntles that must be

resolved to achieve a commercial thermal breeder plant were described

 

 
 

recently 1n report ORNL-h5hl and 1n Nuclear Appllcatlons and. Technology
The Molten-Salt Reactor Experlment (MSRE)a - a 7,5- MW(t) reactor — was
operated from December l96h to December 1969 to demonstrate ‘the feasibll-
1ty and 1nvest1gate some aspects of the chemlstry, englneerlng, and
operatlon of molten-salt reactors. Although successful operatlon of
the MSRE was a notable achlevement _the power denS1ty was low, the heat
was reaected to alr, and the experlment lacked meny other complexztles
of & power breeder plant | The next step 1n the Program plan for devel-
oping the breeder is the constructlon of a Molten-Salt Breeder Experlment
(MSBE) .4 |

The MSBE would be & lBO-MW(t) reactor that Would have all the “tech-
| n1cal features of a hlgh-performance breeder The maximum temperature
‘(1300°F) and peak power density (llh W/cc) Would be as high or hlgher‘,-
than in the reference breeder design. Supercritical steam'would be
generated in the reactor steam supplyrsystem, and the plant would have
the fuel reprocessing facilities required for a breeder. The purpose
of the MSBE would be to demonstrate on an intermediate scale the solu-
tions to all the technical problems of a hlgh-performance Molten-Salt
Breeder Reactor (MSBR).

An alternatlve approach to the development of a commercial MSBR
has also evoked 1nterest This approach emphasizes more rapid attain-
ment of commerclal smze but more gradual attainment of high performance.
The step beyond the MSRE is construction of a_300-MW(e)_Molten-Salt
Demonstration Reactor (MSDR). The purpose of the MSDR would be to

 

1Molten-Salt Reactor Program Staff, Roy C. Robertson, ed., Conceptual
Design Study of a Single-~Fluid Molten-Salt Breeder Reactor, ORNL-L5L1
(June 1971).

2g, §. Bettis and Roy C. Robertson, "The Design and Performance
Features of a Single-Fluid Molten-Salt Breeder Reactor," Nucl. Appl Tech.,
8, 190 . (1970) , , |

®p. N. Haubenreich and J. R. Engel, "Experience with the Molten-Salt
Reactor Experlment " Nucl. Appl. Tech., 8, 118 (1970). !

"‘J ‘R. McWherter, Molten Salt Breeder Experiment Design Bases, ORNL-
TM-3177 (Nov 1970).

 
 

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demonstrate the molten-Salt reactor concept on & semi- commercial scale

while requiring 1ittle development of basic technology beyond that demon-
strated in the MSRE.

The objective of the study reported here was to prepare a conceptual
design of an MSDR'plant. The overall engineering deSign of the plant and
the details of some aSpects of the:deSign”are described in this report.
Basic information7on chemistry, materials,_neutron physics, and fuel
reprocessing waS'reported recentl& in'ORNLéhshl and is not repeated here.
The problem to which this study was addressed concerns design of a
firstgof-aAkind'reactor plant which could be built with & minimum of

development'and from which higher performance breeder plants could evolve,

Concepts which could not be used in future breeder plants were to be

aVOided

| Two maJor 51mp11f1cations were made in the design of this demon-
stration plant as compared to the de31gn of the breeder plants. First,
the MSDRVhas only such chemical processing as was demonstrated in the
MSRE and has no proviSion for removing fission product poisons on a short
time cycle. This results in a much less complicated chemical processing
plant, although it means that'the reactor has a breeding ratio less than
one and is therefore a converter. The second major simplification is
that the poweridensity was made low enough for the graphite core to last
the 30-year design lifetime of the plant,xthus simplifying the reactor
vessel and eliminating the eqhipment for replacing the core. Other
areas of design were also simplified by the very low power denS1ty of
the reactor as will be noted in the description of the plant that follows.
' 'We believe that the design described here’ represents a molten-salt
reactor plant which is feas1ble to build will produce a significant

amount of electrical power, and would be a major step toward a useful

family of breeder reectors f ‘7'

o GENERAL DESCRIPTION

ThlS plant is designed to produce 750 thermal. megawatts in a single-
fluid molten-salt reactorr. Tho_r:Lum in the prlmary, or_,fuel, salt is
converted to fissile uranium. Because of the simplified salt processing,

the conversion ratio is of the order of 0.9. _The heat generated in the

 
 

reactor is transported to the primary heat exchangers as the salt is
“clrculated through the reactor and heat exchangers by the prlmary pumps.
Figure 1 is a simplified flow dlagram of the system In the prlmary heat
exchangers the prlmary salt glves up. its heat to the secondary salt which
is c1rculated between the prlmary and ~secondary heat exchangers by the ,
secondary salt pumps. The secondary salt has the same compos1t10n ‘
(7L1F-BeFa) as the prlmary carrler salt Slnce it contalns no f1ss1le
or fertlle materlal it is very much 1ess radloactlve than the prlmary
- salt. . , _ , .
The secondary salt 1s cooled in the secondary heat exchanger by a f
thlrd molten salt whlch circulates between the secondary heat exchangers
- and the steam generatlng equlpment This third salt is a eutectlc mix-
ture of nitrite and. nltrate salts (KNOa-NaNOg-Namoa) The prlmary purpose
- of thls th1rd salt loop is to capture trltlum.which is generated in the
prlmary salt and dlffuses through the heat exchange surfaces of the pri-
mary and secondary systems and 1nto the third salt system. It would
migrate into the steam system if this thlrd salt, having oxygen avall-,.
able to tie up the tr1t1um, were not present. The nitrite- nltrate salt
cannot be used as a secondary coolant for reasons that will be dlscussed
later. _
The steam system is conventlonal. It has high-, 1ntermed1ate-'
and 1ow-pressure turbines, coupled to a generator, whlch take steam from
the steam generator-superheaters, and reheaters at a temperature of 900°F.
The high-pressure turbine throttle pressure is 2HOO psia. The b011ers
and superheaters are of the once-through type with rec1rculation of water
through the boiler for flexibility in control. The condenser, deaerator,
water. treatment and feedwater heater chains are conventlonal and Wlll
not be descrlbed The steam.condltlons were chosen somewhat arbltrarlly
as be1ng sultable for & first-of-a-kind plant. R
One of the essential auxiliary systems for a molten-salt reactor
is the cover-gas system for the prrmary salt circuit. Since salt must
be kept free from oxygen, an inert atmosphere (helium) must be maintained
“in the gas space associated with the fuel-salt system. Many of the
fission products are volatile and these highly radioactive gases must be

“cooled, stored, contained, and safély disposed of by either radioactive
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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!fi'i!.lm -
SEPARATOR
o) '
.. § 24,450 gem
T
‘wearpemcy T -
' ; mimasy
RAN TANK
5 .

 

 

 

 

 

 

 

 

 

 

Fig. 1. Simplified Flowsheet

 

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ORNL DWG T2-3586

 

 

 

 

 

 

 

 

 

for 300-MW(e) Molten-Salt Demonstration Reactor.

 

 

 

 
 

decay or permanent storage. This,off-gas:system,.which,will'beAdescribed
-in detail later, must have a guaranteed heat removal system, fission
product-absorber.system, pressure regulating system,.and meenS[of sepa-
rating 1iquid~(salt) entrainment. The off-gas system is quite involved
and.must ‘have an extremely high degree of dependablllty'— a- requlrement
that makes & certain amount of redundancy necessany
A second auxiliary system.whlch is extremely 1mportant is the after-

‘heat removal system. Because ‘the fuel can be drained from the molten-
salt reactor and because of the low power dens1ty,there is no need for
anremergencygcore cooling system,,but the drsln_tank for the prlmary
salt nust have a cooling systefifthafsis posiiiveVend independent of the
-power~sfipply or operating machinery3 if possible. The afterheat removal
system ‘is thus one of the essential auX1liary systems.‘ '

| The fact that -there are no solid fuel elements to be fabrlcated
1oa.ded, reprocessed, and refabricated makes the molten-salt reactor
unique. Many advantages accrue from this fact, but it 8150 mskes neces-
sary an on-site chemical processingfplant'for'maintaining.the”salt in

a cleen\and:operating condition. This chémical processing plant is

another auxiliary system which is essential to the plant.. When a molten-

salt reactor is to be used as a breeder, the chemlcal processing system
becomes relatively involved. In addition to keeping the salt clean and
low 1n,oxygen, and adjusting the uranium inVentory,~fihe fission product
-poisons must be removed from the primary sjstem on,a%fairly short time
cycle;i If, on the other hand, the plant is designed only as a converter
having & breeding ratio of less than unity, the‘fission producfis can _‘
be removed on a long time cycle and: the chemical plant becomes much more
31mple o 3

Even in a converter reactor it 1s uneconomical to allow the fission
product p01sons to remain in the prlmary salt for the 11fe of the plant.
The fuel carrier salt with the fission product poisons is therefore dis-
carded after about 8 years of operation. Provision must be made for
recOvering.the.fissile material and for disposing of the radioactive
carrier salt. Such a chemical processing plant is an integral part of
this reactor power plant design. The'processes inVolved are those which
were used successfully to process the salt in the MSRE.
 

