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Liquid Metal Fuel Reactors
By K. r. A|der*

The advantages and disadvantages of a liquid metal fuel reactor system are outlined
In relation to other possible power reactors. Liquid metals which are possible fuel carriers
and coolants are discussed, and in particular o comparison is drawn between a solution-type
fuel using bismuth and a dispersion-type using sodium. The reasons for the choice of a
sodium dispersion fuel for the A.A..E.C. research project are presented and the problems
posed by such a reactor system are summarised.

INTRODUCTION have good heat transfer characteristics, i.e.,Present trends of development ln nuclear high specic heat and thermal conductivity,

temperatures to improve thermal eiciency, and A number of liquids are available ioi hightowards more irradiation-resistant fuels to temperature operation, and inclu.de oiganicachieve increased reactor reliability and greater compounds, fused inorganic salts and liquidfuel utilisation. metals. Organic liquids such as “Dowtherm"

power reactors are towards higher operating °°up1ed with 1°“ vls°°sitY- '

“High temperature» operatleu requires (diphenyl-diphenyl oxide eutectic) have accept-
denition. Considering existing large-scale able heat transfer properties’ Put °n1Y a Sm?“
power reactors, it may be said that the upper utleful t°nf1Pemt‘~u'° range mdlerent ram?‘temperature llmlts ere about 300°C for pres_ tion_stab1lity. Fused salts introduce problems
surised water units and 450°C‘ for gas-cooled °f mgh n§utr°n captulfe n'md"at‘.°n mstabmtyieysteme Temperatures of 5o0.C and up eeu and corrosion of contamer material.
be regarded 3,5 high in nuglear 1-e3,ctQ1~5_ The heat transfer coeicients of liquid metals

Operation at temperatures higher than 500°C are high, thus allowing high heat ratings per
should be possible with two types of coolant— unit of. reactqr .°°r° ‘mhlme .°r' alternatively’
gases and liquid metals. The Australian Atomic pert Em“ t°f nssned magergzl mveslifid‘ d Ltqlnd
Energy Commission’s research program has as me aid syilemsdneet no . pre.ssu lfse ' dglllmg
its basis the study of such high temperature ‘1’g“Sld°” "an afeglm "’§§melf§;‘§gan§S1§r"-
systems applied to thermal reactors. Advanced glut. meta mare st: te tgnt rbem element‘
types of gas-cooled reactors are discussed by ra‘ 13' .1? ° the e? 31 3* t $1 SDalton (1958); this paper considers the pre- Orl mm Pres Elie“ '. ey ° .n° e°%mp°s°'
ferred alternatives among the possible liquid sgnxlgé or ° erwlse su er In a mac or en"metal s stems. '

y All liquid metals present some problems of
Ll UID ETAL COO TS compatibility and containment when considered

The neeeeelty for lluuld eeoleute to operate for reactor service, and this factor, plus theat lugh temperatures has arisen lrl many lrl_ eect of the coolant on the neutron economy
stances before the advent of nuclear power. °f_ fe _"ea'ct°_r>_ °°“5t1tul'° tw° °f the mamclassical examples are the General Eleetrle criteria in deciding the metal to be used.
Company's mercury boiler for increased Unfortunately, there is no one liquid metal
emliiellcy Of POWEI‘ Eelleratillt dB-ting from 1922 which combines all the desirable attributes for
(Hackett, 1942), and the use °f liquid sodium a reactor coolant. Consequently, some com-
l3O C001 8i1‘C!‘8.fl5 exhaust valves since 1928 (Heron, prgmjse must be reached in making a selection,
1928). and other aspects of the reactor design so

Ideally, a lluuld coolant for e thermal reactor affect this selection that a number of research
Should have low capture cl-oss_sectl°n for prolects on nuclear reactors utilising different
thermal neutrons, 9, 10w melting point, high liquid metal coolants are in progress in the
boiling point, and a low vapour pressure in Western world (Williams, 1954; Siegel, 1955; ,

the operating temperature range. It must be H°m9J1, 1956; Abra-ha-m 1957)-
compatible with constructional and container
materials which it contacts in the reactor core “gum M“-Al"c°°LEDs¢l:.g LIQUH) METAL‘
and external circuit, so that negligible cor- FUELLED S Ms
rosion or mass transfer of these materials Liquid {T189511 ooolants offer the opportunity
will occur. It is desirable for the coolant to of operating reactors with conventional Woo
have good radiation stability and a low level fuel lemellts at high temlloratllres and jlndel‘
of induced radioactivity after neutron irradia- ¢°nd1l71°1"1$ 91? high fuel I‘§1I1$- The addltlollal
tion to avoid the necessity for heavy shielding stel? °f lmlxlng the fuel _1I1_l5}I1_1'at@1Y With theof elreult components external to the reactor coolant increases the possibilities of high rat-
core, and to minimise health hazards should }1’18S,_ and also removes the problem of the
leaks develop. Axiomatically, the coolant must ITYB-11194910“ behaV1°"1' °f fuel elements-

The desire to develop reactors having high
reliability and: good fuel utilisation has focused

Esl;aAl)1l?;!1:rlieBg1t;.At:I!:1lgl§:?f;§yr;3:gleg:‘lls§}g-gob%§fe?;§€ attention on irradiation damage in fuels since
PAGE 346 SECTION 2

