CA1155565A - Production of tritium - Google Patents

Abstract

PRODUCTION OF TRITIUM
ABSTRACT
Tritium and electricity are co-generated in an HTGR. Nuclear fuel material and target particles are disposed in chambers formed in a body of refractory material, such as graphite. The target particles comprise minute cores of a solid lithium compound, e.g., LiA1O2, an inner coating of porous material and an outer gas-tight coating of relatively dense material.
Irradiation of the target particles by thermal neutrons results in transmutation of Li6 nuclides to create helium and tritium that is retained within said outer gas-tight coating, which preferably includes a relatively thick layer of SiC or ZrC. Tritium is recovered by heating the irradiated target particles to a fairly high temperature or by crushing the SiC or ZrC
coating layers.

Description

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PRoc)uc~rI ON OF TRI TI UM
BACKGRC)UND OF THE INVENTION
This invention relates to the production of trltium and more particularly to the economical production of tritium in a power-generating nuclear reactor.
Tritium has generally been produced from the lithium isotope of the mass 6 by the absorption of slow neutrons and the resultant transmutation to produce an atom of tritium and an alpha particle (i.e., helium : nucleus~, which is referrPd to as an (n~cx~ ~ raaction.
~ithium naturally contai~s about 7.49~ o the isotope Li and the remainder of the isotope Li .
Commercial production has been carried out using lithium in ~ts natural or enriched form within a sealed, doub~le-wall container 1hat is hydrogen impermeable which ~s lo~ated in the core of a plut~nium production ~ reactor. Lithium has been used in the form of an alloy : with magnesium or aluminum a~d has also been used in oxide form. These large wa~er~cooled reactors, which have been either graphite-moderated or heavy ~ater-moderated, produce tri~ium essentially as a by~product of plutonium production, and as a result of the e~er-increasing demand for tritium ~which will be magnified substantially when fusion technology ~ecomes economically feasible), additional economic me~hods for the production of tritium are deslrable.
RIEF SUMMP~RY OF THE INVENTION
i I~ has been found that high temperature gas cooled graphite-moderated reactors ~HTGRs~ offer a unlque opportunity for the dual production of tritium and usable electric power, uslng today's technology.
These reactors inherently have a relatively high converslon ratlo which makes excess neutron~ available 3S ~or breedin~ fertile material from ~is~i}e material and ~.~

1 1555~5 . .

-2-have hereto~ore been employed to breed uranlum-233 from ~horium. By using minute coated particles containing lit~ium, which serve as individual pressure vessels that retain tritium, as target material within the fuel elements that constitute the core of a gas-cooled graphite-moderated nuclear reactor, the p~roduction o txitium can be carried out without disrupting the power-generating function of such a nuclear reactor and without the creation of significant safety hazards.
Substantially all of the tritium created in the lithium t~rget particles is retained therewithin; however, ~hould minor amounts of tritium escape outside the coatings, it is recoverable from the primary gas coolant stream. Following the fuel lifetime of the ~uel ~lements, the tri~ium is recovered. Usually the fuel elements are first removed from th0 reactor, and the tritium is then recovered by heating in a facility designed for that purpose. The recovery o~ the tritium may also be carried out as a part of the reprocessing of the overall fuel element and the reclaiming of the remaining fissile uranium.
BRI F DESCRIPTION OF_THE DRAWINGS
FIGURE 1 is a perspective view of a fuel element designed for use in the prismatic core of a gas-cooled nuclear reactor which may be employed for the production o~ tritium;
FIGURE 2 is a diagrammatic view showing a nuclear reactor including a reactor core formed of fuel elements of the type shown ln FIG~RE l;
FIGURE 3 is a ~ragmentary sectional view throuqh one o~ the fuel elements shown in FIGURE 1 FIGURE 4 is a fragmentary sectional view of an alternative embodiment of a fuel element similar to FIGURE 3; and - FX~URB 5 is a view, enlarged in s~ze, of a ~ ......
, . ~ .

