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2,902,424

MoMOGENROUS NucLEAR POWER REAmOR

L. D. P. King, Los Alamos, N. Mex., assignor to theUnited States of America as represented by the UnitedStates Atomic Energy Commission

AppIieation June 6, 1956, Serial No. 58%s37

1 Claim. (Cl. 204-193.2)

The present invention relates to nuclear reactors andmore particularly to homogeneous nuclear power reactorsutilizing a liquid fuel.

The nuclear reactor of the present invention is an im-proved reactor of the homogeneous type, and is describedas particularly suitable for use in power generating facili-ties.

Homogeneous reactors of the prior art generally re-quire extensive fuel handling and gas recombining sys-tems. Thus, in reactor systems utilizing uranium watersolutions the radiolytic dissociation of the water createsan explosive mixture of hydrogen and oxygen which musteither be vented or recombined. In either case extensiveapparatus is required for the safe handling of these gases.Further, such aqueous systems are generally convectioncircu~ated thereby limiting the power level. The priorart homogeneous reactors also generally have wide rangesin operating temperature which produce large undesirablereactivity effects. However, the use of homogeneous re-actors for power production has well known advantages,i.e., inherently safe operation because of the negativetemperature coefficient of reactivity, and the comparativeease in recovering the fissionable material and productsthereof from the liquid fuel.

The preferred embodiment of the present invention doesnot require fuel handling outside of the reactor vesselduring any normal operation including complete shut-down to room temperature, nor is radiolytic gas handlingor recombining apparatus required. The preferred em-bodiment of the present invention utilizes .a. liquid fuelcomprising a uranium, phosphoric acid and water solutionwhich requires no gas exhaust system or independent gasrecombining system, The inherent power limitations ofthe prior art homogeneous reactors are overcome in thepresent invention by utilizing forced circulation of theliquid fuel.

The present invention was designed in accordance withthe following considerations: (a) a minimum activevolume to permit the construction of a high-pressurehigh-temperature reactor of reasonable size, (b) a smallexcess reactivity, and (c) good self -regulating featuresunder extreme operating conditions. These initial con-ditions imply a minimum of absorbing material in thecritical region, high neutron reflection, and a maximumof self-regulation.

Although the description of the preferred embodimentis specitic to a power-level of about 2 megawatts, at whichthe thermal neutron flux would be of the order of 1013neutrons/cm2/sec. using ordinary water as moderator,appropriate changes in the size of the critical region, heatexchanging capacity and volume of fuel may be madeto provide a power output of either larger or smallervalue.

The reacter of the present invention consists generallyof a reactor vessel having four main regions, i.e., a liquidfuel reservoir, a heat exchanger region, a critical regionand a vapor region. These are of the proper size to takegare of large temperature and accompanying reactivity.

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2change by means of a suitable geometry change. Thecritical region is heated between the two non-criticalregions, i.e., a poor geometry vapor region above, anda heat exchanger region below. The amount of solutionin the critical region depends on the liquid fuel tempera-ture, and is full only at the desired operating temperature.

The preferred embodiment of the present inventionprovides for the circulation of liquid fuel by means ofan impeller which moves the liquid fuel through the heatexchanger. The location of the heat exchanger outsideof the critical region but within the reactor vessel reducesthe size of the critical region and minimizes the liquidfuel circulating circuit.

Further, by this arrangement and association of compo-nents within the reactor vesseI the liquid-gas interface un-der normal operating conditions is not within the criticalregion, i.e., not within the geometry which determinesthe critical mass, as it will be located above the bafflewhich defines the upper limit of the critical region. There-fore, disturbances on the liquid surface will have a re-duced effect upon the power and neutron level.

A cold critical volume of fuel, as described in moredetail hereinafter, is introduced into the reactor andfills the fuel reservoir and partially fills the criticalregion. The capacities of the reservoir region and thecritical region are such that upon expansion of the selectedfuel solution at the elevated operating temperature thesolution will completely till the critical region, therebycreating a criticaI assembly. The critical region willnot have a large excess k, since it can never be full ofcold solution. The expansion of the liquid fuel in thereservoir region when the temperature is raised forcesadditional fuel into the critical region. This gain incriticality is partially compensated for by the negativetemperature coefficient of the solution in the critical regionwhen a cylindrical shape is used. Complete compensationwould be possible by properly shaping the critical re-gion. When the critical region is almost full, a slight gainin reactivity will be produced by additional neutron re-flection from the baffle separating the vapor and criticalregions. Any additional rise of the liquid fuel into thevapor region above the baffle will have little effeet on reactivity.

Thus it is apparent that in the reactor of the presentinvention a predetermined relation exists between theratio of cold critical fuel volume and total reactor vol-ume, and hot critical fuel volume and total reactor vol-ume. These ratios, as explained in more detail herein-after, require that the volume of liquid fuel initially putinto the reactor bear a certain relation to the volume ofthe entire reactor vessel, this relation being dependentupon the expansion coefficient of the liquid fuel.

Therefore it is an object of the present invention toprovide a homogeneous, liquid fuel reactor which is con-trolled by the thermal expansion of the liquid fuelutilized.

Another object of the present invention is to providesuch a nuclear reactor which does not require gas re-combining apparatus or liquid fuel handling outside ofthe reactor vessel during normal operation.

A further object of the present invention is to providesuch a nuclear reactor having a minimum critical volume,small excess reactivity, and good self-regulating features,

A stilf further object of the prwent invention is toprovide such a nuclear reactor which is so designed andconstructed that the expension of the solution at the ele-vated operating temperature fills the reactor critical re-gion and makes it slightly super-critical.

A still further object of the present invention is toprovide a reactor which can be completely shut downwithout removing the liquid fuel from the reactor vessel

2,902,424

3and which does not have the critical region full of coldsolution during shutdown.