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In general, high temperature operation No low melting point alloys with high

aggravates the problems of fuel element dam- uranium or plutonium contents and reasonably

age, and to utilise the advantages of liquid low thermal neutron capture cross-section are

metqi cooling it would be highly desirable to known. Consequently, the other attractive

eliminate them altogether. A possible method liquid metal fuels are those based on low melt-

" is to use fuel intimately -mixed with the ing point metals with good heat transfer and

' coolant so that mechanical irradiation damage nuclear properties. Liquid uranium and

~x_»1¥<c<\».\/

the successful operation of the earliestreactors. coolant is also the moderator (AECD3646,

’ Under reactor irradiation, unalloyed uranium 1955). This reactor suffers from two dis-

may distort by surface wrinkling, growth and advantages-the danger of a highly-pressurised

swelling, causing failure of the fuel element of and extremely radioactive circuit, and the cor-

which it is a constituent (Ball, 1955). Heat rosive ns ture of the liquid. Solutions of

treatment and alloying have been developed uranium salts in fused salt mixtures have been

to minimise such damage, and present trends proposed also, but neutron capture in the salts

in ceramic fuels indicate that even greater and the compatibility of the liquid with con-

resistance to irradiation damage is forthcoming. strucional materials pose severe problems.

However, all solid fuel reactors are liable to Circuhtin, fuel systems have potential ad_

the disadvantages of fuel element failure, and vantaqds m°e1immation of the charge_discharge

any system which °ver°_°me5, th_e_se pr°b1e_m5 probleams of solid fuel reactors, and in the possi-

lgasl pgtential advantages in reliability and high bimy of cgntinuous de-p0iSOI1l11g and process-

“e um'“p‘ ing of the fuel. Such advantages would be

One method of forming a “high integrity” lost in liquid metal systems employing canned

fuel is to disperse ssile material in a matrix fuel, with the exception the-t Venting Of the

which is known to be stable under irradiation. cans might allow continuous remove-1 of Xenon

This allows high heat output per unit of fuel, from the core.

blegaéusier$1111;eigiagrgimléljyolge elgsftd 21etixtgigldtiglf Tine choice of liqgid metdl fdr da t<t:1ircul€.ting

- _
' ue sys em is muc more imi e an or a.

The fuel for the Australian reactor HIFAR is 1 1 a1_ 01 d re ctm-_ be X1; 5

of this Woe» being on enriched uranium oom- Lgiileilioe-tsoiifgiiigy ofathe ruegn51:stanbeei;i-

pound dispersed in aluminium. Power reactors pcsed

of high fuel rating may be built, using such
fuels and suitable coolants, e.g., the U.S. Navy POSSIBLE LIQUID FUELS

” submarine “Sea Wolf" uses such a fuel in - -

stainless steel, cooled by sodium (AECL p0€€1Hg_t;;{peST;g5ehg;::1__mem1 fuels may be

e ORR-590). However, so much parasitically _ , _ _ , .

neutron-absorbing material has been added to (1) L°w meltnig p°mt au°ys “ch m “mnmm

this type of fuel that a reactor using it is (°1' P1ut°m“m)3

unlikely to be eeonomioal for civil purposes. (ii) solutions of uranium or plutonium in

‘* Ari alternative is to disperse fuel in part of acceptable coolants;

the reactor moderator. This achieves a high (iii) slundes or dispersions of ssile material

fuel rating by dispersing the heat source, but (metal or compounds) in equilibrium

,, the irradiation stability of such dispersions in with solutions ie added in quantities

moderators suitable for high temperature exceeding the'sa'td}ation solubility. and

Q» operation (graphite, beryllium. beryllium oxide) _ _ _ _ ’

has yet to be studied. The Australian high <1") Slllmes °Y d1sPe1"§1°1=$ °f se materiel

. temperature gas-cooled reactor project involves (metal °r c°mP°“nd-'5) “nth negligible

studies of this type of fuel (Dalton, 1958). s°1‘1b11itY in the °°°1e!1t-

L is either absent, as in a fuel solution, or of plutonium alloys may nd application in the

no consequence, as in a fuel slurry. future as fast reactor fuels, where the range

. . . of possible alloying elements and container

plgcevaglzgggegtfiogi lfaggfrelzgilggscgy 1: t,?c;:,, materials can be extended considerably because

filled with liquid metal fuel solution or slurry. ggcggiggnthgiguggiiuggegapture °mss'

The rst altemative, a canned liquid metal g gy'

solution, is not attractive because of the very Metals which are possible thermal reactor

dilute nature of the available solution-type coolants, and which may be considered for suit-

fuels. The “canned” slurry fuel element, has ability as fuel carriers, are listed in Table I.

; been discussed widely, but has not been ln- Water and some unsuitable liquid metals are

vestigated seriously for thermal reactors be- included for comparison purposes; the undesir-

cause of unfavourable neutrol economy. How- able properties which cause their rejection are

-/5‘, ever, should circulating fuel systems prove underlined.