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3--target particle of the type whlch may be employed in the ~uel element of FIGURE 1.
DETAI~,ED DESCRIPTION OF T~E PREFERRED EMBODIMENTS

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Gas-cooled high temperature graphite moderated nuclear reactors have been developed which utilize heat produced by nuclear fission to produce steam in steam - ~enerators which is then used to drive electricity-producing turbines and also to provide heat or other applications. Such reactors have utilized cores made up of prismatic block-type fuel elements which contain coolant hvles that extend axially therethrough and that are spaced between parallel, elongated fuel chambers, and have also utilized'cores made up of a ~ed of sp~erical graphi~e balls which are located randomly within a pressure vessel and provide passageways for coolant gas through the interstices of the bed (which is generally referred to as a Pebble-Bed reactor core). Particulate nuclear fuel for such fuel elements has been provided in the form of minute particles having coa~ings which ~unction as fission-product-retentive pressure vessels. Such fuel particles include a core of fissile material, e.g., uranium enriched in isotope-235 in carbide form or in oxide form or as a mixture of uranium carbide and uranium oxide. Such cores often also include thorium as a diluent and fertile materia}. Examples of such nuclear fuel particles are described in detail in U.S.
Patent No. 3,649,~52 issued March 14, 1972, to Jack Chin, et al., the disclosure o~ which is incorporated herein by reference. Such nuclear fuel particle designs have been found to be particularly well-suited for gas-cooled nuclear reactorsO
Illustrated in the drawings is a gas-cooled nuclear reactor of the prismatic core type which utillzes ~uel elements 11 in the form of graphite blocks . .

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13 havlng hexagonal top and bottom suraces lS
interconnected by perpendicular side faces 19 as shown ~n FIGURE 1. The nuclear reactor core 21 is formed from a plurality of vertical columns o such fuel elements ~1 s~acked one atop another, arranged within a pressure vessel 23 as illustrated in FIGURE 2. The fuel element block 13 ls formed with a plurality of coolant holes 25 located on a constant ~rian~ular pitch. The coolant holes 25 extend from top to bottom axially through the blocks and provide the passageways for the gas coolant, preferably helium, to extract the heat ~rom the nuclear ~ission reactions~ ~uel chambers 27 of slightly lesser diameter than the coolant holes 25 are located in a triangular array of lesser pitch so ~hat each coolant hole is surrounded by a number of fuel chambers.
To ~acilitate alignment of the indivi~ual fuel ~- elements 11 in stacked columns, ~he blocks 13 are each provided with short pins 31 which protrude from the top end surface of each fuel element which are received in corresponding cavities provided in the bottom end surface. The pins 31 and cavities are aligned with individual coolant holes.
The reactor core 21 made up of these columns of fue~ elements 11 is located within a pressure vessel 23 o~ a pre- or post-stressed concrete or the like, which may llave a steel inner liner 33. A surrounding reflector 35 is prov~ded by un~fueled prismatic blocks of qraphite, as is well known in the nuclear reactor art. The primary helium coolant is circulated downward through the core 21 by blowers or circulators 37 which are preferably disposed in suitable cavities located ln the pressure vessel ~3. To conine the primary coolant stream to the pressure vessel 23, steam generators 39 are also provlded in cavit~es within the pressure vessel. The primary hellum coolant is accordingly . - .. - .
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1 1S~5~5 circulated by the blowers 37 ~hrough the reactor core 21 ~here it picks up heat and then through the steam generators 39 where it exchanges its heat with a secondary coolant stream in the form of water which is being turned to steam. The blowers 37 take suction from ! the steam generators 39 and circulate the primary coolant back downward through the core for another pass~
It has been found that only very minor modifications to an existing ~TGR design are needed when target particles containing Li6 are included along with nuclear fuel in the individual fuel elements ll.
The target particles are neutronically compatible with the nuclear ~ission reac~ion and can be employed in the place of burnable poison (e.g. boron) that would normally be included in such ~uel elements and also in place o~ most o~ the ~ertile thorium. In fuel elements ll of pr~smatic design, the fuel chambers 27 generally define the volume available for disposi~ion of nuclear fuel material. When an HTGR ~s used for the production of tritium, only approximately 3% to about 5~ of the volume of the fuel chambers is occupied by the target particle material, and the remainder o the available volume holds nuclear fuel material.
Li6 has an extremely large cross section, equal to about 953 barns, for the absorption o~ neu~rons in the thermal energy range and the consequent transmutation to produce tritium and helim. As a result, lithium i~ inherently self-shielding, and in order to induce efficient conversion of Li6 to tritium, it is important to dlsperse the lithium throughout the reactor core. Excellent dispersal is achieved by forming small cores or kernels of a Li ~ompound, having a size on the order of about 300 to lO00 microns, an~ spaclng these cores from one another by means of exterior coatings which totally surround the ~ 1~5~5 .