Other objects and advantages of the present inventionwill become more apparent from the following descriptionincluding the drawings, hereby made a part of the speci-fication, wherein:

Figure 1 is a functional diagram of the preferred em-bodiment of the reactor of the present invention helpfulin explaining the operation of the reactor;

Figure 2 is a detailed sectional view of the preferredembodiment of the reactor of the present invention;

Figure 3 is a sectional view of a portion of the circulat-ing pump utilized in the reactor of Figure 2;

Figures 4 and 5 are graphs showing the temperaturecharacteristics of the liquid fuel solution of the presentinvention;

Figure 6 is a graph showing the temperature charac-teristics of the uranous phosphate liquid fuel solution;

Figure 7 is a series of graphs showing additional prop-erties of the uranyl phosphate liquid fuel solution;

Figure 8 is a graph showing the dependency of vaporpressure on temperature for the uranyl phosphate system;

Figure 9 is a graph showing the dependency of vaporpressure on temperature for the uranous phosphatesystem;

Figure 10 is a graph showing the variation in vaporpressure with per cent phosphoric acid at different tem-peratures for the uranous phosphate system;

Figure 11 is a graph comparing the recombinationrates as a function of temperature for the uranyl anduranous phosphate system;

Figure 12 is a graph comparing the total pressure atequilibrium as a function of temperature for the uranyland uranous phosphate system;

Figure 13 is a graph representative of the total pres-sures at given power levels

Figure 14 is a graph showing the dependency of thereproduction constant on reactor temperature for thereactor of Figure 2; and

Figure 15 is a schematic diagram of the liquid fuelhandling system.

SUMMARY OF RE.4CTOR SPECIFICATIONS

Moderator ------------------------Solution:

Composition . . . . . . . . ..-.. -----Power density -----------------Specific power . . . . . . . . . . . . . . . . .Critical mass------------------Total fissionable material... .Hot volume . . . . . . . . . . . . . . . . . . .Cold volume . . . . . . . . . . . . . . . ..-Maximum operating tempera-

tllm. ..Maximum qm-sting pressure_Ga3 evolution . . . . . . . . . . . . . . . . .

Heat exchanger:Ar@a. . . . . . . . . . . . . . . . . . . . . . . . . .Average heat flux. . . . . . . . . . . . .Coolant -----------------------Coolant velocity . . . . . . . . . . . . . . .

Coolant flow rate. -_.. -.... -_Coolant temperature. . . . . . ..-.

Vessel:Over-all volume (less pump)..Vapor volume . . . . . . . . . . . . . . . . .Storage volume and heat ex-

cbmger region.Critical region. . . . . . . . . . . . . . . . .

Over-all vessel length . . . . . . . . . .Vessel and pump . . . . . . . . . . . . . .Composition . . . . . . . . . . . . . . . ..-

Control: Rods (Blo density 1.7). . . .

Shield: Composition --------------Fluxm in core:

l?ast neutrons (maximum, over

.038 C.v.).Fast neutrons (average) -------Fast neutrons (inner vessel sur-

face).Thermal neatrons (maximum).Thermal neutrons (average) ..-Total gamma flax -------------

Homogeneous.Thermal.2 megawatts.About 90 percent enriched VOS

dissolmt in H3POj.Water (ordinary).

.6 M U0,+7.5 M H!PO1.46,5 kw.lliter.470kw./kg. fissionable material4.24 kg.9.2 kg.94 liters (430° C.).621iters (20° C.).455” c. (s50” F.).

6000p.s.i.Equal to recombination by back

reaction.

38.5 Sq. ft.177,0W13.t.u,/sq. ft./hr.Water.Inlet 15 ft./see. at 39OOp.s.i.; outlet

12uft./see. at 3600p.s.i.12g.p.m.In 38° C.; ont 427° C,

122Iit,crs.26.98liters.48,39.

15” dia. x 16” high cylinder, witha volume of 46,21liters.

8.2.5ft.12.63ft.3“ wall stainless steel.4 safets-.1 control.4 ft. HjO+lOr’ Pb+5.5 ft. concrete.

I,4X1014n./cm.2/sec.

9X101$ n,/cm,~/scc.7X1OI3n,/cm,2/sec.

2.6x1013n,/cm.~/sec.I,3XIO13n./cm.!/sec.7X101Z“@X.

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4APPARATUS

The preferred embodiment of the present invention isshown in schematic form in Figure 1, and may be dividedinto five regions for the purpose of description, i.e., vaporregion and manifold section 20, critical region 21, heatexchanger region 22, fuel reservoir 23, and circulatingapparatus section 24. These sections are shown in detailin Figure 2 of the accompanying drawing. Referringnow to the detailed sectional view of Figure 2, the pre-ferred embodiment of the reactor of the present inven-tion consists of a pressure vessel 25, preferably fabricatedfrom stainless steel and plated with gold, which has anupper flange 26 and a reduced diameter impeller section27. The interior of the vessel has a diacritical diametersection 28, a non-critical reduced diameter section 29,having a diameter less than the diameter of the diacriticaldiameter and which extends from the top of the heatexchanger region 22 to the bottom of the fuel reservoir23, and a circulating pump aperture 30 at its lowerextremity.

Attached and sealed to the upper vessel flange 26 isa collant inlet manifold assembly 31. Connected to theinlet manifold assembly 31 is a plurality of heat exchangerlead pipes 32 which are sealed to the manifold 31 andwhich are connected to a source of water (not shown)through inlet water channel 33. A top plate assembly34, is sealed to the inlet water manifold assembly 31,and to a spacer ring 35, by means of a plurality of bolts36 or other well-known means. The top plate assembly34 has a cross section in the form of a T with a centralaperture 37 and bottom plate 38 welded or otherwisesealed to lower portion 39 of the top plate assembly 34.

Fixed to the interior surface of bottom plate 38 andextending upwardly therethrough and through centralaperture 37 is steam outlet manifold assembly 40. Ter-minating in the outlet manifold 40 are outlet lead pipes41 of heat exchanger 52 which are sealed to the outletmanifold 40 and are connected to the steam utilizingsystems (not shown) through outlet channel 42. Theoutlet channel 42 extends up through sleeve 43 which isconnected in any conventional manner to the steamsystem. Supported within the sleeve 43 is a centralcontrol rod thimble 44 which is of considerably smalleroutside diameter than the inside diameter of sleeve 43and has its upper extremity welded to the inside surfaceof the sleeve 43 to provide a seal for the channel 42. Thechannel 42 is connected to the steam utilizing systemthrough an aperture in the upper portion of sleeve 43.Control rod thimble 44 extends downwardly throughsleeve 43, is welded or otherwise sealed to outlet mani-fold assembly 40, and extends to the bottom of fuelreservoir 23.