1 technologically not feasible, the concept of a only three metals, lithimn lead and bis_

camied slurry fuel’ llomd meta1'°°°1ed' sh9u1d muth, have appreciable solubilities for uranium,
receive further attention. It may be particu- and of these bismuth is outstanding in both

lay Suitable f°1‘ a fast r°°'°t°r- this property and low neutron capture. It may

Circulating combined fuel-coolant systems of be possible to iooreese the solubility of

A; several forms have been proposed and in some uranium in lithium and in lead by additions

cases investigated. The aqueous homogeneous of further alloying elements, and work is pro-

reactor is such a system. in which the fuel- ceeding on these lines in Great Britain. The

POWER REACTOR5
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solubility of plutonium will not be considered is a slurr in which the dis rfurther, because supplies of this element are negligible solubility in the liquid? iidlgilifrfnot likely to be available in Australia in the perature gradient mass transfer and consequentnear future. Also, plutonium fuelled systems particle growth is also negligible. Such slurrieshave lower possibilities in conversion factor, could be based on any of the preferred liquidwhich is likely to affect their economy adversely. metal cooiants, and could contain insoluble
The thermal neutron capture cross-section ‘uanlun? °°mp°“nds such 9'?‘ T.Jo=' U0 °r U361»oi lithium is so unfavourable that this metal or uranium metal where this is insoluble. Againwould have to exhibit outstanding advantages bismuth appears most attractive because of itsin other properties to make it attractive for fa'v°umb1e °r°Ss's?°t1°n'.but m°st °f the p'°b'thermal l-eaetem Similar reasoning applies lems associated with this metal are with con-

to lead, its cross-section being much lower Fame!‘ matenam am.‘ lf these can be S°1Ved=than that of lithium, but still high compared "‘ “PPe‘“$ m°‘° 1°g1°a1 t° use bismuth as 3with bismuth In both eases these elements solution type fuel rather than in a. slurry. It
occur in nature as mixtures of isotopes with remams' then‘ t° °°ns1d?' .Wh=ih°1" 9‘ slurrywidely diering nuclear properties, and an in- formed “nth any other hqmd 11.18"‘ has Sui‘teresting possibility is the use of the low cent advantages to compete with a bismuth
capture cross-section isotopes lithium '7 and S°l“t‘°n'
lead 208 as reactor coolants. Present isotope $°dil-1m and DOW-Ssium have negligible $01\1-separation processes are not likely to produce bility T01‘ uranium, and 11611138 0011161 be used aslead 208 economically, but the greater ease of slurry vehicles for the metal or its compounds.isotope separation with light elements may mean P099-Ssillm has Door nuclear D1‘0P61‘¢ie$, and isthat lithium '1 is a feasible coolant in the more diiwli to contain than sodium, and itsfuture, only advantage appears to be a lower melt-

» Both lead and lithium dissolve many other i1l{lgKp°im' Alloys of potassium and sodiumeeal an S . .. ( a ) have been used (Trocki, 1955) as reactor
w‘?h°°J3>§€‘.‘i§23"m§&ii.1§?“$%“‘l’l§§Z fgggngii ,i;;m.m,beinihadd@i todiculties appear not insuperable (Homan, 6 me g pom ° e coo an‘

1955, 1956). However, bismuth also exhibits T," *1?em_1a1 n°11'¢r°11 capture cross-seqon ofslight solubility for many other metals, and it s°dmm '5 hlgh compared, w1_7h that °f b1sm“th~should he emphasised that the magnitude oi but reference to_'I'able I indicates that the heatthe_eon.osi°h problems in the use Oi this metal transfer properties of sodium are much better
was not apparent from a study of its properties, than th°$e °f Plsmuu?‘ Th“ m"?9‘ns that lessbut emerged as a result of a considerable re~ S°di",m °°°1a'nt 15 requlred for 3' g1Yen heat Wt‘Search program (Weeks. l955)_ put, l.e., the poor nuclear properties of sodium

are partly offset by its good thermal properties.Liquid fuels of higher uranium content may In an insoluble sliu-ry type fuel, the fissilebe made by adding amounts of ssile material material concentration is not dependent on aexceeding the saturation solubility in bismuth metallurgical property of the coolant. A-liquid,and lead. In both cases uranium forms an in- metal fuel could be made, based on sodium andtermetallic compound with the coolant (U'Bi,, containing uranium, in which the neutron lossesUPb,) and the result is a slurry of the relevant to the coolant would be no greater than in acompound in equilibrium with liquid metal bismuth solution fuel, and in which a similarslilgtioniil The solnbiliitly of li.lli:,nium in lite liquid fuel rating could be achieved.p se creases rapi wi rise of m era-
ture in both systems, gnd this causes dillculty COMPAMSON BETWEEN MSMUTH ANDwhen the slurry ows in a circuit of varying SODWM A5 FUEL CARRERS
:1ei‘!1€)8I'8,l2i;l1‘8S. as woulélmocicur insa. iieactortaiid Metallurgical aspectsea exc anger com a ion. ma par ices - -dissolve preferentially in the hot region be- se§,c§°§;‘1e”§§éf,a§§§§§,fi"§° ntfigalggflgfgiln rsiicause of their greater surface to volume ratio, nding suitable container materials for a his_
and P1'e°iPita'ti°n in the °°ld regmn tends to muth-uranium fuel solution Workers at Brook-occur °n the remaining °°arse Pa1'ti°1°S- The haven National Laboratory.(Weeks 1955) haveresult is particle growth to such an extent that shown that the most promising ’end readily
dep°Sm°n and eventual bmcking (“Plugging”) available constructional materials for bismuthof the circuit occur. Any system containing circuits are steels containing 25 — 5 per centparticles in equilibrium with a solution is prone chromium and some molyhdeiium hut low iii
to this eecti which is 3' case °f “tempemture nickel and carbon However the eyxtent of cor-gradient ma-SS transfer.” For example, the rosion and mass‘ transfer in these materialssyst°m’th°rmm“bi$muth has been mvestigated has led to investigation of corrosion-inhibitingfor possible use as a neutron absorbing blanket additives in the bismuth Additions of mag_for thermal reactors, but the thorium content hesium as a deoxideht end zirconium as an
required is such that 9- slurry °f ThBi= in bis‘ inhibitor have been found benecial and themuth is ne°e55ary- The alloy systems th°ri“m' mechanism of inhibition has been proved to bebismuth and umni“m'bismuth are Very snmlar» the formation of a lm of zirconium nitride onand particle growth of the ThBi, is p1’°‘/3118 the surface of the steel the nitrogen being an:i.Bsei‘ious problem in such a blanket design inherent impurity in all steels. Horsley, work-ar on, 1957). ing at. Harwell, has shown that deliberate ad-The "nal alternative for a liquid metal fuel dition of further nitrogen to the steel prolongs
POWER REACTORS