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cores. By locatin~ some of the target material in each o~ the fuel chambers 27, overall dispersion 19 still further enhanced.
. In the fuel element design depicted in FIGURE
3, short sticks or rods 41 of target particles joined together by sui~able bonding material, such as carbonized pitch, are located adjacent to .the top and bottom ends of each of the fuel chambers 27, with ~imllar rods 43 of nuclear fuel material filling up the lQ major portion of the fuel chambers 27 in the regions between the top and bottom target material rods. .
Graphlte plugs 45 close the upper end, and . heat-decomposable plastic spacers 47 are provided. This arrangement facilitates the selective recovery of tr~tium by physically slicing the upper and lower e~ds o~ ~he fuel elements 11 from the remainder and then reprocessing these sllced por~ions separately for the recovery of tritium, as discussed hereinafter. An : . alternative version of a fuel element 11' is illustrated in FIGURE 4 wherein the target material is ~ormed into : ~airl~ flat wafers 41' which are.then disposed alternately between adjacent nuclear fuel rods 43 t~roughout the entire length of each fuel chamber 27.
Such an arrangement facilitates even better dispersal 25 throughout the nuclear reactor.core; however, it.doe~
not facilitate selective tritium recovery by processing only portions of the indiv.ldual fuel elementsO
- .~. Examples of representa~ive target particles 51 . ~are. depicted in ~IGVRE S and ~nclude minute cores or Xernels 53, pre~erably spheroidal in shape, which are between about 300 and about 1000 ~m in diameter. The kernels 53 are surrounded by an inner coating region of a generally p~rous nature which acommodates the build-up o~ helium and tritium from the transmutation of the lithium and wbich is in turn ~urrounded by an outer ~ 15~5~

coatlng reglon which forms the gas-tlght barrier that prevents escape of the hellum and tritium.
The cores 53 are formed ~rom a solid compound of lithium which is preferably stable at the temperatures employed for the vapor-deposition of the surrounding coatings. Lithium in oxide form, elther by ltself or in a combination with another refractory-like element, may be employed as the kernel materials.
~xamples are lithium oxide, lithium aluminate ILiA102), and lithium silicates (Li2Sio3) ~Li4Sio4). The l~thium compound should have a melting point and other characteristics which render it compatible with the coating processes. It can be employed in any form in which the cores have sufficient mechanical stability to render them physically suitable to treatment in a vapor-deposition coater. For example, small kernels can be formed by a powder agglomeration process or by cold-pressing in steel dies and then ` ~intered to provide strength and higher density. For example, lithium aluminate powder can be cold-pressed in a 2ie at about 3000 psi and then sintered in a cacuum at ~bout 1200 C. for eight hours. Kernels made by powder agglomeration can also be sintered to provide mechanical ~trength. I~ high density is desired, the sintered kernels can be made spheroidal by be~ng dr~pped through a hot zone at between 180~ C. and 2200 C. to cause them to melt and densify into spheroidal shapes in accordance with known technology.
Generally, the kernels 53 are densified to at least about 70% of theoretical density. By theoretical den~ity i~ meant the maximum density for that part~cular stoichlometric compound. ~he preferred iithium compound is lithium aluminate ~hich has a theoretical density of about 2.55 grams per cm3~ Although densification to a 35 ~ density approaching theoretical density is possible, it -. ; ;"
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may be preferred to employ kernels 53 in the range of about 70% to ~0~ of theoretical density from the standpoint both of spa~ial dispersion and ultimate ~ccommodation of the gaseous products vf the lithium transmutation.
To prevent the reaction of lithium aluminate with subsequent vaporous materials to which there will be exposure durinq the coa~ing operations, an impervious carbon seal layer 55 is applied at a temperature below 1100 C. and preferably at a temperature not higher than about 1000 C. Such a seal coating may be depositecl in a particulate bed 1uidized by gas flow or in a rotating drum or other ~ype o~ agita~ed bed coater. Pyrocarbon seal layers should have a density of about 1.8 to 2.0 gram/cm3 and are preferably oriented. A thickness of ~bout 10 to 30 microns o such pyrocarbon prov'~es an adequate seal coating and can be deposited from a mixture of acetylene plus an inert gas such as argon~
~he porous layer 57 that is provided for the accommodation of the helium and tritium within the minute pressure vessels is preferably pyrocarbon having a density between about 0.9 and 1.2 gram/cm3. The thickness o the poro~s pyrocarbon layer 57 is dependent upon the amount of Li6 included within the kernels 53 and the pressure which the ou~er gas-tight coating is designed to wlthstand.
If there are no constraints on the amount of ~pace occupled by the target particles in the nuclear reactor core, larger amounts o~ porous material can be included so as to prevent the build-up of high gas pressures wi~.hin the gas-tight outer barrier. On the other handl if particular constraints llmit the amount o space, a lesser thickness of the porous pyrocarbon may be employed along with a slightly thicker outer coating, which will withstand the higher ga~ pres~ure .. .. _ , ~;