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A nuclear power plant requireslother<auxiliary systems such as

control, instrumentation, and safety systems. Basic and inherent safety

'features of the molten-salt reactor permit the safety system to be con-

siderably less complicated than 1t is for some nuclear reactors. Because

" of the necess1ty to bring the reactor to a temperature above the melting

point of the molten salt before the system can be loaded, a startup heat-
1ng system is required

The MSDR plant is housed in two buildings the reactor building and
the turbine building Since the latter is a conventional mill-type
structure, 11ttle effort was put into designing a building for this part
of the plant

Considerable attention was given to des1gn of ‘the reactor building.
It consists of 8 cylindrical shell with a. hemispherical top. Contain-
ment integrity is provided by 8 1/2-1n -thick steel liner, completely |
surrounded by concrete. The concrete is for biological shielding as
well as providing strength for resisting tornadic winds and miss1les,
in accordance with the accepted standards for design of reactor buildings.

One unique feature of the reactor building shown in this report is
that it 1s supported by & large circumferential concrete ring, with
about one-third of the structure hanging from this ring below grade and

‘two-thirds of the building extending above the support ring It was

felt that this building configuration would have better seismic res1stance
thah one: which totally extended above the support. Obviously this feature
is not mandatory and the design of a particular reactor building would

'depend on the topography, geology, and seismology of the site

h The eylindrical part of the building contains all the radioactive
systems and components, such as the reactor, primary and secondary heat |
exchangers, off-gas system, chemical processing plant and primary drain
tank, The cells for this equipment have a sealed steel containment in |
addition to the building containment On the thick concrete ring, but

joutside the building containment are located the steam generator cells,

the control room, and water-cooled heat sinks that prOVlde for afterheat

firemoval.{';;”',

 
 

10

. PRIMARY SYSTEM
Reactor

The reactor 1s one of the s1mplest components 1n the entire plant

due to the b&SlC S1mp11c1ty of the circulating fuel concept.' In preV1ous )

molten-salt concepts the reactor has not been fully described ‘For this
reason the mechan1ca1 design of the reactor is given s more thorough o
treatment 1n th1s report. Other parts of the plant have recelved a much
more cursory treatment since these 1tems have been more fulky dlscussed
1n prev1ous design studies There 1s 11ttle heat transfer that ‘takes
place within the reactor 1tself the flssion heat be1ng transported out-
side the reactor vessel by the circulatlng fuel salt. The only heat
transferred w1th1n the reactor vessel is that produced by gamma and
neutron heating of the graphite and the vessel walls By making the'
power dens1ty of the reactor low, the graphite w1ll not undergo exces;fj
sive radiation damage during the 1ife of the plant. Hence, the reactor
tank can be an all—welded vessel Also, s1nce the vapor pressure of .
the fuel salt is 1ow even at high operating temperatures, the reactor
need not operate at a high pressure and the walls can be relatively
thin. The temperature coefficient of expansion of graphite is onLy
about one- third that of the Hastelloy of which the vessel 1s made,
however and this requires some des1gn ingenuity to prevent the differ-
‘ential expan51on from causing problems when the reactor is brought to
temperature | I
The reactor con81sts of a cylindrical vessel with dished heads,
which is filled wath 8 matrix of graphite slabs that form flow passages
for the fuel salt. The 1nner surface of the vessel is lined W1th an
average thickness of 2-1/2 ft of graphite as a reflector.r This reflector
conserves neutrons by reducing leakage to a very low value and thereby “
also reduces the amount of radiation reaching the vessel wall.' The{lurl
reflector 1s attached to the vessel so that 1t moves w1th the vessel as
they expand during heatup _"7 - | f
. The 1nternal structure of the reactor is made up of the core matrix
the axial and radial graphite reflectors, and two internal metal dished
heads which locate and support the graphite of the axial reflectors.
-y

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1

 

11

Figure 2 is an elevation of the reactor vessel showing the internal
grephite structure. Figure 3 is a plan view of the reactor vessel.

Tt is important to maintain controlled flow passages in the reactor
vessel and to regulate the flow so as to get a uniform temperature rise -
in all flow passages as the salt flows through the reactor. Since the
power distribution is non-uniform, this requires different velocities
of salt in the various flow passages.”It is also desirable to minimize
cross flow and to have axial flow fromfbottom to tophof the reactor core.
The flow in the reflector and along the vessel Wall should be straight
through from ‘bottom to top.

The graphite should be mounted Within the core in a manner that
will preserve the geometry of the flow passages yet, at the same time,
accommodate the dimensional changes due to temperature differences and
radiation damage in the graphite. The factor of sbout 3 x 10~® in. [in./°F
difference in thermal expansion between graphlte and Hastelloy must also
be taken care of in the design. A further limitation is that graphite,
of the requlred grade, cannot_be,made'in-large monolithic blocks.

The reflector should have a minimum salt volume in.it in order to
keep the fission rate low. Ideally,_onlysuchsalt'as is necessary to
cool the reflector’shouldtbe'present;' This makes it desirable to have
a few large pieces of graphite in the reflector rather than many small
pieces. For this reason the graphite of the reflector is laminated of
smaller pieces cemented together, baked and machlned to shape after
baking. The low flux encountered in the reflector permits this fabri-
cation technique to be used. By this lamination procedure, blocks of

the correct size and shape for the axial and radial reflector sections

‘are made up from graphite slabs approximately 2 in. thick by 12 in.

W1de and from 4 ft to 21 ft in length.

In order to eontrol the unavoidable gaps that result from the expan-
sion of the vessel relative to the graphite, both radial and axial
reflector blocks are attached to the vessel and move with it while the

- core matrix remains stationany. The method of aCcomplishing this will

become evident in the desoription of the’reflectors that follows.
Modified dished heads having a flat center area and an overall size

slightly smaller than the vessel heads are used to hold the graphite

 
 

 

 

12

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Reactor Vessel Plan.

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14

of the axial reflectors. Top and bottom heads are exactly alike and
have ribs welded on their convex surfaces to form bearing surfaces that
fitvagainst the reactor vessel heads. The concave surfaces bf these
- heads are machined to form concentric‘rings having flat "lands" or |
surfaces about 12 in. wide to which gfaphiteISectors are attached. Thei
graphite sectors, having radii corresponding to the radius of the ring
to fihich they fasten, are mounted on these'heads with gaps of’varying
widths between the concentric rlngs formed by the graphite. Spaces
.equal to the gap width between rings are also left between the ends of
adjacent graphlte sectors in each ring as they are assembled on_the head.

The method of mounting fihé*Séctors*bn the:héédsfis?shbfifi infiFig, L.
Three holes are drilled in each graphite sector fram-the.Cénter of»the.
surface that is to be in contaét with the head. One is at the center .
“of the séétor and'one is near the end. Into these holes aré]placed
Hastellqy plugs 2 in. in diameter bjjh inI lbng. These'plugs are are. -
 drilled end tapped axially for 3/h~in, cap screws, They also hévé
3/h—in.-d1am‘transverse holes at a distance of 3 in. from the tapped
" end. The graphlte sectors have horizontal holes so that 3/h~in.-d1am
pins 1nserted through those holes will pass through the transverse holes .
in the metal plugs and fix them in the_graphite. Holes in the hgads '
permit the graphite sectors to be fixed to the heads by cap screfis that
engage the metal plugs in the graphite. The holes for the screws that
engage the plugs in the ends of the graphite sectors are slight1y overg
size to permit differential expansion between the graphite and the metal
head. | | o e
The widths of the slots between the concentric rings of reflector
were computed to provide salt flow to the core in accordance with the
radial power distribution in the core.men making these computations,
the AP across the reflector was assumed to be 5 psi.' The AP across
the holes in the head under the slots was also assumed to be 5 psi and
the number of holes required to furnish the required flow was calculated
with the results shown in Table 1.

After the graphite has beeh installed in the reflector head the
complete bottom assembly is lowered into the reactor vessel. Worklngr
through the bottom hole in the vessel, the center gussets of the
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Axial Reflector Mounting.

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ORNL DWG T2-4030

 

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16

Teble 1. Axial Reflector Hydraulic Data.

 

Rings from Slot Width Velocity @~ Number of 1-In.