PAGE 349



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the inhibition of corrosion (Horsley, 1957). How- sion product release would be expected to pro-ever, the eects of high ux irradiation on the duce some solids (e.g., halides) and gasesinhibition mechanism, and on the long-term (e.g., xenon, krypton). Continuous fuel pro-corrosion resistance of a reactor circuit, are cessing outside the reactor core would keep thesestill unknown. Research at Ames Laboratory to low equilibrium values. On the other hand(Fisher, 1956) has shown that bismuth can be a sodium slurry presents new problems in thecirculated for long periods at high tempera- circulation of the fuel itself, because there istures (1000°C) in loops of pure tantalum, but little experience of maintaining dispersion inthis metal is expensive, dicult to fabricate, ll ht li ' Y
g quid systems containing ne denseand has an undesirably high thermal neutron particles. Possible fuel additions are discussedcapture cross t‘ 22-sec ion ( barns). Generally, later, but the density range of these is betweenit appears that the problem of circulating bls- 4.4 gm/c.c. and 19 gm/c.c., whereas the densitymuth for long periods under reactor conditions of sodium at 500°C is 0.8 gm/c.c. The problemsin materials which have desirable engineering, of maintaining dispersion in s h li ' 'uc quid-solidnuclear and economic characteristics, has not systems are being studied by Cairns (1957),yet been solved.
'

'
using tungsten and other powders in waterOn the other hand, sodium and NaK have a~? _a"a1Pgu°S _f°1' uiaiiiiim and oiiiei fuel ad-been circulated in loops and in reactor circuits d1t}"e5 in _$°d1m,n- Imtlal resuits mdlcaife iii?-ifor long periods at high temperatures with umf°,rm d15Pe1'51°n can be malntained m_°§1‘-little diiculty. Several reactors have been culatmg systems at 1'eas°n”'b1Y 1°?” Vei°°1t1e$built and are operating (Siege1_ 1955, AECL of about 6.5 ft./sec._for tungsten particles smallerCRR-590) using sodium as coolant and stainless than 10 mlcmns dlameter m Water-steel of the 18/8/1 type as constructional This analogue work is to continue becausemateria'1- It is remgmsed that Qxygen as an it is necessary to investigate the behaviour

‘I

impurity has a profound eect on the corrosion of such a dispersion under an conditions likely to
5

°f many metals by S°dium (Liquid Metals Hand" be encountered in a reactor circuit. The be—
book, 1955), but adequate measures for control havigur of a, reactor fuelled by uranium par-°f °XY3en cmtent in $°dium as a °°°1am have ticles dispersed in sodium has been studiedbeen <i<-‘>v¢1°P¢d- Thus, on the basis Of <=<>m- theoretically by Dalton and Thompson (1957)Patibmty with °°ntain°1'5' s°di“m has distinct who concluded that short-term variations ofadvantages °"e1' bismuth fuel concentration by more than 2 per cent.Engineering “necks in the circulating ‘fuel/coolant would have

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An L» a mes :::;:i.::::"t:i.:i:.:: °%?m’iZ’“i1§§%%l§it%‘;‘§re°'°t°r design is the “ta! investment °f ssile necessary to ensure uniformity’ in the fuel feed
material in the reactor circuit. Because fuel must be establishedmust pass through heat exchangers, pumps, 'processing vessels, and other components ex- W01‘k With aqueous SIISPBII-$10115 may I101? giveternal to the reactor core. the total amount of adequate i11f°1‘mii°Ii 011 the 5061111111 b8-58dfuel in a system based on bismuth may be three systems. because the wetting characteristics areto five times that in a reactor core (Dalton, fiiefeiiti and ionic Phenomena do I101; 006111‘1957). As the fuel rating of the reactor in 111 8_0d1i1m- Th1_1-S, Similar Studies Of Particle be-megawatts per kilogram of ssile material must havwllr in owing sodium must be iindcriakcm -
be considered on the basis of total fuel invest- Small 100118 '50 d0 ibis. Using eleciroiagneiicmerit, the calculated rating of the core must be Dumps, are being built as part cf the mctallmerdivided by the ratio of total hold-up to core cal research program at Lucas Heights. andinvestment

a larger loop has already been constructedIn the case of sodium, a fuel slurry having by the chemmal engmeermg S°°t1°n' I
neutron losses corresponding to those in a bis- Sodium has several minor advantages over