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bu~ld-up. In general, for cores in the 300 to 500 micron range, it is likely tha~ at least about 75 m~ crons of porous pyrocarbon would be used.
In the outer coating which provides the gas-tight barrier to prevent the escape o tritium, the key layer 59 is one of dense silicon carbide or zirconium carbide. The reactor may be operated so that the temperature of the target particles may be in the range of about 900 to 1000 C.~ at which both dense 10 silicon carbide and zirconium carbide provide an effective barrier to the passage of tritium. As ln any ~uch barrier material, the thicker ~he material, the more efective the barrier, and it is expected that at least about 50 microns of SiC or ZrC would be used~ For axample, a silicon carbide layer having a thickness of 9~ microns or even greater might be employed. The carbide barrier layer should ha~e a density of at least 95% ~ theoretical densityO Deposition of silicon ~arbide from a vaporous atmosphere can be ~airly readily carried out to produce deposits having greater than 99~
of theoretical density. For example, for SiC, which has a theoretical density of 3.22 grams/cm3, densities greater than 3.20 grams/cm3 can be achieved.
Disposed immediately interior and exterior of the carbide layer 59 ara layers 61, 63 of isotroPic pyrocarbon having a density between about 1.7 and about 2.0 grams/cm3 and havin~ individua} thicknesses of between about 35 and 45 microns. Such isotropic ~oatings are deposited from a mixture of acetylene, propylene and inert qas at a temperature of about 1350 C. which have a BAF IBacon Anlsotrophy Factor) of less than about 1.05. The interlor pyrocarbon layer 61 retards the outward diffusion of mater$als from the core to the silicon carbide during irradia~ion in the reac~or core and, during the process when the sllicon carbide is .. . .
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--10-- . , being deposited~ serves a~ a further barrier to prevent chlorine (which is present ~n the coating atmosphere) ~rom reaching the kernel where undesirable chemical reactions may occur. The exterior pyrocarbon layer 63 has a larger strain to ~racture than the relatively brittle silicon and thus provides mechanical handling strength for the target particles following completion of the coating operation, particularly dur~ng bonding of - the particles with pitch or the like to form short rods before loading into the fuel chambers. During operation ln the reactor core, the isotropic pyrolytic carbon layer 63 undergoes a con~rolled shrinkage as a result of exposure to hi~h temperature and ~ast neutrons, and it shrinks onto the silicon carbide layer 59 placing it ln compression and increasing its strength as a minuta pressure vessel.
- - Although the employment of an outer coating which includes a layer of silicon carbide or zirconium carblde sandwiched between isotropic pyrocarbon layers is preferred, other su;table ~as-tight coatings can also be employed. For example, oriented pyrocarbon has proved to be extremely effective in retaining tritium, and it is contemplated that a single layer of such pyrocarbon might be disposed exterior of the porous layer 57. For example, pyrocarbon having a BAF of about 1.1 to 1.4 and a density between about 1.85 and 2.0 gram/cm3 may be employed. Such a layer should have a thickness of at least about 70 microns, and depending upon the size of the llthium kernels and the amount of porous pyrocarbon employed, a layer up to about 200 microns thick might be used. Such oriented pyrocarbon la~er can be deposited from an atmosphere of acetylene, propylene and argon at a temperature o about 1350 C.
in a fluidi~ed bed coater uslng a lower coating rate than is used to deposit lsotropic pyrocarbon.