 

Center (in.) (fps) Holes per Ring
2 0.14 5.1 - 23
3 0.22 6.6 18
4 0.25 7.1 36
5 0.24 6.9 51
6 0.24 7.1 63
7 0.22 6.9 L
8 0.20 6.8 76
9 0.16 6.2 62
10 0.16 6.2 63
11 0.11 5.0 143
12 0.07 4.6 28
13 0.03 1.2 1
1l 0.02 0.7 1%

 

®Substitute many small holes for equivalent area.

reflector head afe welded to matching overlapping gussets in the vessel
head, rigidly attaching the reflector head to the‘vessel at the center.
The periphery of the reflector head has 8 slots which permit it to
be lowered into the vessel, the slots sliding over the 8 vertical ribs
on the cylindrical wall. After having been lowered into position the
reflector head is pinned at these 8 points, tying the. head to the vessel
but permitting radial expansion as the pins move in radial slots in the
ribs. Figure 5 shows this method of attachment. ‘
The radial reflector is made up of laminated blocks of two different
shapes, plus some odd-shaped filler blocks used to wedge the reflector
to the vessel wall. The outer reflector is made up of laminated wedges
about 10 ft long and about 3 ft high. - These wedges are machined to fit
the vessel wall at the 8 places vhere they are installed around the
inside vessel perimeter. By fitting each wedge approximately'l/l6 in.

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17

/‘}hfil. COosIR2E WELO

 

Jor REpECTOR MISTRLLATION

 

osacroe VEssaL

 

 

 

LBorros _AEL CECTOR JLSTRLLABTLION

 Flg. 5. Reflector Attachment.

ORNL DWG T72-3580

 

 

Secrron £-8

 

LLT7TTA

 

 

ey

 

 

 

77

 

 

yren A-A

 

 
 

 

18

from the-wall, any out-of-roundness of the vessel is accommodated. When
all pieces have been put into place, & retaining "T" iron is fixed to
the vertical metal ribs and wedging pieces of graphite are inserted be-
hind the "TI" iron. Thus the outer reflector is made to move out with -
.the vessel wall as it expands. Figure 6 is a planvview of the core
‘which shows the reflector graphite as well as the reactor core matrix.

The inner layer of reflector is made up .of columns of graphite
-approx1mately ‘1 £t wide by 2 ft thick by 21 £t 1ong These blocks are
also laminated from slabs similar to those used elsewhere in. the core.
These vertical blocks are doweled into the bottom axial reflector pieces,
but the dowels are loose enough to permit the columns to float up from
the axiallreflector while still being tied to it radially. The columns
are installed with 1/8-in. gaps between columns and the back wedges and
between adjacent columns. Also the bottom edge of each block has & 2-
in. by 1/4-in. slot machined on it to provide flow access to the'vertical
slot should any block fail to float when the reactor is full of salt.

At the top of these columns there are horizontal slebs of graphite
approximately 1 in. thick by 3 in. wide by sbout 2-1/2 ft long. These
slebs'fif in milled slots and plug the gaps between columns at the top.
They are doweled to one of the columns eand te the wedge?of graphite
:adjacent to the tank wall in order to provide orificing and tie the
columns to the wedges at the top to maintain the de31red location of
the columms. The slabs rest against the top reflector when -the reactor
is filled with salt, and they provide a l/h-ln.-thlck salt plenum over
the radial reflector graphite.
| The core matrix is made up of a square array of slabs approximately
1-3/4.in. by 9-3/8 in. in cross section. These slabs are stacked in
square cells approximately 11-1/2 in. by 11-1/2 in. The slabs are sepa-
-rated by dowels fihich‘provide verticel flow passages approximately 0.1h42
in. wide rumnning from bottom to top of the core. The plan arrangement
of the core is shown in Fig. 7 and Fig. 8. In addition to the slabs,
there are posts approximately 2—1/2 in. square that define the corners
of the cells. These posts have 6~in. dowels 1-1/h4 in. in diemeter on
the bottom end which fit into holes in the corners of a graphite "egg
crate" grid that-rests on the bottom axial reflector. This grid defines
 

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Fig. 6.

19

ORNL DWG T72-2826

 

Reactor Core and Reflector Plan.

 

 
 

- 20

ORNL DWG T2-2830

 

 

 

vay

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

nes”
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IE D 7 7
S
oL AR = - ~
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Fig. 7. Core Cell Plan.

 
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Deres RFE ~

 

 

 

 

 

 

 

 

Fig. 8.

Core Peripheral Cell Plan.

 

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ORNL DWG 72-357T

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22

the core and prevents accumlation of radial expansion tolerances in any
region of the core. Figure 9 is a detail of the grid. |

Around the top of the core is a graphite "collar" which prevents
spreading of the vertical core pieces into the annulus between the core
and reflector. This annulus, approximately 0.75 in. wide, results from
" the differential expansion between the metal vessel and the gréphite;
The collar also blocks this annulus to prevent excessive flow in this
path of low pressure drop. Flow from the annulus has;fo_éross,throuéh
the radial slots in the reflector and out the exit slots above the wedge
reflector pieces. Figure 10 is a detail of this collar structure..

The graphite core is assembled in the vessel and then qrifice plates
are put on top of the core to makerthe core flow match the iadial power
g distribution. These flat plates are approximately 11-1/2 in. by 11-1/2
Cin. by 1-1/2 in., with undercut edges to fit down into the cells formed
- by the vertical posts. These plates have no intentional gaps and are
fitted as closely as tolerances permit. Resting on top of the cell side
plates,rthey leave a 1/8-in. gep between their underside and the top of
- the five slabs making up the cell structure. Short dowels on the under-
_side of the plates prevent the slabs from floating up and closing this
- gap. In the center of each plate is an orifice. The diameter of this
hole is 2.05 in. for the center cells and 0.7h4 in. for the peripheral
cells, with intermediate cells having holes of diameter proportional to
the radial power distribution. .

These cover plates, in addition to providing orificing for the
different cells, perform an'additional‘function,of'tying the core to-
gether at the top and preventing accumulation of expansion opéning in
one location. On one side of each plate is a 3/l-in. dowel extending
1-1/2 in. sbove the top of the plate. After these orifice plates have
been placed on top of the core, rectangular tie blocks l;l/éiifl; fhick,
3 in. wide, and 6 in. long, with two holes in them, are placed over the
~dowels of adjacent plates to tie the cells together. At the same time,
the ties are not rigid and will allow some movement of the core if
needed. These links cannot float free of the dowels even if some cell
or cells stick and do not float to the underside of the top axial re--
flector. Figure 11 shows the orificing and tying of the graphite.
 

W

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Bottom Graphite Grid.

 

 
 

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- ORNL DWG T2-2828

 

 

 

 

 

 

 

 

 

 

 

Fig. 10. Graphite Collar.
»

Fig.

11.

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Core Orificing and Tie Plates.

ORNL DWG T2-2827

 

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26

Six cells in the core are different from the other 375 cells. These
contain the sockets into which the six control rods are, inserted. They
~are made up of solid square prisms, 1am1nated out of the core slabs, and
bored and machined according to the design shown in Flg 12 These blocks
are made in sections as long as pract;cal to machine, and the sections
are then joined to make a continuous cylindrical hole into which each

control rod is inserted. The holes are 7'in. ifi diameter end have a
1/2 in. orifice in the bottom to permlt proper flow of salt to the
channel.

~ The control rods are cruc1form in shape and 6-1/2 in. w1de This
provides a l/h -in. radial clearance between the rod and the hole into
which each is inserted. The rods are made of 1/8-in. Hastelloy N en-
closing a 1/2-in. filler of boron carbide. When fully inserted, the
rods extend to within 3 ft of the bottom of the'eore. When fully with-
dfawn; the bottoms of the rods_eiefjuet within the top reflector.

Pipe extensions around the eontrel rod extensions penetrate the
top of the reactor cell shieldihg andithe control rod drive mechanisms
are located outside the reactor cell on top of the shielding. The rods
will have to be replaced periodically as the high flux will destroy
the ductility of the Hastelloy. One rod is sufficient to shut down the
reactor, and only one rod will be-used at any one time for control of

the reactor.

Primary Heat Exchanger

Primary salt is pumped tfiieugh the three primafy loops by thiee
salt pumps. Each pump discharges salt into two primary heat exchangers
in parallel. There are two heat exchangers in each leg in order to make
the fabrication of these units more practical. Eech exchanger is about

- 26 in. in diameter and is in the form of a "U" with a tube length of
~ sbout 30 ft. Bach has 1368 tubes of 0.375 in. OD with a wall thickness
of 0.035 in. Table 2 gives the data on the primary heat exchangers.
Table 3 gives the physical properties of the salt on which the design of
the heat ekchangers is based. Figure 13 is a view of the heat exdhafiger
and Fig. 14 shows the detail of the head closure.
*2

 

 

 

 

 

 

 

 

 

Jecpiod A-4

Fig. 12. Control Rod Cell.