*

muth solution must have a. higher uranium con- bismuth in the engineering of reactor circuits.
-

tent In the reactor core th‘ ' t‘ f
'

. is is sa is actory The lower melting point of sodium results infor heat removal because of the high specic less diicult “heat tracing" of circuits to ensure 1
heat of sodium, but in the heat exchangers this that all reg-ions are molten at start-up. Heat

.

same property makes necessary a greater heat tracing for bismuth circuits must be carefully
‘

transfer surface per unit of fuel than is re- designed because the metal expands on solidi-quired for the bismuth solution. As a result, cation and may fracture pipework if allowedthe minimum possible volume of heat exchanger to freeze indiscriminately. Another factor in ‘for a given power output contains more fuel favour of sodium is its high electrical con- ,for a sodium slurry than for a bismuth solu- ductivity, enabling it to be pumped by electro- ’tlon. Advances in heat exchanger design may magnetic means far more readily than can ,,:
reduce the actual value of this extra fuel hold- bismuth. Finally, the sheer weight of large ‘iup, but preliminary calculations (Berglin, 1957; volumes of bismuth constitutes a disadvantage ’
Dalton, 1957) indicate that the ratio of total in necessitating heavy pipework and supports, 3
investment to core investment for a sodium whereas sodium is lighter than water.slurry system may be between 10 and 15. Thus,

‘i
the overall fuel rating in a sodium system is ¢|,e,n;“| uspesprobably inferior t0 in 3- System. There is no clear distinction between

;

A . . . .
.circulating liquid metal fuel solution would chemistry and metallurgy when considering

'

be homogeneous liquid initially, although s- phenomena in liquid metals. However certain ’7PAGE 350
SECTION 2

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problems of both the liquid fuels may be termed Liquid sodium has some disadvantages whenchemical. compared with bismuth, in that a fuel slurry
The bismuth fuel solution under study in is m°1'e dlmcule t° elreulete than 5 9011310",the U_S_A_ twtthemei 1954; weeks, 1955) eoh_ and the higher neutron absoption leads tolarger

i

te; 600 m of th-ehtum Under reeeter fuel investments in the reactor system as aeegigttiehs, "End operating eh either U233 or whole. However, the fact that sodium has
U235, other non-ssile uranium isotopes would extremely !°w s°l“b.mty for m°st °°“¥a?*!erhe produced and would build up in the met materials gives promise that the compatibility
thus hmttmg the eee eehteht if the eatum_ problems of a sodium-based system will be less
tion solubility is not to be exceeded. This dlmeuni to 5°1ve- The sedmm 51""? eyetemproblem does not eeem. in e ehm.y met, heeause for thermal reactors is not being studied else-
the ehewehle eeheenttatieh does net depend where, and for these reasons it was selected as
oh so1ubihty_ a suitable project for the Research Establish-

Continuous chemical processing of bismuth- mirgegf the Australian Abomlc Energy Com-
based fuel is being studied, using molten saltcontacting teehhiques, whieh are promising There are many possible combinations of
(Dwyer, 1955)_ Ne simple system of eehtmu_ moderator and fuel _additions for_ use with awe processing for Sodium ehhriee has been de_ sodium coolant, and it is not possible to study
vised to date, and it may he neeeeeery to all of these with the staff and facilities
separate ssile material from coolant before ev9'11eb1e- Th“-F, _$°me _e11mmae1°_n was neees‘processing A relatively simple meehe of sary, although it 1S realised that in the process
separation exists in the hydroclone (Berglin, 5°me pr°m‘smg °_°mbme't1°ns _meY have been
1957). and work on this principle is proceeding delete‘? The ehmee °f_ materlels $01‘ deteedin the A_A_E_c_ reeeareh lehomtertee study is outhned briey in the following sections.

A further point favouring bismuth fuels from
the processing viewpoint is the possibility of a _THE CHOICE OF MQDERATPR
simple operation to replenish the fuel with F°r 111811 lemvorotulo ocmoo-_Pooo1b1c reactoruranium produced in e heutrOn_eapturihg moderators are graphite, beryllium and beryl-
breeder blanket surrounding the reactor core. hum °X1de-If a thorium-bismuth slurry (containing The smaller critical sizes of reactors using

‘_ 'I'hBi,) is used as blanket, U233 is produced beryllium and its oxide as moderators give thesein the ThBi, particles. Raising the temperature materials advantages over graphite in permit-
of the alloy to the point at which all thorium ting lower fuel inventory and power output.

. and uranium is in solution, and cooling to re- Further, there is a reasonable possibility of
constitute the slurry, results in signicant con- making the fuel/coolant compatible with beryl-/centration of uranium in the liquid phase, which lium or its oxide, whereas graphite and sodium
could be further concentrated and added to the are incompatible. In the only sodium-cooled

i core (Dwyer, 1955). The particle growth prob- graphite moderated reactor built to date, iti lems of a thorium-bismuth alloy have been W9-S necessary to can the graphite in Zirconiumdescribed above, but there is considerable in- (Siegel, 1956) because the available graphitese centive to solve them. were all penetrated and disintegrated by liquid. Finally, the effects of neutron irradiation on

%

sodium.
g the coolant itself lead to chemical problems in Beryllium exhibits a nuclear property whichthe case of bismuth. The product of neutron enhances its value as a moderator. but which
‘ capture in bismuth is polonium, a highly active may have serious mechanical consequences. Two