The outlet and inlet manifold assemblies, as describedabove, are separated by a distance of about 18 inches inthe preferred embodiment so that gradual temperaturegradients are possible, and so that the thermal stressesin the top plate assembly 34 and vessel flange 26 arereduced. It should also be noted that the main vesselseal through inlet manifold assembly 31 and spacer ring35 is well above the critical region 21. The flange 26may be water cooled by cooling jacket 46, as is thesurface of the central apertures 37 by cooling jacket 47.The main vessel seal region is further cooled by the inletmanifold 31. In this manner, the activation of the sealregion, which should not exceed a temperature of 50” C.,will be low, with about 18 inches of steel available toattenuate neutrons and gamma rays. Seal welds areprovided although with the low temperatures existing inthis region neoprene or metal O rings or similar sealingmeans may be used. Channel 48 between the vessel 25and the lower portion 39 of the top plate assembly 34serves the dual purpose of separating the hot and coldmanifolds and of providing a restricted region where

75- vapor condensation may take place.

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2,902,494

5Additional or fewer safety rod thimbles 49 may be

provided. In the preferred embodiment these thimbles49 are four in number, are symmetrically placed aroundthe central control rod thimble 44, and extend only tothe bottom of the critical region 21. Thimbles 49 aresealed to the bottom plate 38 and extend upwardlythrough the central aperture 37.

Supported by the central control rod thimble 44 is aliquid fuel flow directing baffle 45. The baffle is madeheavy to decrease gamma ray heating of the cover, toserve as a poison for the vapor region, and to providea narrow region above which the liquid fuel can risewithout producing a change in criticality due to a volumechange of the reactor core.

The central thimble 44 also supports a spider 50 whichis attached to the bottom of thimbles 49. In this mannerthe upward thrust caused by the circulating liquid is dis-tributed over all of the thimbles. The spider 50 is madeup of several diametric supports which support a platinumfunnel 51, heat exchanger 52, draft tube 53 and poisonreservoir 54.

The patinum funnel 51 serves to guide the liquid fuelagainst the baffle 45 and to prevent vortexing of theliquid entering the draft tube 53. The funnel 51 is pro-vided with openings (not shown) to permit convectioncurrents in the vessel during start-up and before thecirculating pump is turned on.

The heat exchanger 52 consists of twenty-two similarlyshaped, tightly wound spirals which are closely spaced,e.g., %(r inch and staggered for maximum efficiency. ThecoiIs are made of VIBinch O.D., % inch I. D., stainlesssteel tubing which is clad with a few roils of gold. Theheat exchanger is supported by inlet pipes 32 and outIetpipes 41. However, the spider 50 provides support againstupward movement resulting from the forced circulationof the liquid fuel.

The draft tube 53 extends downwardly from the spider50 through the heat exchanger 52 to the bottom of thefuel reservoir 23. Attached to the draft tube 53 is apoison reservoir or can 54 which contains highly com-pressed and sintered normal boron carbide, or otherneutron absorbing material, the purpose of which will beapparent hereinafter. Attached to the lower extremity ofdraft tube 53 is a flow directing eIement 55 which isshaped to give an efficient suction inlet and turn-aroundfor the liquid fuel.

The reactor vessel 25 is surrounded by a reflector(not shown) which consists of, in the preferred embodi-ment, four feet of water, which also serves as a neutronshield. It should be noted however that the reflectormay be of any materiaf known in the art as a neutronreflector or the reflector may be absent if sufficient fission-able material is present.

Figure 3 shows a detailed cross-sectional view of aportion of the circulating pump. The pump is of com-mercial design and therefore no detailed description ofthe pump assembly is included herein. Referring toFigure 3, an impeller 56 is attached to a shaft 57. Theimpeller 56 is designed to draw tbe liquid fuel downfrom the draft tube 53 into the area below the flowdirecting element 55 and force the liquid upwardly intochannel 58. The pump is inserted through pump aper-ture 30. The motor is of the sealed rotor construction,designed to take up to 10,000 p.s.i. pressure. The bear-ings are of the liquid floating type. A small integral im-peller circulates a lubricant and also COOISthe bearings.The stator is cooled by water circulating in the tubularelectrical conductors. A labyrinth type seal is providedto reduce mixing between the hot radioactive liquid fuelin the vessel and the similar low temperature liquid flow-ing in the pump circulation cooling system.

The critical region 21, heat exchanger region 22, andthe fuel reservoir 23 of the reactor vessel 25 are sur-rounded by a retort jacket assembly 60. The jacketassemblv contains insulation to minimize the temperature.

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6gradient between the vessel and the surrounding watershield during normal operation, and cooling coils to takecare of additional gamma heating resulting from short,higher than normal power runs or errors in calculations.Electrical heaters may also be incorporated in the retortjacket assembly for initially heating the soup shouldthis be required for start-up procedures.

LIQUID FUEL SYSTEM

The preferred liquid fuels for use in the present inven-tion are solutions of enriched uranium phosphate inphosphoric acid and water, although other Iiquid fuelshaving similar characteristics may be used. The pre-ferred solutions include uranyl phosphate and uranousphosphate in phosphoric acid and water, i.e., U(VI) andU(IV), respectively. The uranium is preferably en-riched in the isotope UZSSto a value of about 90 percent,however other enrichments of U~35 or Uzss, as well asD20 or mixtures of H20 and D20, may be utilized inthe liquid fuels of the present invention, The accom-panying drawings, Figures 4 through 12, illustrate someof the properties of these solutions. with particular ref.erence to Figure 4, there is shown the dependence ofthe relative volume of the liquid phase in percent of thetotal volume of the vessel upon the temperature indegrees C. for the solution of 0.491 M U(VI) as UO~in 7.5 M H!P04.