ORNL DWG 72-3576

   

 

 

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28

Tsble 2. MSDR Primary Heat Exchanger Design Data

 

Type

Rate of heat transfer per unit
o ,
Btu/hr

Tube~-side conditions
Hot fluid , ‘
Entrance temperature, °F
Exit temperature, °F -
Pressure drop across exchanger, p81
Mass flow rate, lb/hr

Shell-side conditions
Cold fluid E
~Entrance temperature, °F-
Exit temperature, °F

Pressure drop across exchanger, psi -

Mass flow rate, 1b/hr
Tube material
Tube OD, in.
Tube thlckness, in.
Tubesheet-to-tubesheet dlstance, 't
Shell material
Shell thickfiéssx ip}
Shell ID, in.
Tubesheet material
Number of tubes
Pitch of tubes, in.
Total heat transfer area, ft?
Bagis for area calculation
Type of baffle
Number of baffles
Baffle spacing, in.
Disk OD, in.
Doughnut ID, in.

Overall heat transfer coefficient,
U, Btu hr-* £t~

Volume of fuel salt in tubes, ft®

U-tube, U-shell, countercurrent,

one-pass shell and tubes with
disk-and-doughnut baffles

‘125

4.3 % 10°

Fuel salt

1250
1050°
127 - '
6.6 X 10s

" Coolant salt (2 LiF-BeF@)

900
1100
115 ‘
3.7 x 10°

Hastelloy N
0.375_
0,035

300,

Hastelloy N

0.5
1'26532

Hastelloy N
1368

..0 672 (trlangular)
'hoeh B

Outside of tubes
Disk and doughnut
L7

7.7

19.0

18.6

- .T00

20.8

 
( t " . LA £ o (

Table 3. FPhysical Properties of the Fuel and Coolant Salts Used in the MSDR

 

Composition | IiF-BeFs -ThFs ~UFs (71.5-16.0-12.0-0.5 mole %)
Density, 1b/ft§ | | N | 236.3 — 2.33 x 10°°T (°F)
. . -1 et | oy 7362
Viscosity, 1b hr ft S | . 0.2637 exp 55T + T (°F)
Specific heat, Btu 1b7:(°F)™* 0.32.

Thermal conductivity, Btu hr™® £t (°F)"? 0.75

LiF-BeFs Coolant Salt

 

Composition  7LiF-BeF (66-34 mole %) (99.99 + % 7Ii)
Density, 1b/ft®> - . 138.68 — 1.456 x 1072T (°F)

Viscosity, 1b h_r-]- 71 | ' | 0.2806 exp E%TFT

Specific heat, Btu 1b7} (°F)7 | 0.57

Thermal conductivity, Btu hr™* ft~* (°F)™* 0.58

Nitrate-Nitrate Coolant Salt

 

Composition (eutectic) - KNOg -NaNOg - NaNOg- (44-49-7 mole %)
Density, lb/ft® | 130.6 — 2.54 x 10727 (°F)
: i -1 1 3821.6
Viscosity, 1b hr * ft | 0.1942 exp 557 5 T (°F
Specific heat, Btu 1b~* (°F)7? 0.37

Thermal conductivity, Btu hr t ft ! (°F)? 0.33

 

62

 

 
 

30

ORNL DWG

T1-5032

 

 

 

.

 

 

PIeIMtARy Sasy MKET

 

 

~
A

 

 

 

 

| s28”

 

 

 

Almot Tronsrer Pdk - SEE MW .
AB. Fubes - IFE8. .

Tube Sice-Y 0.0 <.035 woll
Tube Btct -.67 4

Fig. 13. Primary Heat Exchanger.

 
 

31

ORNL DWG T72~358)

 

~SELL WELO

 

 

 

 

 

R

 

JOBE SHsEy

v //////// //// ////

 

- Fig. 14. Primary Heat Exchanger Head Closure Detail.

 
 

“ The heat exchangers are mounted hbrizontally, one leg of the "U"
being under the other. The tubesheet’endseof the exchanger are access-
ible through a hole in the cell wall by removing a shield plug from the
hole. The head is flanged and bolted with metal "O"-ring seals backed
up by a thin’éeal weld. This permits relatively easy removal of the
heads for plugglng a tube in the event of a 1eak g '

Primary,ggggsl

‘Sump-type centrifugal pump5jfpr-eircfilatingemblteneSaits?haVe'been'
employed for many years in experimental rigs and he#e been used success-
fully in two molten-salt reactors, the ARE® and the-MSRE,_ Theselmolten—
salt pumps are described in the 1iterature,§ and tnep-material will not
be repeated here. -

In the MSDR the pumps are mounted symmetrlcally on the top of the
reactor vessel. Each pump is submerged in a tank that has excess gas
volume to accommodate expansion of the salt. This volume commnicates
with the primary sysfem.by & 6~in. line from each pump bowl to the center
dome in the_top'of the reactor vessel. During normal operation, flow
down these lines returns the fountain purge flow from each pump to the
top head of the reactor. The pump suction line commnicates with the
pump sump through a small leakage path in order to drain the sump when
the primary system is drained.

Each pump has a capacity of 8100 gpm end develops a 150-ft head. In
order to pfirge xenon from the salt; about 10% of the flow from each
pump is bypassed directly from pump outlet to inlet. The bypass line |
contains a bubble generator for admitting gas to the salt and a bubble
stripper'for rembving'gas. The gas stripper removed about two volumes
of salt with each volume of gas removed, and this mixed stream is

directed into the drain tank. A venturi in this line is provided with

 

5J'am.es A. Lane, H. G. MacPherson, and Frank Maslan, eds., Fluid
‘Fuel Reactors, Chapter 16, "Aircraft Reactor Experlment " Addison-
Wesley Publishing Company, Inc. (Sept 1958). :

®Molten-Salt Reactor Program Staff, Roy C. Robertson, ed., Conceptual
Deszgn Study of a Single-Fluid Mblten-Salt Breeder Reactor, ORNL o541, '
58 ff (June 1971).
»

L1

 

33

a l-in. connection to the gas space 1n the pump bowl. This arrangement

“makes 1t pos31ble to mix the. pump. seal purge gas with the stripped gas

and to send it to the drain tank and thence into the off-gas system.
This line also serves as a syphon break to prevent draining the system
through the gas separator discharge line in the case of a stopped pump
in any of the primary circuits. ' |

In addition to providing the primary selt flow and the bypass flow,
a line from each pump discharge is manifolded into.a line which drives
two jet pumps in the drain tank. Each of these lines has a ball check
valve inserted in it-npstream of the manifold. The jet pumps in the
drain tankfreturn_the_entrained liquid in the gas ‘separator discharge
to the primary circult, as will be described in the discussion of the
drain tank. )

SECONDARY CIRCUITS

Heat from each primary clrcuit is transferred to a secondary salt
system. The salt in each\secondary loop is circulated through the shells
of the primary heat exchangers and the tubes of the secondary heat ex- -
changers by a pump similar‘to the primary pump. The secondary circuit
is designed'to contain arrelatively small volume of salt. o

The salt used in the secondary loop is 7LiF-BeFs (66-34 mole %).
Although it would be possible to use a different salt in the secondary
circuit, it was believed that the advantages accuring from using the
7LiF_‘--BeF‘z salt Justifies its useuin the first demonstration plant. By
using this salt, any primary heat'exchanger_leak will be much less
troublesome'than-if5a”different-secondary salt were used. Als6, the -

LiF-BeF: salt will remain°nonradioactive, except for very. Short-lived'

activities, and’ thus maintenance of the secondary heat exchangers will

: be less complicated.

There are +two secondary heat exchangers per secondary.loop. These

- are "U" tube exchangers of the same design as- the primary exchangers.
;Because of the differences in the salt heat transfer properties, they .

are slightly. larger than +the primary exchangers.-_Table L gives the
design data for these exchangers. e -
It may be noted that the secondary circuit isolates the primary cir-

cuit from the rest of the system. Because of the sensitivity of the

 
 

 

3k

‘Teble 4. MSDR Secondary Heat' Exdhanger De51gn Date

 

Iype .

Rate of heat transfer per unit
MW | :
“Btu/hr
Tube-side conditions
Hot fluid L s
Entrance temperature, °F
Exit temperature, °F Lo

Pressure drop across exchanger, psi -

Mass flow rate, 1b/hr

Shell-side condltlons
Cold fluid
Entrance temperature, °F
Exit temperature, F ' o
Pressure drop across exchanger, pSl
Mass flow rate, 1lb/hr

Tube material

Tube OD, in.

Tube thickness, in.

Tubesheet- to-tubesheet dlstance, £t
Shell material

Shell thickness, in.

Shell ID, in.