K alpha emitter, which would constitute a severe nuclear reactions occur in beryllium under fact
- health hazard in bismuth circuits. Neutron neutron irradiation; $11958 8-1'9 the I1, 1 and I1,

-1; irradiation of sodium produces the isotope Na24, 2n reactions, both of which produce heliumj 5 short-lived gamma emitter, which decays to as one end product. Calculations based on the
magnesium 24; this is no prQblem and may available data shows that a “neutron enhance-

; even be benecial is some reactor circuits as merit” Of Freverl Der cent. is Dessible by tho

sodium slurry liquid metal fuel other elements (“B81‘Y11i11lI1,” ASM, 1956), and this lllcre-86$
magnesium is a. powerful deoxidant. In the n, 2n reaction in c beryllium moderated system

.1 present are likely to have higher alriity for the possibility of breeding 011 the ‘H1232-U233
Qxygen, cycle in a uranium-sodium-beryllium reactor.

ii; On the other hand, the eect of the helium
summm-y-4-ho Augfrqgn |__M_|=_R_ P.-ojett atoms on the beryllium is not known. Calcula-

; Of the two most promising liquid metal fuels, ti‘-ms by the alleh°1‘- and late!‘ by Hickman
the bismuth solution has been chosen by workers G958)» have indicated that at high tempera"i in both the U.S.A. and England on the grounds tures the beryllium {nay Swen, as a 1'*?$“1t_°f
of low thermal neutron capture cross-section, hehum bubble f°m1at1°n- Expenmemel lmedle‘
ease of handling in comparison with a slurry, time °f beryllium at high tempelietures ,m

.1‘ and the promise of simple high temperature HIFAR are bemg undertaken’ and Wm P1'°‘“de
chemical processing. Unfortunately, the prob- b°th fundamental and te°hn°1°gi°a'1 data °n

E lem of corrosion of structural materials by liquid this pr°b1em'

, was anticipated and, indeed, may prove in- compared with metallic beryllium; the neutron
surmountable. enhancement by n, 2n reaction is likely to be

bismuth _has proved much more dicult than Beryllium oxide has several disadvantages

E

i POWER REACTORS PAGE 351
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less, the material is more difcult to fabricate fuel is its density, 18.3 gm/c.c., compared withinto shapes of low porosity and the thermal that of sodium at 500°C (0.83).$h°°k resistance is i11f°1‘i°1‘-
» Uranium oxide U0, has the advantage ofThe mechanical eects of helium production being readily prepared as a ne powder. Iton beryllium oxide are diicult to predict. On is wetted by sodium at about 380°C (Taylor,one hand, the relatively open lattice structure of 1955); a dispersion of U0, in sodium is being.the oxide may accommodate helium atoms studied by Abraham (1957) at Argonne Nationalmore readily than the almost close-packed Laboratory as a possible fast reactor fuel. Ametallic lattice; and, on the other, the possible disadvantage of U0, is the liberationoxide must be considered as a brittle material, of oxygen to the system by fission of uranium.so that sumcient internal strain set up by the The most serious compatibility problems ofpresence of many foreign atoms would cause sodium are those involving oxygen as an im-cracking and possible crumbling. purity, and this is particularly true of circuits

- in which metals with a high ainity for oxygene.5ii".%‘;i‘éi$i°i%§‘ieietviiiiiiiisaoie present ea beryllm The muence orfeasibility of a sodium-based system, because °XYg‘*%‘ 1’°1‘?ased f.’°m .U°= °n “Fe °°'.‘°51°n °fit has the best combination of nuclear proper- Pcrylilum m s°d,mm '5 t° be lnvesngated byties, it is relatively easy to fabricate, has good 1"”'d1at1°n tests m HIFAR"resistance to thermal shock, and it is negligibly Uranium carbide may have a. similar dis-soluble in sodium. The disadvantages of the advantage; if carbon is released by ssion,metal, compared with its oxide, are mainly the it will undergo mass transfer in sodium to anytemperature limitation imposed by the low creep metal forming a, stable carbide. 'I‘h.is is anstrength of beryllium at high temperatures, example of “concentration gradient mass trans-and corrosion problems -caused by oxygen in fer,” in which even minute solubility of athe sodium. Beryllium oxide corrosion in substance in a liquid metal will cause appre-sodium is being studied in France (Grison, ciable transfer from a “source” to a “sink” in1956), and the results of this work are of con- a circuit, even in the absence of a temperaturetinuing interest to the Australian group. gradient. No work is being done on uranium
carbide because it does not possess any out-Posslm-5 FUEL ADD|T|°N5 To SOMUM standing advantages over uranium metal orThe fissile material may be uranium metal UBe13~or any compound capable of being prepared as The most interesting fuel addition is U'Be,_,,c ne pcwclcr which is Wcttcd by sodium. and the inter-metallic compound formed betweenwhich does not contain elements of unaccept- uranium and beryllium. It has the advantageably high thermal neutron capture cross-section. of low density (4.4 gm/c.c.) compared withBecause ssion of uranium atoms in a c0m- uranium metal, and this reduces settling outpound will ultimately release the other com- problems of the dispersion. The compoundbined atoms, these must not be of a sub- is relatively easy to prepare by heating nely-stance which will cause corrosion of the m0d- divided mixed powders of uranium and beryl-erator or circuit materials. Three uranium lium to 1,100°C. Further advantages of thisccmncunds have acceptable nuclear prcperties. fuel addition emerge when considering metal-are readily prepared and readily wetted by lurgical aspects of the sodium-uranium-berybliquid sodium. These are U0, UC and UBe,,. lium system.