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It is contemplated ~hat the core of a 1000 ~Wt HTGR could be designed w1~h approximately 10V0 kg of U-235 and 140 kg of lithlum ~measured as lithium metal). Operatlon of such an HTGR for about two years would result in fissioning of about 80% and 90% of the fissile isotopes of uranlum, in the transmutation of over 90~ of the Li6 nuclides and in the recovery of about 5 to 6 kq o tritium. Although such a batch-loaded cycle wherein all the fuel el~ments are loaded and discharged at the same time appears the most promising, several fuel management schemes seem feasible, including a 3-year graded cycle in which every year 1/3 o the fuel elements are reloaded~ The batch ~cheme has the advantage of having a flatter power across the core and therefore lower fuel temperatures.
The HTGR has the uni~ue advantage to allow var~ous such schemes through balancing the reactivity effect of the uranium depletion with th~ shieldin~ of the lithium. During operation in all such fuel management schemes, the HTGR produces electricity in the same manner as the existing HTGRs, which are economically compe~titive on the basis of electricity production aloneO Of course the fuel cost would be somewhat higher because it would not be offset by breeding ~airly large amounts of U-233 from the fertile thorium; however, this additional fuel cost would be more than of~set by the economic value of the tritium produced. Accordingly, the dual operation of the ~TGR
~oth as an electricity producer and as a tritium p~oducer appears to be extremely attractlve economically.
As earlier indicatedt the lithium target particles 51 could be restrlcted to zones adjacent to the top and bottom o~ prlsmatic fuel elements, which ¢ould then be severed and reprocessed separately for the recovery o~ tritium, or the target material could be 5 ~ 5 .

-1;~ ' ' distributed throughout the ~uel chambers. In either lnstance, heating of ~uch target particles to a temperature o about 1300 - 1400 C. or above effects the relatlvely prompt diffusion of tritlum througb a ~ilicon carbide barrier layer, which was very efective in restraining passage of tritium at lower temperatures9 For example, autoclaving o~ the fuel elements, or the remains thereof/ at temperatures of 1500 C~ can be employed to release the tritium into a lQ controlled atmosphere. Alternatively, if the tritium recovery is to be effected as a part of the overall recovery treatment of the spent fuel elements, the hex block fuel elements can be crushed and burned in a controlled atmosphere at temperatures below about 800 C~ to destroy the graphite blocks and the isotropic - pyrolytic carbon exterior layers. Tbereafter, crushing the carbide barrier layers in a controlled environment, coupled with heatinq to a temperature of at least about 500 C., woula quickly e~fect release of the tritium 2Q being held therewithin, and thi~ procedure is preferred ~or ZrC coatings which are more resistant to hydrogen dif~usion at hlgher temperatures. ~ltimate reaovery of tritium ~T) from a gaseous atmosphere is pre~erably effected by conversion of the tritium to T2O by oxidation using a suitable oxygen source, such as copper oxide. T20 has physical characteristics quite ~milar to ordinary water and is then removed from the gas stream by a molecular sieve or by freezing in a ~uitable cold trap, such as liquid nitrogen.
Alternatively, tritium can be recovered as a hydride, instead of being oxidized, by exposure to zircanium or t~tanium sponge metal~
Although the foregoing containment syYtem is preferred wherein the target particles are provided with ~as-tight barriers which retain therewithin the tritium ~55~5 which is bred, i~ it is desired to continuously recover trit~um from a producing HTGR, a less efective coating system may be employed so as to permit the controlled release o~ ~ritium throughout the life of the reactor core. In such an instance, a helium purification system 71 of a substantially greater capaci~y is employed to continuously treat a side stream of thle circulating pr~mary gas coolant stream to remove tritium therefrom, employing one of the tritium recovery schemes just described~ Moreover, with the employment of such a tritium recovery system as a part of the overall reactor design, and using target particles having either controlled release characteristics or excellent retention characteristics based upon SiC barrier layers, lt migh~ be pos~ible to recover the trltium wh~le the fuel elements remain in the reactor. In such an lnstance, the temperature o the reactor might be raised ; to heat the target particles to about 1300 C. for a ~u~ficient period~ e.g., a week, to assure the dif~usion of substantially all of the tritlum into the helium a~tmosphere, whence it is removed by ~he recovery system.
The following example illustrates a presently preferred embodiment of target particles ~or the production and retention therewithin of tritium;
however, it should not be understood to in any way limit the scope of the invention which is deflned solely by claims at the end of this specification.
EXA~PLE
LlA102 powder is cold-pressed into small 3a cylindrical pellets using a steel die and about 3000 psi. After ~intering in a vacuum for about 1200 C. for a~out an hour, the 3intered pellets are dropped through a zone heated to about 2200 C. in an inert atmosphere to cause them to spheroldize. The resultant particles are found to have a dens~ty equal to about 99% o~