Tubesheet material -

Number of tubes

Pitch of tubes, in.

Total heat transfer area, ft° .
Basis for area calculation
Type of baffle
Number of baffles
-Baffle“SPacing. in.

Disk -OD, in.. -

Doughnut ID, in.

Overall heat transfer coefficient, U,
Btu hr™* £t72

Volume of 2L1F-BeFa salt in tubes, ft3

. U-tube, U-shell counter-

' current, one-pass shell

= and tubes_W1th disk-and-
doughnut baffles .., .

1z
ook, 3 X 10

;2LiF-BeF3,§alti,e'
1100 .

900

80

3. 7 X 106 -
Hitec
T00

1000
80

3.8 x 10°
.. Hastelloy N - L
0375 .
' .0.035

- 37.5

Hastelloy N - -
015_ ’

Hastelloy N . -
160k |

- 0.7188 (trlangular)

OutSide.offtubeg_;fl,*;‘

- Disk and doughnut. - -

8.6
o2.0 . -

500

305

 
1

 

35

primary salt to oxygen, this is a desirable, if not necessary, feature.
Also, the secondary and tertiary salt circuits provide a double barrier -
between the low-pressure fuel-salt system and the relatively high-pressure

steam system.
'TERTTARY SALT CIRCUIT

- The primary purpose of the tertiary salt c1rcu1t is to prov1de a
tritium trap to prevent diffus1on of tritium.from the primary circuit
into the steam system v1a the circulating salt systems The tertiary
circuit uses a eutectic mixture of KNOa—NaNog-NaNoa (hh 2-48.9-6.9
mole %), which has the commercial name of "Hitec " The oxygen in this
salt comblnes with the tritium to form tritiated water, Whlch can be
recovered from the system, . o

Although the primary function of the tertiary saltiloop is to trap
tritium, it also has certain advantages which offset, at least to a
degree, the complication of the additional pumps and heat exchengers
required for the third loop. The advantages derive chiefly from the
fact that the liquidus temperature of the nitrite-nitrate eutectic is
288°F, which relieves cons1derably the poss1ble problems of salt freeze-
up in the steam generators. An additional advantage concerns the fact
thet water‘will simply vaporize from the nitrite-nitrate salt‘and_the_
consequences of a steam leak ere thus much less severe In fact, this
salt could possibly be used with steam. as & cover gas.

~ The nitrate-nitrate salt cannot be used as a secondary coolant

_because 1t would react W1th the primary salt to precipitate thorium

and uranium oxides in. the event of a primary heat exchanger leak.

'There 1s the additional danger of a reaction W1th the graphite of the
”core should nitrite-nitrate eutectic get into the primary system..w

The nitrite-nitrate salt will remain nonradioactive, except for
some tritium, and thus the tertiary loop can penetrate the building
containment. This permits the location of all the steam system equipment
outside the containment and makes it access1ble for direct maintenance
It also removes the possibility of a pressure rise within the contain-
ment due to a leak in the steam systen.

The corrosion resistance to Hitec is a consequence of the estab-
lishment of a passivating film on the containment metal. Cheaper

 
 

36

materials can be used for the piping'andeteamegenerating equipment of

this~loopthan is reQuired inithefprimaryfand secondary systems. ~It:is

‘intended to make the secondary exchangers out of 316 stainless steel -
because of the secondary,salt in the tubes. The shells and piping, -

however, could be made of Croloy. ?he‘tertiary;pumps could also be fab-

ricated of Croloy.

As preViously mentioned the low melting pOint (228 F) of the Hitec
permits ‘the feedwater to enter the boiler at temperatures that are in
line with current steam plant practice Since the wall At must be held
to a reasonable value (< 200 F), the salt temperature at the ex1t of the
bOiler must be of the order of 700 F if the feedwater enters at about |
500°F.f This 700 P salt cannot be returned to the secondary heat exchanger
because the freezing point of the secondary salt is 856°F.- To av01d o
this low temperature and still maintain an acceptable At across ‘the tubes
in the bOiler, a bypass stream.of hot tertiary salt is mixed‘With the'
cool (700°F) salt to raise the temperature of the mixture to an accept-
able level before returning it to the secondary heat exchanger The‘"‘fi
total flow 1n the tertiary system is about hO 500 gpm of this, 27 OOO
gpm flows through the steam generating equxpment and is cooled to 700 F.
The bypass loop mixes 13 500 gpm of lOOO F salt With the 27 OOO~gpm o
T00° F stream and raises its temperature to 800°F before it enters the |
secondary heat exchanger A throttle valve in this bypass 1ine permits
temperature control for partial loading and trans1ent conditions 'y o

Data on the decompos1tion and corrosion of Hitec are insufficient |
to be absolutely certain of the highest temperature that is allowable |
and the type of cover gas required over the salt It is intended to
limit the temperature to 1000 F and to prov1de an Ng overpressure until
more experience is gained Indications are that a tenperature of 1100 F
is tolerable, and, if this is verified steam conditions for & molten-
salt plant could be Significantly improved A report ORNL-TM-3777,_ o
which is in preparation, Wlll present the known information on’ Hitectygm

temperature limitations.
 

»

v

37

- STEAM SYSTEM .

The steam system condltions at the turblne throttle Were chosen to
be 900 F/2h00 ps1 ‘with 900°F reheat but the steam system was riot specialky
designed or optimized for the demonstratlon plant. Steam flows and tem-
peratures are shown on the simplified flowsheet of Fig. 1.

The steam generatlng equipment is, of course, quite different fram |
that in a f08811-fired plant in that the heat 1s extracted from a circu-
lating nciten salt On a completely arbitrary bas1s the steam 1oad for
the turbine-generator is diV1ded between 6 b01ler units 6 superheater
units,'and 6 reheater unlts A1l these units are heated by the three
salt circuits’ comprislng the tertiary salt system of the plant The
boilers are divided into two units each simply because this results in

units of a size more eas1ly handled. The entire steam c1rcuit is a

‘once-through system where both the water and salt s1des of the preheater,

boiler, and superheater are in series.

Feedwater from the conventional feedwater chain enters the tube side
of a ULtube preheater at a temperature of h80 F and a pressure of 2600
psi. It passes from this unit first 1nto a similar U~tube boiler and
from that into the U-tube superheater. These three units each have 100
tubes in shells about 12 in. ID. Additional data are given in Table 5.

Table 5. Steam Generating Data

 

No. of Length Diameter Wall Steam AP Salt AP Duty

 

tifunit; Tubes ‘ (ft):_ | (in ) (in ) L (psi)'u (PSl) (Btu/hr)
Preheater —€100 V'¢32%7'-‘ 50:5—15* 0.050 7.2 . 23.8 :451,x:106a-
" Boiler 100 32 j-o ‘50,050 17.6 23.8 68 :106 -

Supérheater 100" 3h;8.z;a?o;5*ff 0:.050 . 73.8 " -:23.8 57 X 10°
Reheateri Jér16o_;tfh3;5'”’u50551c*~f0;050--v'*6O-rx1*5'3h5"1**7.2'x 108

 

At the'side—stream eXit'of'the ‘boiler secticn‘there is a bypass
through a recirculator pump: to the inlet of the preheater.» ‘This. bypass
is for the purpose of .control for partial load operation and for such

 
 

_38.

contingencies as a breaker trip!from.fulléload. The shell sides of all
‘these units (preheater, boiler, superheater) are also connected in series
8o that the 1000°F tertiary salt flows countercurrent to the water and
leXits fram the preheater at 700 F at des1gn load._: L e
The steam generating cells are outside the reactor containment  ;(3

building but are closely adgacent to provide close coupling._ The 1nterlors
of the steam.cells are not heated 1ike the reactor cell but heaters and
1nsulation are, 1nstalled on the piping and components. All steam.and ‘
water 11nes are run from each piece of equipment in the steam.cells from
manifolds, stop valves etc 3 located outs1de the steam cells. The |
-rupture disecs on the tertiary salt system for relieving steam.pressure .
in the event of a leak are installed in the steam cells 1n order to _con=
tainthe saltw:Lthinthe cell. ', o S

. The turbine-generator 1s shown as a tandem.unit with one high-pressure,
one 1ntermediate~pressure, and two low-pressure caSings on one shaft
drivang a single generator unit The Specific turbine-generator has not
been des1gnated As has been stated the usual extraction p01nts and |
conventional feedwater handling are emplqyed. No effort has been made :_
to detail the regenerative feedwater heating system or other aspects of

the steam system

REACTOR ‘BULLDING

The. reactor building follows what has come to be a rather traditional
containment structure. Bas1cally it is a cylindrical reinforced concrete
structure with a hemispherical - dome. - A sealed steel membrane, 1f2-in. -
thick, lines the entire building and provides the containment. The con-
crete provides. protection‘againSt-missiles.resulting*frumttbrnadic‘winds.
The building is about 150 ft tall and about 112 ft in diameter. - - - -

The building is suspended from a large concrete ring about 176 £t
in outside diemeter and sbout 8 £t thick. This arrangement gives & low
center of gravity to the building. Although seismic analyses_have not
been made, it is believed that this method of construction may be more
stable'to'a*seismic disturbance. The same basic design?of‘thepbnildinga*
could be used if subsequent-analyses-or considerations related to:agpar-fi
ticular site indicate that the building should be set on a foundation
»

£

"

39

at the bottom of the structure. Figures 15 and 16 show the elevation and
plan of the reactor building. The steam cells are appended symmetricélly
outside the containment and rise from the concrete ring. Also mounted
on the ring and external to the contaifiment are the three water tanks
which provide the heat sink for the drain tank cooling system. The
control room is also built on this ring outside the containment building.