Curve 63 at an initiaf filling of 52 percent shows thatat increasing temperatures the relative volume of thevapor phase tends to level off, i.e., the liquid does notexpand sufficiently to fill the entire volume of the vessel.However, this leveling off is dependent upon initialfilling.

Curve 64 at an initial filling of 60.5 percent showsthat the liquid expands with increasing temperaturethereby filling a greater percentage of the total volumeuntil at a temperature of about 5250 C. the meniscusdisappears. This phenomenon is interpreted to meanthat at the critical temperature, i.e., the point where themeniscus disappears, the uranium becomes soluble inthe gas phase in the upper portion of the containerformerly occupied by vapor only. This amounts to asudden dilution of the uranium at this transition andthe reactor would become subcritical. Thus for theparticular solution and percent initial filling the maximumoperating temperature could be built into the reactor,thereby controlling the reactor.

Curve 65 with an initial filling of 61.8 percent showsthat for the particular sohrtion the phase critical phenom-enon is no longer present. Thns such a solution fillingcould be utilized in a reactor where it was consideredundesirable to have the phase critical phenomenon pres-ent in the reactor system. A simiIar effect is obtainedby increasing the phosphoric acid concentration asdescribed below.

Curve 66 shows the effect of a greater initial fillingon the maximum operating temperature. As can beseen by comparing curves 65 and 66, the effect of anincrease in initial filling in this range of about 5.2percent decreases the temperature at which the entirevolume is occupied by the liquid phase from about500° C. to about 450” C. In this manner the maximumdesired operating temperature can be built into the re-actor by varying the initial filling.

Figure 5 shows the effect of varying the concentrationof phosphoric acid with approximately constant uraniumconcentration and initial degree of filling wherein theabscissa and ordinate are the same as Figure 4.

Curve 67, for a solution of 0.483 M U(VI) as U03in 4.10 M H3P04 with an initial filling of 59.3 percentwhich is approximately equal to the filling for curve 64of Figure 4, shows that the effect of a decrease in theconcentration of phosphoric acid for approximately thesame uranium concentration results in the phase criticalphenomenon becoming more pronomced, appearing at

2,902,424

7 8a considerably lower temperature, and having a negative Curve 77 shows the relation between total uraniumslope portion. This is due principally to the lower con- molarity and the least percentage of filling requiredcetttration of phosphoric acid. to avoid the maximum in the relative volume curve

Curve 68, for a solution of 0.462 M U(VI) as U03 (compare curves 64 and 67) before the meniscus dis-in 5.61 M H3P04 with an initiaf filling of 60.1 percent 5 appears. This curye..ts fur a constant. phosphoric acidmay be compared with curve 66, 0.491 M U(VI) as molarity of 5.6. Points slightly above the curve giveU03 in 7.5 M H3P04. An increase in phosphoric acid the phase critical phenomenon without a maximum. inconcentration, i.e., from the solution of curve 68 to that the curve. Thus, for a reactor wherein the phase criticalof curve 66 results in a solution which has a relative phenomenon is not to be utilized, the combinations ofvolume of the liquid phase of 100 percent at about the 10 uraoiu.rn molarity and percentage fdfing which are com-same temperature as does a solution with lower initial sidcrably above the curve must be utilized. Further, itfilling and lower concentration of uranium and phos- is apparent that a minimum fi!ling of 55 percent of thephoric acid. totaJ volume is required to avoid the phase critical

Curve 69, for a solution of 0.480 M U(VI) as U03 phenomenon with a maximum even with no uranium,in 12.7 M H3P04 at an initial filling of 60.3 percent, in 15 Cu.rye 78 is rel.at@ to curve 77 in that in. the soh~-comparison with curves 67 and 68, shows that the gcn- tions of crmve 78 the uranium molarity is held. con-era! effect of increasing the phosphoric acid concentra- stant and the effect on the minimum percentage fillingtion is to materially reduce the relative volume of the to avoid the no-maximum phase critical phenomenonliquid phase at a given temperature and for a particular of variations in the phosphoric acid m.olarity are shown,initial filling. Thus the expansion of the solution is also 20 The critical filling required for a constant uraniumrelated to the phosphoric acid concentration. Such a molarity of 0.48 is approximately 59 percent. Thusrelation enables a. determination of the percentage of the phosphoric acid concentration does not appear tothe volume of the reactor which contains liquid fuel appreciably affect tie existence of the phase criticalto be made by remote temperature indicating devices. point, although the temperature at which it takes place is

Referring now to Figure 6, a series of curves is shown 25 affected. Similar curves for other uranium molaritifiindicating the relation between temperature and the can be worked out by skill of the art techniques.relative volume occupied by the liquid phase, in percent The series of curves 79 through 82 depict the relation-of the total volume for enriched uranium (IV) in the ship between temperature and relative volume of theform of dissolved UOZ. liquid phase in percent for two specific solutions with

Curve 70, for a solution of 0.4 M U (IV) as UOZ 30 and without the use of an atmosphere of gas over thein 17.8 M H3P01 with an initial filling of 62.2 percent solution. Referring in particular to curves 79 and 8Qshows that the uranous system as compared with the for a solution of 0.462 M U(VI) as U03 in 5.61 Muranyl system exhibits the property that a higher phos- H~P04 and of 0.45 M U(VI) as UC)3 in 5.56 M H~P04,phoric acid concentration materially reduces the expan- respectively, and an initial filling of about 62 percent, itsion of the solution over the same temperature range. 35 is seen that the solution of curve 80 reaches a higher

Curve 71, for a solution of 0.40 M U(IV) as U02 temperature at 100 percent liquid volume than does thein 14.1 M H3P0A with an initial filling of 64.8 percent solution of curve 79. This change does not result merelyshows the same properties as curve 70 and has the from the minor changes in concentration, but is a resultsame general curvature. However, in the case of curve of the utilization of a 200 p.s.i. overpressure of oxygen71 a hydrogen-oxygen recombination catalyst, copper, 40 over the solution of cume 80. This overpressure ofhas been added in the form of 0.10 M Cu as oxygen is used to help prevent corrosion to the reactor