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theoretical denslty~ ~n impervious layer of orlented pyrocarbon of about 10 microns thick and having a density of 1.9 yrams/cm3 is applied in a rotating drum coater using a mixture o~ acetylene ancl argon at a S tempera~ur~ of about 1000 C. The particles are then transferred to a ~luidized bed coater and, at a temperature of about 1100 C., are coated using a mixture which is about 90 volume percent acetylene and 10 volume percent helium. Spongy pyrocarbon having a L0 dens~ty oE about 1.1 grams~cm is deposited, and a layer about 80 microns thick is applied to the kernels whioh averag~ about 500 microns in diameter.
Following depositivn o the porous coating, the tempera~ure is raised to about 1350 C., an~ a mixture O~ propylene, acetylene and argon is employed to deposit about 35 microns o~ isotropic pyrocarbon having a ~ensity o~ about 1.9 grams~cm3 and a BAF of about 1.02.
The temperature of the coater is then raised to about 150Q C., and hydrogen is employed as tile ~luidizing gas. Approximately 10% of the hydrog n stream i5 bubbled ~hrough a bath of m~thyltrichlorosilane. Under these conditions, silicon carbide having a density of about 3.20 grams/cm3, wh~ch is beta-phase SiC, is deposited to create a layer about 90 microns thick.
Thereafter, argon is again used as the ~luidizing gas, and the temperature is lowered to about 1370 C. A mixture of acetylene, propylene and argon is then employed to deposit about 45 microns of isotropic pyrolytic carbon having a density o~ about 1.85 grams/cm onto the silicon carbide layers.
Therea~ter, the particle~ are slowly cooled in a stream o~ inert ga3 until they approach room ~emperature and are removed from the coater.
35 . The particle~ are mixed with p~ tch to form a ... , ~ . .
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paste which is lnjected into molds to form short sticks or rods in the same ~anner as heretofore employed to produce nuclear fuel sticks~ Baking these green ~ticks at a temperature o~ about 1600 C. for about one hour drives o~f the volatiles from the pitch and leaves short cylindrical rods having a diameter of about 1.57 cm.
wherein the coated target particles are securely bonded to one another by the car~onized bonding agent.
These bonded target particle rods are then irradiated in a suitable capsule subjecting them to a thermal neutron bombardment at about 1000 C.
Irradiation is continued until a sufficient dosage of neutrons has been encountered so that more than 95~ of the ~i6 isotopes should have been transmuted to tritium and helium. Monitoriny of the aapsule atmosphere shows that barely measurable amounts of trit~um are present during irradiation.
After removal of the capsule from the reactor, the targe~ rods are removed from the capsule and disposed in an autoclave which is supplied with a controIled recirculating helium gas atmosphere. The autoclave is heated to about 1500 C. and held at this temperature ~or about 10 hours. The circulating helium atmosphere is passed over zirconium sponge material, and the tritium which is released from the target particles in the autoclave is absorbed on the metal zirconium as zirconium hydride. Following completion of the absorption, examination of the zirconium sponge shows tritium has been recovered in an amount equivalent to about 90% of the Li6 isotopes present in the target material. Accordingly, such target particles are capable o~ producing and retaining tritium when exposed to thermal neutronsr which tritium can be released ther~from by heating to about 1500 C. These target particles are considered to be well-suited for use in an - .

~;'' ' 1 ~5~5~5 -16- .
. HTGR designed for the co-production of tritium and : electrical energy.
Although the ~nvention has been described with regard to certain preferred embodiments, which . 5 constitute the best mode presently known to the ; applicants, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing ~rom the scope o~ the invention which is defined in the la claims appended hereto. Various features of the lnvenLlon are emphasized ln che olaims whloh follow.

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Claims (23)

The embodiments in which an exclusive property or privilege is claimed are defined as follows.

1. A method of making tritium which comprises forming minute coated particles including cores which are spheroids between about 300 microns and about 1000 microns in diameter and contain a lithium compound including Li6 nuclides, an inner coating of porous material and an outer gas-tight coating of relatively dense SiC or ZrC, irradiating said coated particles with thermal neutrons in the core of a gas-cooled nuclear reactor which is at a temperature of at least about 900°C. to cause the transmutation of a major proportion of the Li6 nuclides to form helium and tritium so that the pressure of tritium within said outer coating is at least about 5 atm., and recovering said tritium from said particles.