Inside the reactor building are the cells containing the radioactive
portions of the plant. There are 5 sealed cells which have a common
atmosphere: the reactor cell, the 3 heat exchanger cells, and the drain
tank cell. These cells have water-cooling coils embedded in the concrete
walls with water plena at the ends of the cells for removing the heat
that leaks through the thermal insulation. Each cell is completely
lined with;a;l/2-in.-thick—30h stainless steel membrane Wfiich seals the
cells from the remainder of the buiiding;..PenetratiOns into the cells
have been kept to a minimum and the'interconnecting sleeves between cells
have bellows which permit differential movement between the membranes of
the various cells. These sleeves provide the necessary passages for the
salt piping. Flgure 17 is an elevation of the resctor cell and one of
the 3 heat exchanger cells. Figure 18 is an elevation and plan of the
drain tank cell. : a

The reactor is hung from a ledge by means of a large flange near
the top of the vessel. This is seen in the elevation section of the
building, Fig. 15. In the heat exéfianger cells all units are mounted
from & superstructure.rising from the floor of the cell. This mounting
method is shown in Fig. 19. The drain tank sits on the floor of the
drain tank cell, therload being.carfied through a skirt and 8 lugs which
transmit the load through the seal membrane and insulation to the con-
crete botfam.of the cell. .

CELL HEATING AND COOLING

All 5 cells are heated by 01rculat1ng the cell atmosphere (mostly
nitrogen) over electrical heaters. The c1rculation is accomplished by
3 large blowers,'eaCh of which discharges 32,500 cfm of gas at a tem-
perature of 1050°F into the reactor cell. The reactor cell has 4 gas

outlets, one to each heat exchanger cell and one into the drain tank

 
 

 

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ORNL DWG 72-3578

 

Fig. 15. Reactor Building Elevation.
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L5

cell. Return ducts frdm each of these cellsrto the blower suction make
a closed circulation system which is part of the cell containment. Perti-
nent data for this cell heating system are shown in Table 6.

| Heating the cells by forced circulationNgreatLy reduces the number
of cell penetrations necessary when compared to heating by space heaters
in the cells. In addition to heating, it is desirable to cool the cells
at times. The data for cooling,'nsing the same circulation system, are
shown in Teble 7. The circulation duct is divided into two sections, and
a simple'butterfly'type damper directs the'air-through either the heater
section or the cooling section. | |

The electric heaters and the reentrant tube cooling water units each

fit into thimbles which are welded into the top of the duct. In each
case (heating or cooling), heat transfer is by radiation. Neither the
heaters nor the water piping penetrate the ducting so that repairs can

be made without breaking the containment.

DRAIN TANK SYSTEM

One of the most important systems in the molten-salt reactor plant
is the fuel-salt drain tank with its cooling system and provisions for
gas holdup. (It is somewhat analogous to the emergency core cooling
system of a solid fuel reactor.) It provides automatic removal of.the
afterheat in the event of g failure of the main circulation sy stem.
Also during normal"operation the volatile fission products and the
stripping gas are held up for about 6 hours in the drein tank. The .
decay heat from this operatlon 1s removed by the natural convection '
coollng system. . | | |

- The drain tank con31sts of two separate tanks. Theiprimary_tank
81ts 1n31de & secondary tank or. crucible. The drain tank has a flat
top with thimbles located in a symmetrlcal pattern. Sixty'anits of 6
tubes each cover the top of the drain tank. Flgure 20 is a plan view
shOW1ng the arrangement of these thlmbles W1th the cooling headers 1n
place Figure 21 1s an elevation of the drain . tank ‘In: order to

stabilize these thimbles against seismic shock they are interlocked
at the bottom in such & way as to allow differential axial movement
but no individual vibration. The 360 thimbles, therefore, act together

 
Table 6. Heater DeSign Data for MSDR Contairment Cells

 

,Estlmated normal containment heat loss (total), KW
Design heat 1oss, KW

Number of heater cells

Capacity of each heater cell, kW

éirculétigg gas

fGaSginiet-outlet temperatures at heaters, c"F

Gas flow rate through each heater'cell, cfm

Heater element

Heater length, ft
Number of heater elements per heater cell

Heater element arrangement per heater cell

Pressure drop in circulating gas, in. HgO
Heater cell width and depth, ft

Thermal conductivities for cell wall (k), Btu hr -1 f£7r (°F)7?

1/2-in. stainless steel cell liner
~5=in.-thick fiber glass cell insulation
Prestressed concrete with 2-in. sched LO

carbon-steel watér cooling pipes on 6 in. centers located L in.

from inside concrete face

Assumed heat transfer coefflclent 1n'water plpes, Btu hr™t Tt -2 (°F) R

:Maximum.concrete temperature, °F

600

1200

3
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1000~1100

32,500 (at 1050°F)
1-in.-0D cartridge with
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150 |

12 rows of 12 or 13-
elements on 3-in. A'pitch

9. 6 -
| 3.25_x'2.61

12.4

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25.9 (steel)
- 1_5'0 .

 

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Table 7. Cell Cooling System

 

Tnitial heat rate, Btu/hr

Number of cooler cells

Number of thimbles per cell
Length of thimbles, £t

Diameter of thimbles, in.

Pitch of tubes in squgre arfay, in.
Temperature of cooling water, °F
Gas flow'rate per cell, cfm

AP acrst_thimbles;_in..fléQ |
Bmissivity of thimble

Volume of'gas.tobe quléd;ftaz
Temperatfire.N§ at stért, °F

| Tem@eratfiie Né after 2.75 hr, °F
Temperature reactor atsfart;ffF\

Temperature reactor after'2f75_hr, °F

3.5 % 10°

225
4

2

3

225
32,500
10

0.7
150,000
1000

740

11000
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50

'e*asia%unit as far as vibrationis:concerned,andthe.natural frequency‘_ )
of this-Structure is high enough to provide protection against,earthquake
damage ., - e o B

A cooling module. cons1sts of & manifold and 6 reentrant tubes for -
each of the 60 groups of thimbles which penetrate the draln tank. Each
“of these cooling modules is connected by two h-in pipes to a section of
pipe . in the Water heat dump outside ‘the building containment - These
cooling modules contain NaK as the clrculating heat transport fluid

Each module has an expans1on tank and shutboff valve so that each can

. act independently of the others.

' Three water tanks 25 ft X'25 £t X 12 ft deep are located outS1de "
| the reactor building. Water flows through these tanks in an open cireuit.
.JThe water is taken from ‘the same source as the turbine condenser cooling
water and, after going through the tanks, it is returned to ‘the stream,

Data on these water tanks are given in Table 8, and the tank arrangement

Table 8. .Dump Tank Heat Sink Date

 

Size of tank, ft 25 X 25 X 12

Depth of water over tubes, ft 5 _ .
Number of tubes ' 576 for each of 2 tanks, 672'for‘third
Arrangement of tubes 418 tubes per row, 12 rows for 2. tanks,
. | and 48 tubes per row with 1h rows for
_ - , third tank.
Total volume of water for 12,920
all tanks, £t° | - - S
Thimbles tn row =~ 3-in. sched-10 pipe on 6-in. centers
" Row spacing, in. 1 - “
NeX pipes in thimbles . 2-in. sched-lLo pipe
'NaK entering temperature, °F 532 |
NaK exiting. temperature, °F 450

Water enterlng temperature, °F 80
Water exiting temperature, °F 100
Nak flowfrate,~gpm S - 2800 -
Water flow rate, gpm 39

 
 

"

51

is shown in Fig. 22. Two of the 3 tanks are sufficient to provide cool-
ing under maximum heat load conditions. Heat is transferred from the
drain tenk to the NaK, and from the NaK to the water by radiation. Thus
the NaK has a double barrier between it and the contents of the drain
tank and also between it and the water in the cooling water tanks.