CU3(P04)2.3HZ0vessel and also functions to keep the uranium in thepreferred valence state during the operation of the

This would make it possible to operate at somewhat reactor, as is explained in more detail hereinafter.

lower temperatures if required, since the recombination 45 As can be seen by comparing curves 81 and 82 this

rate would be higher with the catalyst. overpressure also increases the temperature at which

Curve 72, for a solution of 0.364 M U(IV) as IJ02 the phase critical point phenomenon occurs, i.e., 479”in 16.3 M H~POA, with an initial filling of 65.4 percent, C. for the solution of curve 81 and 4910 C. for the

follows the same general curvature as 70 except that in solution of curve 82.

the higher temperature ranges the expansion is relatively 50 In the case of the uranous system, i.e., the tetravalent

larger. system, an overpressure of hydrogen is utilized which hasCurve 73, for a solution of 0.40 M U (IV) as U02 the same general effect as oxygen does for the uranyl

in 14.1 M H3P04 with an initial filling of 73.1 percent, system, i.e., prevents corrosion and aids in maintainingshows that in the temperature range up to 6000 C. the uranium in the proper valence state.this initial filling percentage of about 73 percent is about 55 Figure 8 shows a series of curves for the uranyl sys-minimum if the liquid is to occupy the entire volume. tern wherein the vapor pressure is plotted against tem-

Curve 74 is for the same solution as curve 71, only perature for various concentrations and percent initialthe initial filling percentages being different. It sliould fillings. Specifically, cw-ve 83 is for a solution of 0.764be noted that curve 73, for a solution witbout a recom- M U(VI) as U03 in 5.28 M H3P04 and an initial fillingbination catalyst, and curves 71 and 74 have the same 60 of 51.6 Percent; cuwe 84 is for a solution of 0.309 Mgeneral curvature, and that no adverse effect on the U(VI) as U03 in 2.90 M H3P04 with an initial fillingexpansion of the liquid results from the use of such of 58 percent; curve 85 is for a solution of 0.76 N?recombination catalysts. U(VI) as U03 in 5.28 M H3P04 with an initial filling of

A solution of 0.343 M U(IV) as UOZ in 15.4 M 58 percent; and curve 86 is for a solution of 0.75 MH3P04 with rur initial filling of 78.1 percent has the same 65 U(VI) as UG in 7.50 M H3P04 with an initial filling

general properties as the solutions of curves 73 and 74. of 58 percent.Referring now to Figure 7, several graphs indicate Comparing curves 83 and 85 it can be seen that for

additional properties of the uranyl system. essentially the same solution the vapor pressure is re-Specifically, curve 76 shows the relation between lated to the initial filling percentage. Comparing curves

phosphoric acid molarity and the temperature at which 70 84, 85, and 86, it is apparent that increases in the phos-the meniscus disappears, i.e., the phase critical point phone acid concentration result in lower vapor prea-temperature, This curve is for a constant uranium sures for a specific temperature. Thus the higher themolarity of 0.48. Thus the general increase in the phosphoric acid concentration the lower the internal re-phase critical point temperature with increasing phos- actor pressure.phqric ac~d m,olarity is apparent. 76 T@s it, is deskqble.to..o~@ip. w high a concePt$a@q, 0$

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@~cmp~oric acid as is possible. With the uranyl systemit is progressively more difficult to keep the uranium insolution as the phosphoric acid concentration is increased.However, in the uranous system the opposite is true, i.e.,it is at the lower concentrations of phosphoric acid thatdifficulty is encountered in keeping the uranium in sohr-tion.

Specifically it has been found that about 0.6 M U(VI)as U03 is soluble in from about 3 M H~POA up to ap-proximately 7.5 M HtP04. However, in the uranous sys-tem, U(IV), with uranium of about 0.4 molarity, thenrainum is soluble from 99.9 percent effectively 100 per-cent, H~P04, i.e., 18 M H3P04, down to about 15 molarOr 90 percent H8P04. In the intermediate range of 7.5to 15 M H3P04 the properties of the solutions are similarexcept for the volubility of the particular valence state.

The vapor pressure curves for the uranous system areshown in Figure 9. In this figure the vapor pressure isplotted as function of temperature for 0.5 M U(IV) inthe form of U02 with an initial tilfing of 62 percent.Curve 87 represents the variations in vapor pressure foran 85 percent concentration of phosphoric acid, i.e.,14.0 M H3P04. Curve 88 is for a 95.7 percent concen-tration or 16.7 M .H3P04, and curve 89 is for a 100.7percent concentration or 18.3 M H3P04. It is apparentfrom these three curves that increasing the phosphoricacid concentration lowers the vapor pressure at a givenoperating temperature.

The series of curves in Figure 10 are similar to thoseof Figure 7 except that the temperature is held constantfor each curve and the concentration of phosphoric acidis varied.

Curves 90 through 95 are for solutions of 0.5 M U(IV)in the form UOZ with initial fillings in all cases of 62percent and for temperatures 300”, 400”, 4500, 5000and 600°, respectively. It is apparent from the series ofcurves that increasing phosphoric acid concentration hasa greater effect in reducing the vapor pressure of the solu-tion as the temperature is increased.

As was pointed out hereinbefore, one of the outstand-ing advantages of the enriched uranium-phosphoric acidand water systems is that there is little net radiolytic gosproduction, when operated at high temperature, i.e., thegases are recombined without the necessity for conven-tional catalytic recombining apparatus. Figure 11 showsa plot of the recombination rate constants as kr as afunction of temperature for the uranyl and uranous sys-tem. The .recom.bination rate constant is defined as thefractional recombination per hour. Curve 96 is for asolution of 0.5 M U(IV) in 2.9 M H3P04 but curve 97is for a solution of 0.5 M U(IV) in 95.7 percent or16.7 M H3P04. It can be seen that for the dilute sohL-tion, curve 96, the recombination rate is a rapidly vary-ing function of temperature, while for the concentratedsolution, curve 97, this variation is not as rapid. Further-more, the dilute solution requires a minimum operatingtemperature of about 300” C. before the recombinationrate is sufficient at one megawatt of power. However,in the concentrated phosphoric acid solutions the recom-bination rate .is considerable, even as low as room tem-perature.