The particle size of the powder is determined METALLURGICAL PROBLEMS OF 1-HE
“Y *‘”° m°°°"”‘ SODIUM-BASED FUEL/COOLANT(i) To enable ssion products to escape from Mention has been made of the inuence ofthe particles, with the attendant possi- oxygen impurity on corrosion and mass 1;;-3,9319;bility of continuous de-poisoning of the of metals in owing sodium, and the eecg is

1'59-°t°,r» Particles less than 10 mlcmns in greatest for metals of high anity for oxygen.diameter are desirable (Hickman, 1958). In systems using sodium as a coomnt 0n1y~,(ii) To mantain uniform dispersion, it is de- several methods have been developed to main-sirable to have particles with minimum tain a low oxygen concentration in the liquidsettling velocity, i.e., as small as possible. metal.
Consideration of settling rates of metallic One such system is “cold trappmg," consist-uranium in sodium led to the choice of particles ing of passing the sodium stream, or a. portionof less than 10 microns for initial studies of it, through a region at temperatures just(Kelly, 1953), above the melting pomt. Because the solubility
“mum metal may be Prepared in the Mm °i;;’§ii§“ ‘§L.i°iiZi?e£“‘$i ";’é'2§%‘3.f°f.°,.’i‘Zi‘;’go$§.;'ieof particles smaller than 10 microns by repeated P th '15’ t P nd ltration or sewing in ahydrlding and dehydriding of massive uranium m . e °° ran a . ."- - f low velocity can retain this oxide.(Kelly, 1958). Uramum powder is wetted by regwn. °

. is dliquid sodium at approximately 400.0 and By this means oxygen content can be minim e
does not de-wet on cooling (Taylor, 1955). F0 less ghacgepatgtscgglsnlgggéaigiglg ggllgUranium is not corroded in liquid sodium pro- $i;1°oxiggs more stable than sodium oxidevided that the oxygen content of the liquid ‘ 'metal is kept suciently low (Mogard, 1955). A second system is to “hot trap the sodiumA disadvantage of elemental uranium as a (Siegel, 1955) by passing it through a region
PAGE 352

- SECTION 2



J

containing material with a higher anity fol (ii) Only two liquid metal fuels are attractive;1 .
.

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oxygen than any other material in the circuit. these are a solution of uranium in liquid
Results to date indicate that it is necessary to bismuth, and a dispersion of uranium or
run hot traps higher in temperature than the a compound in liquid sodium. Bismuth
material to be protected, particularly for re- solution systems are being studied in detail

; active materials such as beryllium or zirconium. in at least two other countries, whereas
‘ Attempts have been made to protect beryllium sodium fuels are not receiving much
" in sodium circuits by hot-trapping with thorium attention.

(Bett, 1958). Theoretically this should be eec- .

tive if both metals are at the same temperature, <1“) s°d.m.r9 pre§ents less. pmblems .°f °°m'
because thorium oxide is more stable than patibllity with contamer materials than

1 l ‘ beryllium oxide. However, the results indicate (1995 bismuth‘ It is in this eld that the
that considerable oxidation of beryllium occurs msmuth system has °n°°‘mtered ‘ts m°St

critical technological barriers.
ant factor is the rate of oxidation. 'I:he kiiieti<>$ Not all of the many possible combinations of

I of oxidation of reactive metals in sodium will be fuel, moderator, and container maieriais have
i studied by chemists and metallurgists as part of been dismissed, but the aim has been to imii.

”*‘* I the Austmllan P1'°Je°t- cate the main reasoning behind the current
Z” . research programme, and the nature of theIn the case of a liquid metal fuel containing . -

ne particles, neither of these systems can be problems Involved‘
. a°1°Pted1 because b°th depend 5°!‘ 1’em°"a1 °f Sufficient design calculations have been done
“~ °xYgen °n a 1°W‘Ve1°citY 1'ea°ti°n Vessel» which to indicate that the sodium-based system is a

would allow settling of the fuel. A third alter- promising one if the iecimoiogicai pi-0b1em5 min
native is tn‘? use °f _a' sQ1u'°1e “gettelm in Fhe be solved. Design of a liquid metal fuel reactor
sodium, of which calcium is the most promiS1l1g- experiment must await satisfactory completion

é P1'e1in_1ina1'Y Wm‘? mdi_°3'te$_ that ¢°1'1'°$i°¥1 °f of compatibility and uid ow studies. which
beryllium metal m sodium 15 reduced ¢0nS1de1'- involve considerable metallurgical, chemical, and
ably by the presence of calcium (Bett, 1958). engineering efiom
However, calcium and nickel interact, and the
effect of calcium additions on nickel-bearing
stainless steels is being studied. Initial results REFERENQ5
indicate that the quantity of calcium required ABRAHAM’ B_ M" FLOTow_ H_ E_ and CARLSON‘
£01’ ‘g9tte1'ing" has negligible eectfl on 13/8/1 R. D. (1957).—Second Nuclear Engineering and
stainless steel (Bett, 1958)_ Science Conference, Philadelphia, March, 1957.