The crueible in which the drain tank is suspended is a double-walled
tank. It is also cooled by NaK, having its own independent lines to the
water tanks. The crucible cools the drain tank walls by radiative
transfer. -

The drain tank connects with the primary system in three completely
different ways in order to perform the three distinctly different func-
tions for which it is used. The first of these functions is to provide
a receptacle into which the prifiary salt may be drained whenever it is
necessary for any reason to remove the salt from the primary system. A
6-~in. line runs from the bottom of the reactor in a generally horizontal
direction into the drain tank pit. At this point a drain valve is
installed and a vertical line connects the drain valve to the drain
tank.

The drain valve is a critical paft of the system since it operates
but infrequently and yet must operate when requlred To provide some
redundancy, the drain valve is bypassed by a rupture disc valve which
can be used in an epergency if the drain valve fails to open. Both
the drain valve and the rupture disc valve are replaceable by remote
maintenance methods after the prlmary system is drained. The drain
valve is a comblnatlon mechanical valve -fTeeze valve.‘ As shown in
Fig. 23, the poppet does not seal mechanlcally'but relies on a frozen
salt film to seal. Thls seal is frozen by czrculation of a coolant

in the body of the poppet . It is thawed by c1rculat10n of hot fluid.

The poppet is actuated by a posztive electrlc drlve and the movement
is sealed by & bellows which cannot be contacted by salt but only by -
the cover gas. o o ,

The second use for the draln tank 1s one whlch 1s extremely
unlikely but nevertheless must be provided for safety considerations.
If there is any leak of"primary salt from the primary system,‘the salt

must be put into the drain tank. Therefore, the reactor cell, heat

 
 

ORNL DWG T72-3583

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5h

exchanger cells, and the salt piping above the drain tank celi are equip-
ped with catch pans which eommunicate with the drain tank through a rupture
disc in the top of the tenk |

The third use of the drain tank ¢oncerns the holdup of the fission
gagses and the separation of entrained liquid from these gases. About_8 MW
of heat from these fission products must be disposed of and the drain;tank
cooling system is used to dissipate this'heat. This is a continuous func-
tion of the drain tank while the plant is in operation. -

As has been mentioned, the primary salt is SParged with helium to
remove xenon from the salt. This sParglng is accompllshed by introducing
ga.s bubbles into a salt bypass loop around each pump of the primary cir-
cuit. In this bypass ercentriffigalrseparatof extracts the gas from the
salt. The separator constantly extracts about 10 gpm of salt along with
the gas. This two-phase stream from each?pump is sent to the drain tank
where, because of the size.offthefdraifi éenk, the salt separates from
the gas by gravity, and, after an average residence time of about 6
hours, the gas passes through a partlcle trap. About half the gas is
returned directly to the bubble generator and the other half passes
through a cleanup system and is returped to the pump purge system by
means of & gas compressor. o

The flow of liquid to the draln tank may vary over & range from O to
about 32 gpm. A means must be‘provldefi for returning this salt -to the
primary system without returning any,bf5ffie’radioactive gas. A system
of jet pumps with automatic flow control is used to_accomplish this.

This jet pump system is installed as a unit in the center of the drain
tank and can be removed for replacement or repair shofild any of the

unit become defective This unit is shown in p;en and elevation in Fig.
2h. A 19-ft -long by 6-1n -diam pipe, closed afi'the'bottom end and open
at the top, acts as a sump in the center of the drain tank. On the out-
side of thls sump there are two jet pumps whlch are driven by a common
-salt source taken from the output of the three primary pumps. A line
from the dlscharge of each primary pump, after passing through a ball

check valve, is manifolded into one common line which feeds these two

jet pumps.
PHOTO T9087

 

   

 

 

 

 

  
 

 

 

 

 

 

 

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Fig. 24. Drain Tank Jet Pump Assembly.

6§

 
 

56

One of the jet pumps has its suction in a small sump in the bottom
of the drain tank and discharges tangentially into the 6-in.-diam.sump
near the top. This pump has a capacity greater than the flow of liquid
into the tank so it keeps the drain tank sump-empty. Because.the“Capacity
of the pump exceeds the liquid flow, this pump at times pumps‘gas, but
the gas simply discharges back into the drain tank atmosphere.
| The second jet pump is located about mi dway up the 6-in.-diam sump
and sucks out of the bottum of this pipe. The discharge of this;pump
goes into one of the pump .bowls. of the: prlmary system. The jet pump
suction can change from about plus 14 £t to a mlnus 5 ft. Therefore,"
the pump dlscharges salt at a rate proportlonal to the suction head and
this rate can be anythlng between O and 32 gpm Thus only liquid and

. never hot gas is returned to the primary system. This automatic regu-

lation takes care of the liquid pumpback for all condltlons-of operation.
A third jet pump is mounted with this assembly, but it 'is used only
for filling the primary system or for transferring the contents of the
drain tank to the chémical processing cell. This operation is described
below, along with the chemical processing system. '
| Because this plant is operated as a converter having a breeding
ratio less than unity, the chemical processing reqfiired is very much
less involved than that required for a breeder. Three things are re-
quired of the chemical treatment: TFirst, means must be provided for
removing oxygen from the primary or secondary salt in case either becomes
contaminated. Second, when the fuel salt has accumilated enough fission
products to cut significantly into the neutron economy, this salt mist
bé'replaced with unconteminated carrier salt. The uranium must be
extracted for return to the reactor before the old salt is discarded..
Third, there must be provision for storing the discarded carrier salt,
at least as long as cooling of this salt is required. In addition to
these three primary functions there is a further requirement for.a
storage faéilityuwhere primary salt may be placed if it is necessary to
do maintenance of any kihd on the drain tank. Since this storage tank
is required in any event; it was decided to use it as the chemical pro-

cessing tank for fluorination of the priméry salt to remove uranium.
 

[}

a*

27

The cooling system.for the storage tank can be quite simple since
the salt can be held in the drain tank until the heat rate is of the
order of 1 Mi. This situation exists after approximately 30 days of
cooling. The 1 MW can be successfully transferred from the surface of

‘the storage tank by radiation to the NaK-cooled jacket surrounding the

storage tank. ‘
After removal of the uraniumg the contaminated carrier salt must be
put into disposable cans. These cans, about 2 ft in diameter by 10 ft

long, are'placed in tanks located in an ennulus around the top part of

the storage tank. Figures_25 and 26'are plan and elevation of-the stor-

age tank and the disposable cans. These cans—haVe interconnecting l-in.
lines so that salt can:he'transferred to them frdm.the_storage tank by
gas pressure.  After cooling, the lines can be cut-and welded, sealing
the spent salt permanently in the disposable,oans. After the cans have
been sealed they can be stored under the reactor in the storage vault or
transported to salt mlnes or other permanent storage facility. New cans
can be installed by dlrect malntenance once the full cans have been re-

moved from the location

FF-GAS SYSTEM

Helium.is used as & .cover gas over ‘the prlmary and secondary salt.
It is also used as a purge, flowzng inwerd eround the pump seals, and

as & sparge for remov1ng xenon from the primary salt. Thls gas system

is a closed system, and storage and cleanup eqnlpment must be provided.

The bubble injection and separation and the holdup of the gas in

- the drain tank have already been descrlbed When the gas leaves the

drain tank it goes to one of two partlcle traps “These traps can be
isolated by valves for removal from the systam for repair. The purpose
of these partlcle traps is to catch the solid daughters and granddaughters
of the noble gases and also such noble metal particles as are carried by
the gas as it leaves the drain tank.: These particle traps have adequate
size to accumulate solid particles for about three months before excess-

“ive pressure drop would require replacement of the trap. To remove the

heat generated by these particles, the traps have NaX cooling; The heat
load is of the order of 400 kW. These particle traps have not been

 
 

A

 

    

- DU SOOSAL, CAn e

rei

8s.

 
 

L)

273

|Lrsro sS4 i

29

ORNL DWG T0-12199

    
     
  
      
      
  
  
 

- DrsposaL Cau
——3 Ccocav/
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=

     

Essnvg TunE -

" Loorans

 
 

Srofage &
LROCESSNG JANK

'/
z&az Z

J
|

= LOOLING ’i./d:r'r_/,f,"

A5

-~ Al EROMAL LIS AT, D

Y

 

 

e

 

 

-~ JACKET _CoOeANT

Fig. 26. . Processing and Storage Tank Elevation.