However, the recombination r,ate is dependent uponpressure, i.e., a certain equilibrium pressure must bereached before the recombination of gas is equal to theproduction. Figure 12 shows the variation of equilib-rium total pressure with increasing temperature. Curve98 shows the comparative variation in equilibrium pres-sure for the uranyl system, i.e., dilute phosphoric acidsohrtions, specifically 0.5 M U(VI) aa U03 in 7.5M HaP04, and curve 99 shows this variation for the con-centrated solutions, specifically 0.5 M U(IV) as U02“in 17.7 M H304, for a power level of one megawatt.

Figure 13 shows the variation of total pressure in thereactor ~s a function of temperature for a solution of0,5 M U(IV) “as U02 in 7.5 M H3P04 with 0.001 M Cu

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-10added. Curve 100 represents the totai pf’esshft iii dpower Level of 1 megawatt while curve 101 is for a powerlevel of 30 kw. It is apparent that at temperatures at orabove 4000 C. the total pressure in the reactor vesselfor the two power levels is about the same. As evidentfrom Figure 12, a graph for the uranous systems, i.e.,U02 would have considerably lower total pressures. Seeco-pending application Seriaf No. 589,835, filed June 6,1956, entitled Nuclear Reactor Fuel System, by BurtonJ. Thamer et al. for further explanation of the charac-teristics of the preferred liquid fuels.

Thus it is apparent that the liquid fuels utilized in thepreferred embodiment of the reactor of the present in-vention have the following characteristics:

(1) The expansion of the solution is dependent uponthe percentage of the total vessel volume which is ini-tially filled with the solution.

(2) A certain minimum initial filling is required inorder to obtain 100 percent of the volume to be occupiedby the liquid phase.

(3) Certain concentrations and initial filling percent-ages exhibit a phase critical phenomenon.

(4) The general effect of increased initial filling is toreduce the temperature at which the entire volume isoccupied by the liquid phase.

(5) Decreases in phosphoric acid concentration gen-erally moves the point of 100 percent liquid volume to alower temperature. The expansion of the solution istherefore related to the phosphoric acid concentrationfor a given temperature range, i.e., higher phosphoric acidconcentration reduces the solution expansion.

(6) The presence of a recombination catalyst does notmaterially effect the properties of the solutions,

(7) Increasing the phosphoric acid concentration in-creases the temperature at which the phase critical pointis exhibited.

(8) There is a miriimum and maximum percentageinitial filling between which the phase critical phenomenonwill take place for any solution.

(9) The phase critical point is related to uraniummolarity, phosphoric acid molarity and percent initialtilling.

( 10) The vapor pressure is related to the percent ofinitial tilling and to the concentration of phosphoric acid.The higher the phosphoric acid concentration the lowerthe vapor pressure.

(11 ) The internal gas recombination rate for the U-(IV) system is materially greater than the U(VI) system.

Thus, the selection of a liquid fuel for a particularreactor will require the selection of the uranium con-centration, the phosphoric acid concentration, the initialfilling percentage, the desired operating vapor pressure,the operating temperature, the recombination rate, andwhether it is desirable to operate in the phase criticalregion.

For example, if the phase critical phenomenon is notto be utilized and an operating temperature of 450” C.is desired the solution of curve 65 may be utilized. Thisparticular solution requires an initial filling of 61.8 per-cent of the total volume to be occupied by the liquid fuel.The phase critical phenomenon is avoided since the ini-tial filling is appreciably greater than 59 percent as indi:cated by curve 78 of Figure 7, and for a uranium molarityof 0.491 the initial filling is considerably above curve 77.The vapor pressure for this solution at 450” C. wouldbe similar to curve 86 of Figure 8, and the recombina-tion rate constant, see Figure 11, would be of the orderof 5, with a total vapor pressure at one megawatt ofabout 5000 p.s.i. (see Figure 12). If a lower vapor pres-sure is desired, then the uranous, U (IV), system may beutilized which may require a higher phosphoric acidmolarity.

The effect on the reproduction factor k on reactortemperature and initial fillirw is shown in Figure 14.Cu~es 102, 103, and 104 sho~ respectively the ~empera-

11t~e dependence, for solutions of 0.6 M U(IV) as in 5.6M HxP04 in a cylindrical critical region 15“ diameter~d 15” high, for initial fillings of 58, 59, and 60 per-cent, respectively. The dotted curve 105 is for 59 per-cent initial filling but shows the slight correction resultingfrom the expansion of the vessel. Curve 106 representsthe increase in reactivity due to the baffle 45. It is thusapparent that the IxdlIe has little effect on the reactivity.It is apparent from Figure 14 that at about 4300 C. the. re-production constant is approximately 1.00. It should benoted that over the temperature range of from room tem-perature to about 300” C. for the cylindrical shaped ves-sel, there is a positive temperature coefficient of reactivitywhile beyond this point the coefficient becomes negative.Thus, a sufficient number of control rods should be pres-ent so that an equivalent of at least about .08 k can beinserted in cases of emergency shutdowns. However, thepositive temperature coefficient may be overcome bychanging the vessel geometry in the critical region fromcylindrical to a geometry wherein the walls are slightlyconcave inwardly so that there is a geometry compensat-ion for the positive temperature coefficient of reactivity.

Critical region

The critical region lies between two noncritical regions,a vapor region above and a heat exchanger region below.Both of these latter regions, as well as the fuel reservoirbelow the heat exchanger region, are maintained sub-mitical by poison and poor geometry. The selected liquidfuel, at its elevated operating temperature, completelyfills the reactor core region and makes it just critical.Thus only a portion of the critical region is filled initiallywith the cold fuel solution. The portion filled is deter-mined by the percentage initial filling which in turn isdependent on various factors as described hereinbefore.For a particular filling, 59 percent for example, the liq-uid level would be in the lower portion of the criticalregion 21.