Rt» .a-

,;_£ _, _ , A.E.C.L. REPORT ORR-590, Catalogue of Nuclear
* The use of UBe,,, as fuel addition may provide Reactors.
* a!1°'1hel‘ method Of Oxygen 0°lltl‘01- Fission Of BALL J. G., (1955).—J. Inst. Met., London, a4, 229.

umnium at°"{*$ in the °°mP°“_nd_ Panicles will BAR'1:ON, P. .1., and GREENWOOD, G. w., (1957).-
; E release beiylllum atoms, and it is hoped that AERE M/R,2310,

in the higher chemical activity of this beryllium BERGLIN, Q ;,_ w_,_-(195-7)_ Private commimi¢,,__
~ 3 compared with the moderator material will re- tlon.

sult in continuous “gettering" of oxygen with BETT, F. L., and DRAYCOTT, A., (1958).--Private
lliil.-u; ,.

out appreciable corrosion of the moderator. This ¢°mm11Ill¢&';l°n-
~ hypothesis will be evaluated by irradiation ex- CAIRNS, R. 0., (1957).—Prlvate communication.

periments in HIFAR. DALTON, G. 0. J., (1957). Private communication.

T In addition to the problems of oxygen im- DALTON, G. c. J., (1953). Proceedings of this
Ti purity, ssion products_ released within the sYmP°s1“m-
1 r sodium may cause corrosion and mass transfer D“;YER- °- E--1§'=66'f11-» (1955)-— ’°°- “ - °“ - ~

l . - En 955 vol 9
.. I . Th " ' ' 'it ;. 9f moderator and mrcuit- matfrims k -e fiyfem FISHER, R. w., and wmonns, G. R., (l956).—-

; .. ' : Y ,

v,;7~;..

M.
t»

is so complex that experlmen a wor , in u ing Am L b t Iowa state C°"ege_ Contact
~ irradiation tests, will provide the only real guide. w -§’fo5_ Hg“ $7
its It may be seen m general that a °P“$1d°'?b1e GRISON, E., (C.E.A., France), (1956).--Private com-

amount of metallurgical research is required munlgatign,
before the fuel-coolant, moderator. and con- HACKETT, M_ N_ (1942)___.'1‘i-gn5_ Am Soc, Mech

If structional materials for a reactor can be Eng1's.,64i64'1.
' 2 dened. HERON, s. D., (1928).—U.S. Patent 1610965.

, HICKMAN, B. s., (1958).-—Private communication.
5 - SUMMARY oiioiiglgiiin, E. E. and MANLY. W. n., (1956).-

fj The reasons for the choice of a sodium-based HOFFMAN, E. E. and MANLY, W. Di. $11955).-

L 1 liquid metal fuel system as a high temperature lfgléglear Ens 8v Science Congress. Cleve an , ec.,

* ‘z reactor study in Australia may be summarised
P . as f°11°w$=- AERE M/Bis-la. '

HORSLEY G. W. and MASKREY. J. T.. (l957).——

: (1) A liquid metal fuel system has potential x::i.i.Y. J. w., (1958).—'1‘he Prep-l'ati0i:l of mic
1"'?i; - - ' - uranium powder by the hydride process. Proceed

§ l advantages in high fuel rating, high lugs of this symposium.

I’ bl-l1'!1'11Pl and high tempera-t“1'e operation’ MOGARD, H, (1955)-Observations on the corro-t l A the latter leading t0 high thermal slon or uraniuin in liquid sodium. Proc. Int. Cont.
K » emciency. At. En-. 1955. vol. 9. P5118-

. POWER REACTOR5 PAGE 353

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SIEGEL, S., CARTER, R. L., BOWMAN, F. E., HAY- THE METAL BERYLLIUM, (1956). —— AmericanW, »' WARD, B. R., (1955).--Basic Technology or the Society for Metals.
> Sodium Graphite Reactor. Proceedings of Geneva

Conf., 1955, Vol. 9, P.a21. TROCKI, '12, et a1.,_ (1955).—S0dium and potassium
alloy tor reactor cooling and steam generation. Prue.

TAYLOR, J. w., FORD, s. 1)., (1955).—Solid Metal Int <1<>11f- Ah En-- 1955, V°1- 9, P241-—Liquid Metal interaction studies, Part II, Contact U.s.A.E.C., (1955). — Liquid Metals Handbook.§It;1g%l:i72I;e1atlonships for Sodium on Solids. AERE sodium-NQK Supp1ement_
U.S.A.E.C., (1955).—The Reactor Handbook. V01. ‘

.= TEITEL, R. J., (1952).—The Uranium-Lead System. 2, Engineering» A-E-C-11 3646~ .

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J!'- Metals 4- 4. 397, API11, 1952- WEEKS, J. R., et a1., (1955).—Corrosion problemsFROST. B. Rh T" MASKSEYI J_ T" (1952)___The with bismuth uranium fuels. Proc. Int. Cont. At.
System Uranium-Lead. AERE M/R.1027. f§"J§f;5'1;§§§'_ 9' P341‘ (see M“ N“°1°°m°s 12' 7'

TEITEL, R. J., GURINSKY, D. H., and BRYNER, WILLIAMS, C. and MILES, F. L., (1954).—L1q'nid
s_ J_, (1954).-Liquid Metal Fuels and Breeder Blan- Metal Fuel Reactor Systems for Power. Nucleonicl
kets. Nucleonics 12, '7, 14, July, 1954. 12, 7, 11.

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