 
 

 

 

 

60

designed since the exact calculations on which a design is based have
notiBeen made. Past experience with MSRE shows that the design is not
complex and the configufetion does not present a problem. |

The gas line downstream of the particle trap divides into two equal
branches. About 1 cfm of the gas goes directly to the bubble generators
in each of the three pump bypass lines. When this gas is removed by the
bubble separators, it recirculates to the drain tank, through the particle
trap,‘and to the bubble generators again., This gas is very radioactive,
the only loss in radioactivity being that which has decayed in the
approximately 6-1/2 hr transit ‘time for the loqp. |

The other gas stream from the partlcle trap enters one of two char-
coal absorber beds. The two beds are valved so that either can be used.
It is imperative that the krypton and xenon in this part of the gas be
held up for 90 days because it must be essentially clean before it goes
to the gas compressor and storage tank for recirculatlon to the pump
‘seals. The head end sectlon of the holdup bed is cooled by NaK while
the downstream sections of the bed can be cooled by natural convection
of the gas atmosphere in the off-gas cell. These beds are in the form
of calandria with the charcoal surrounding the tubes. The aotual design
‘of the charcoal beds, as in the case of the particle trap, has not been
done. Again, there has been enough experience with such absorber bede
at ORNL to show that they are feasible and are not excessively large.

For details concerning gas cleanup systems, reference is made to ORNL~

45kl (ref. 6).

CONTROL RODS

There are 6 control roanpenetrations'in the reactor core. These
are T=-in.-diam holes in graphifeiblooks<in whioh cruciform control rods
are suspended. There is a 1/4-in. radial clearance around the rod, and
it is free to rotate in the hole. Salt flows up around the control rod
at a velocity of about 7.5 fps and keeps it cool. A control rod is made
of up to 1/2 4n: of B4C clad with a 1/8-in. thickness of Hastelloy N.
Each control rod is worth about 3% AK/K and any one rod will shut the

reactor down.
-9

"

 

61

‘Each rod is suspended on a Hastelldy ceble which is taken up on a
12~in.-diam drum driven through a worm gear drive. The motor is outside
the reactor cell shielding, but the winch and cable are located within
the containment. The motor and drive is also enclosed and & positive

gas pressure is maintained to produce inleakege around the drive seal.

A spring loaded monitor cable attached to a synchro proVides position

indication for the rod. One end of the cable is attached to the top
of the rod and by actuation of the synchro gives remote indication of
the rod position at all times. This cable is not strong enough to sup-
port the rod; therefore, it cannot prevent the rod from falling into
place. ' ‘ | , _

The control rod has a finite life and so must be replaced after it
hes received a fluence of about 10°! neutrons/cm®. The redundancy fur-
nished by the 6 rods makes it possible to hang the rods on cable without
providing a positive drive for rod insertion. A magnetic clutch can be
provided between the winch and the gear-drivé if it is desirable to
drop the rod instead of relying on the slower insertion provided by the

drive mechanism.

INSTRUMENTATION

Instrumentation for the plant has received little attention since
it probably poses few problems that are different from other nuclear
plants. One exception is that the reactor is so heavily reflected that
for initial startup a'fission-chamber;will probabxy.have to be inserted

in. one of the control rod,socketsrin.orderatoAget{a'good endugh signal .

in the'initial:criticality;run. rThe nuclear,insfirumentation should.not

be more complicated then for other nficlear_plantsrfor-the¢reactor;has a

prompfi;negative,temperature,coefficient;-and_the vérywlarge‘heat capacity

‘of the primary system,prevents;rapid.temperaturelexcurSions. ‘A thorough

study'fifuthe control'and“safety'instrummntationumust be done.._Ififappears
that the'inStrumentatioh reqniréd will not exceed the existing art.

 
 

 

 

 

62

. MAINTENANCE

" A ecirculating-fuel nuclear reactor is inherently a highly radio-
active system, and the radioactivity is more dispersed than it is under
normsl conditions in a ‘solid-fuel reactor. For this reason, all main- -
tenance on the primary circuit must be done by remote means. By design-
ing the MSDR to operate at a low enoughrpower:defisity so that graphite |
replacement is unnecessary, the maintenance is greatly" 31mplif1ed '
Also by designing the heat exchangers ‘so that tubes can’ be plugged the
maintenance is made more practical. ' o ' T

Experlence with the MSRE showed that remote maintenance of the
radioactive equipment and systems of a small molten-salt reactor is
feasible and practical. The techniques ahd'types of tools successfully
employed there should be satisfactory for many of the remote maintenance
dperatiens on the MSDR, but they will have to be adapted for handling
the larger and heavier equipment and piping. New tools will have to’
be developed for such operations as remote cutting and welding of piping
and plugging of heat exchanger tubes. Experience with the development
of automatic cutting and welding equipment at ORNL indicates that these

more difficult operations can be accomplished.

PERFORMANCE

A summary of the nuclear data with fuel cycle costs is shown in
Table 9. No credit was taken for reclaiming the discarded carrier salt
after it is removed from the reactor. This salt has some very valuable
constituents in the 7Li and beryllium end when molten-salt reaCtors-comer
into general use it is probable that this salt could be reclaimed at

an ‘economic benefit.

Tt is also almost certain that future reactor plants could utilize

-supercritical steam systems and realize the improved efficiencies of

these systems. As has been stated, this "first-of-a-kind" demonstration
plant was designed on a conservative basis. E '

The question of plant safety has not been treated here. Certainly

safety considerations were of prime concern in the design study described

in this report, but the subject is so important as to merit separate
 

-l

 

63

Table 9. Lifetime Averaged Performance of a 750-MW(t)
Molten~Salt Demonstration Reactor

 

Identification A38
Description
Dimensions, cm
Core, radius 340
Plenum-annulus, thickness 2.54
Reflector, thickness 75.9
Salt volume fractions
Core ' : 0.10
Plenum-annulus ' 1.0
Reflector 0.01
Composition of carrier salt, mole %
LiF | 67
Belz 23
ThFe¢ 10
Core moderator ratio, atoms C/Th 277
Plant factor 0.8
Thermal efficiency, % 36.6
Core life, efp years 2k
Processing, batch cycle time, efp years 6
Fuel enrichment, 3%y, ¢ 93
Volume of fuel salt, ft°
Reactor (spherical model) _ 760
Piping and heat exchangers 320
Total 1080
Performance _ a
Conversion ratio, lifetime averaged 0.88
Fuel cycle costs,  mills/kWhr
Inventory
‘Fissile 0.62
Salt 0.071
Replacement
Fissile 0.18
Salt | 0.13
Total fuel cost | 1.0
Processing cost, estimated 0.1
Total fuel cycle cost - 1.1
Lifetime fissile material balance, kg :
Initial inventory 710
Further purchases 9Lo
Recovery at end of life \ 700
Net fuel requirement | - - 950

 

,%AsSuming a processing loss of fissile uranium of 0.01% per cycle.

bFrom a present-worth calculation of fissile, fertile, and carrier
salt purchases and/or sales over the reactor lifetime, compounded quarterly
at a discount rate of 0.07 per annum. Inventory charge rate 0.132.
Fissile nuclide value in $/kg, 11.9 for 236U, and 13.8 for 223y.

 
 

 

6k

study. An 1n-depth safety study of the mol‘ben-salt concept is in progress L
and will be the subject of an extensive report
 

211

1-209.
210.
211.

212,
213.
21k.

- 215.

US~-AEC

216.
217.
218.
219.
220.
221.
222-223.
224225,
226-227.

228.
229-230.

Industry

231.
232.

- 233.
234-236.
237«
238.

239.
210,

65

DISTRIBUTION | ORNL-TM-3832

MSRP Distribution D

"W. B. Cottrell

H. C. McCurdy
A. J. Miller
M. Sheldon

D. A. Sundberg
G. D. Whitman

D, F. Cope, ORO-SSR
D. Elias, RDT, Washington, D. C.
N. Haberman, RDT, Washington, D. C.

W. H. Hannum, RDT, Washington, D. C.

J. Neff, RDT, Washington, D. C.
M. J. Whltman RDT, Washington, D. C.
Director, Div1sion of Reactor Licensing, Washington, D. C.

- Director, Division of Reactor Standards, Washington, D. C.

Executive Secretary, Advisory Committee on Reactor Safeguards,
Washington, D. C.

- Research and Technical Support Division, ORO

Technical Information Center

J. A. Acciarri, Continental 0il Co., Ponca City, OK

R. M. Bushong, Carbon Products Division, UCC, P. 0. Box 6116,
Cleveland, OH L4101

G. C. Clasby, Byron Jackson, P. O. Box 2017, Terminal Annex,
Los Angeles, CA 9005L

D. R. deBoisblanc, Ebasco Services Inc., Two ‘Rector Street,
New York, NY 10006

B6hpefir1ng, Black & Veatch, P. O. Box 8405, Kansas City, MO

11 ’ -

| Bernard Mong, Babcock and Wileox, P. 0. Box 1260, Lynchburg,

VA 24505 |

T. K. Roche, Cabot Corp., Kokomo, IN 46901

A. E. Swanson, Black & Veatch P. 0. Box 8L05, Kansas City,
MO 611y -