The quantity of fuel inserted in the initial filling isreferred to herein as the cold critical volume, that is,when the liquid fuel level is in the lower portion of cri-tical region 21 as shown by the numeral 125 in Fig. 1.Thus, when the cold critical volume of fuel is introducedinto the reactor vessel the fuel reservoir 23 and heatexchanger region 22 are completely filled with liquid fuelwhile the critical region 21 is only partially filled. Asthe cold critical volume of fuel is heated by nuclearreaction, as explained hereinafter, the liquid fuel expandsuntil the critical region 21 is completely filled, the fuellevel being indicated by numeral 126 in Fig. 1. Thisincreased volume of fuel is called the hot critical volumeof fuel. The reactor, when containing the hot oriticalvolume of fuel, is considered then to be “hot critical,”a critical assembly having been created.

The critical region for the preferred embodiment ofthe present invention is a right circular cylinder havinga 15” diameter and 16“ height. The necessary condi-tions for criticality may be calculated by methods well-known in the art with consideration being given to theparticular factors described above in the section “Liquidfuel system.”

Fuel handling system

The liquid fucI handling facilities are shown sche-matically in Figure 15. The vessel 25 has a solutiontransfer line 107 extending to the bottom of the fuelreservoir. Transfer line 107 is connected through awater cooling jacket 108 and a valve 109 to sampling line110, which is connected to conventional sampling appa-ratus 111, and to pipe 112. The pipe 112 is connectedto the bottom of metering, non-critical reservoir tank 114.The liquid level in the metering tank 114 indicates the~ve~ of the liquid fuel in the reactor vessel. The maxi-

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12mum solution removal rate, with the reactor at full pres-sure, is 6 liter/rein. since transfer lines 107 and pipe112 are % I.D. pipe. This removal rate permits coolingof the soup by cooling jacket 108 from the fuel operat-ing temperature of about 4500 C. to less than 100” C.In this manner the corrosive effect on the apparatusbeyond the cooling jacket 108 is materially reduced anclthere is no need for precious metal cladding or plating toprotect the pipes, valves and other components.

Connected to the top of metering tank 114 is a pres-sure line 113 connected t~ a gas pressure supply 115.Pressure from supply 115 forces the liquid fuel into tbereactor vessel 2S.

An emergency dump line 116 is provided and extendsinto reactor vessel 25 to the level of the heat exchanger,i.e., below the level of the critical region. A rupture disk117 is provided in line 116 which is set to reiem.e the solu-tion when the vessel pressure reaches 7500 p.s.i. The re-lease of rupture disk 117 permits the liquid fuel to flowout of the reactor to an emergency dump tank 118,which is of a non-critical geometry and is located i.n ashielded remote place. The air in the vessel is replacedwith an over-pressure of the desired gas through gas tube119 which is connected to a system 120 which includesa vacuum pump and a source of the desired gas.

All components of the liquid fuel handling facilitiesare chos$n to provide an ever-safe geometry for the liquidfuel.

Safety circuits

The control rod and the safety rods are enriched boronrods which move inside the platinum-clad heavy walledstainless steel thimbles 44 and 49, respectively, as shownin Figure 2. The safety rods are about one-half inchin diameter and extend through the critical region only.The central or control rod is about 0.75 inch in diameter.The region of thimble 44 which lies below the heat ex-changer serves as a container for part of the fuel reser-voir poison. This poison, although removable, is notconnected to that portion of the control rod which ismovable into and out of the critical region.

The control rod mechanisms and safety circuits aresimilar to those of the prior art, see Principles of NuclearReactor Engineering, Samuel Glasstone, chapter VI (D.Van Nostrand & Co., 1955). In general, the controlrods are moved in their vertical thimbles by two-phase,two-pole induction motors. The motors are controlledby level switches. The rods are attached to the withdraw-ing mechanism through D.-C. lifting magnets which arede-energized during a “scram” to allow the rods to failfreely into the reactor under the acceleration of gravity.Each rod hanger actuates a limit switch in the full-inand full-out position, this information being displayed ona control console.

Any leaks in the reactor vessel, abnormally high pres-sure in the steam line, power failure, excessive soup tem-perature, circulating pump leak, or failure of the feed-water pump, will automatically result in all safety andcontrol rods being released.

The above-described components and circuits are well-known in the art and are therefore not illustrated in thedrawings.

Fuel circulation

The liquid fuel circulation cycle for the illustrated re-actor is shown in Fig. 1. In general, the fuel is circu-lated by the impeller 56 upwardly into channel 121 ex-tending between the walls of vessel 25 and the outer sur-face of funnel 51, through the heat exchanger region 22,the critical region 21, and ~nto the flow-directing surfaceof baffle 45 where the direction of flow is reversed, thefuel then flowing downwardly through channel 122 de- —fined by funnel 51, where it is again agitated by impeller56.

A significant contribution to the criticality of the re-actor is rn~de by the liquid fuel as it circulate throu@

13thediacritical diameter section 28 of the reactor vessel.The boron in poison reservoir 54 absorbs a portion ofthe emitted neutrons from the fuel circulating throughthd reduced diameter section 29 of the reactor vessel,thereby reducing the reproduction factor to a valuebelow unity. In addition, the reduced diameter of sec-tion 29 also contributes to the reduction of the reproduc-tion factor in that section.

At normal operating temperatures, about 450” C., forthe illustrative example, there is no temperature differen-tial between the liquid fuel in the fuel reservoir and theliquid fuel in the critical region. Thus, the liquid fuelmay be circulated in a direction opposite to that shown inFigure 1, if this is desirable and the circuiting pump ischanged.

Circulation of the liquid fuel reduces the number ofdelayed neutrons which are emitted in the criticaf region.For a circulation rate which changes the solution in thecritical region twice a second, the reactivity differencebetween delayed and prompt critical is approximately 51percent as large as it is without circulation. Thus thereproduction constant is reduced approximately 0.4 per-cent by virtue of the removaI of the delayed neutronsfrom the hot critical region by the circulating apparatus.

Operation

The start-up operation of the reactor of the present in-vention is as follows: The reactor vessel is evacuated bysystem 120 and the overpressure gas is admitted to thevessel, i.e., oxygen o: hydrogen, so that at operating pres-sure the proper overpressure, i.e., 200 p.s.i. will be pres-ent. Vafve 109 is opened. Gas pressure, 150 p.s.i. ofoxygen, flows from source 115 through Iine 113 intoreservoir 114 thereby forcing the liquid fuel through trans-fer line 107 into the reactor vessel at a rate of about oneliter per minute. The amount of solution transferred tovessel 25 depends upon the percentage initial filling re-quired for the particular liquid fuel and operating con-ditions. The amount required for any particular solu-tion, i.e., the initial filling percentage, has been defined asthe cold critical volume. During the liquid fuel additionat least some of the control rods and/or safety rods arein their out position so that shutdcwn can be effected ifthe counting rates are too high or if the reactor shouldsuddenly go critical. For the particular liquid fuel beingused, cold critical with the remaining rods in should bereached when the liquid fuel reaches a level about8 inches above the heat exchanger. All valves to thereactor are closed.

As the remaining rods are removed the core region ofthe vessel becomes supercriticrd. The liquid fuel in thecore will be heated by the nuclear reaction. The remain-der of the liquid fuel will be heated to a uniform tem-perature by convection circulation. The control andsafety rods in their out position extend into the vapor re-gion to poison this region. However if the vaporized fuelin the vapor region is non-critical by geometry the rodsmay be removed from the vapor region. As the liquidfuel in the entire vessel heats it will expand to its hot criti-cal volume, thereby filling the entire critical region. Asthe fuel level rises from its initial position 125, as shownin Fig. 1, which is the liquid level at the cold criticalvolume of fuel, to the IeveI at the hot critical vohrme,shown as 126 in Fig. 1, the liquid forces vapor and gasespresent above the liquid upwardly through spaces in baffle45 and around the edges of bafile 45 through whichthimbles 49 pass, into the vapor region 20 above thebaffle 45.

The circulating pump is then turned on and the waterflow rate through the heat exchanger is increased untilthe desired power extraction rate is reached. The liquidfuel will be circulated up channel 121 around the heatexchanger 52 into the critical region and down channel122. The specific reactor described at the prescribedoperating temperature will develop about 2 megawatts of

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14heat. The internal pressure will be less than about ~otl~p.s.i.

Thus it is apparent that the reactor of the present in-vention has a novel arrangement and association of com-ponents which results in added safety and ease of controLThe utilization of a fuel reservoir provides a volume ofliquid fuel which when heated will expand into the criticalregion. Thus, it is possible with the reactor of the pres-ent invention to accomplish a complete shutdown of thereactor without requiring large control of reactivity, suchas, numerous rods or removing liquid fuel from the re-actor vessel. The solution will contract and upon cocrl-ing to room temperature the critical region is no longertiflecf with Iiquid fueL In this manner there is never apossibility of a large excess reactivity if the circulatingpump is not turned on until after the solution has heatedto operating temperature.

Although a particular embodiment of the present in-vention has been described it is apparent that numerousmodifications may be made without departing from itsscope. Thus, if a research reactor of a moderately highneutron flux is required, the reactor described may bemodified by omitting the circulating pump and relyingon convection circulation for heat extraction purposes.Furthermore, other liquid fuels than the ones describedmay be used. The only requirement, as to liquid fuelsto be used with the present invention, is that they have anegative temperature coefficient of reactivity, i.e., thatthey expand upon heating. Thus, other fuels having ap-propriate expansion coefficients and similar characteris-tics may be used in the present invention. Therefore,the reactor of the present invention is not limited to thespecific embodiment disclosed but only by the appendedclaims.

What is claimed is:A homogeneous nuclear power reactor comprising a

vertical cylindrical sealed reactor vessel having an upperportion, an upper-intermediate portion, a lower-interme-diate portion, and a lower portion, the upper end of saidupper portion terminating in a removable manifold plate,a fuel flow-directing baffie supported in the uppermostpart of said upper-intermediate portion, the lower sur-faces of said flow-directing baffle being annular troughshaped to alter the direction of flow of liquid fuel imping-ing thereon, said flowdirecting baffle having selectivelyspaced apertures therein to provide communication be-tween the upper and upper-intermediate portions of saidreactor vessel without substantially affecting the flowdi-recting characteristics of said baffle, heat exchanger meanssupported in said Iower-intermediate portion, fuel circu-lation means supported at the bottom of said lower por-tion, said vessel containing a quantity of liquid fuel whichduring normal reactor operation at a predetermined op-erating temperature and pressure is sufficient to fill saidlower, lower-intermediate, and upper-intermediate por-tions, said liquid fuel comprising an aqueous solution con-taining a sufficient concentration of a fissionable isotope toconstitute a critical assembly when substantially filling theIower, lower-intermediate, and upper-intermediate por-tions of said vessel at said operating temperature where-by a critical region is established in said upper-intermedi-ate portion, said critical region being boiinded on the bot-tom by the top of said heat exchanger means, on the sidesby the walls of said reactor vessel and on the top by saidflow-directing baffle, substantially cylindrical means co-axially supported in the lower, lower-intermediate, andupper-intermediate portions of said vessel for directingforced circulation of said liquid fuel whereby when saidfuel is agitated by said fuel circulation means the fuelis circulated upwardly along the outside of said cylindri-cal means, passing through said lower portion, around saidheat exchanger means in said lower-intermediate portion,and up through said upper-intermediate portion where itimpinges on said flow-directing baffle and is redkecteddownwardly through said cylindrical flowdirecting means

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15 16t~.said fuel circulation means at the bottom of said re- OTHER REFERENCESactor vessel. U.S. Atomic Energy Commission, L. D. P. King, LA-

1942, April 13, 1955, pages 4-17. Copy can be securedReferences Cited in the file of this patent from Technical Information Services, OalC Ridge, Term.

5UNITED STATES PATENTS

Proceedings of the International Conference on thePeaceful Use of Atomic Energy, vol. 3, pp. 175-187,

2,820,753 Miller et al. ------------ Jan. 21, 1958 263–282, 283–286, August 1955, United Nations, N.Y.