2. A method in accordance with Claim 1 wherein said lithium compound is an oxide of lithium.

3. A method in accordance with Claim 1 wherein said compound is lithium aluminate.

4. A method in accordance with Claim 3 wherein said cores are initially coated with a layer of pyrocarbon having a density of at least about 1.8 g/cm3 at a temperature not greater than about 1000° C.

5. A method in accordance with Claim 4 wherein said inner coating includes pyrocarbon having a density of not greater than about 1.2 g/cm3 and a thickness of at least 75 µm.

6. A method in accordance with Claim 1 wherein said outer coating includes SiC and said recovering of tritium is effected by heating said particles to a temperature of at least about 1300° C. to cause diffusion through said SiC
layer.

7. A method in accordance with Claim 1 wherein said particles are removed from said reactor and breaking of said carbide layer is effected to recover said tritium.

8. A method in accordance with any one of Claims 5, 6 and 7 wherein the pressure of tritium within said outer coating is between about 5 and about 20 atm. at the time when said irradiation is completed.

9. A method in accordance with Claim 1 wherein said coated particles are disposed in fuel chambers in graphite fuel elements and wherein nuclear fuel material is also disposed within the same fuel chambers.

10. A method in accordance with Claim 9 wherein said coated particles contain a layer of silicon carbide at least 50 microns thick having a density at least about 95% of theoretical maximum density.

11. A method in accordance with Claim 10 wherein relatively thin regions containing the lithium compound cores are alternated with relatively thick nuclear fuel regions within a longitundinally extending fuel chamber.

12. A method of creating tritium in a nuclear reactor and recovering same, which method comprises forming cores of a lithium compound containing Li6 nuclides, coating such cores with an inner layer of porous material and an outer gas-tight layer of relatively dense silicon carbide to form spheroids between about 300 microns and about 1000 microns in diameter, irradiating said lithium-containing spheroids with thermal neutrons in the core of a gas-cooled nuclear reactor for a length of time sufficient to cause the transmutation of a major proportion of the Li6 nuclides to form helium and tritium such as to cause the pressure of tritium within the outer coating to reach at least about 5 atms.
recovering said tritium by heating said spheroids to in excess of about 1300°C. and maintaining said temperature for a sufficient time to allow diffusion of said tritium through said silicon carbide layer.

13. A method in accordance with Claim 12 wherein said pressure of tritium attained within said spheroids is between about 5 and about 20 atms.

14. A fuel element for use in a gas-cooled nuclear reactor which comprises a body of refractory material having relatively good thermal conductivity and neutron moderating characteristics, chamber means formed in said body, nuclear fuel material and target particles contained in said chamber means, said target particles comprising minute spheroids between about 300 and about 1000 microns in diameter of a solid lithium compound including Li6 nuclides, an inner coating. of porous material and an outer gas-tight coating of relatively dense SiC or ZrC at least 50 pm thick, whereby irradiation of said target particles by thermal neutrons results in the transmutation of Li6 nuclides to create helium and tritium that is retained within said outer gas-tight coating.

15. The invention in accordance with Claim 14 wherein said lithium compound is an oxide of lithium.

16. The invention in accordance with Claim 14 wherein said lithium compound is lithium aluminate.

17. The invention in accordance with Claim 14 wherein said cores are surrounded by an adjacent seal layer of pyrocarbon at least about 10 microns thick having a density of at least about 1.8 g/cm3.

18. The invention in accordance with Claim 17 wherein said seal layer is surrounded by a porous layer of pyrocarbon having a density of not greater than about 1.2 g/cm3 and a thickness of at least 75 pm.

19. The invention in accordance with either Claim 17 or 18 wherein said SiC or ZrC layer is disposed immediately between a pair of layers of isotropic pyrocarbon having a density between about 1.7 and about 2.0 grams/cm3.

20. The invention in accordance with Claim 14 wherein said body is a block which has a pair of parallel flat top and bottom end faces and a plurality of sides which are substantially perpendicular to said end faces, and said block also contains a plurality of coolant holes which extend axially completely therethrough from end face to end face and a plurality of said chambers which are elongated and extend parallel to said coolant holes.

21. The invention in accordance with Claim 20 wherein said target particles are disposed in zones at the top and bottom of said chambers and nuclear fuel material is located therebetween.

22. The invention in accordance with Claim 20 wherein said refractory material is graphite, said block has a cross section shape of a regular hexagon, and said fuel chambers are disposed in a triangular array.

23. A nuclear reactor core comprising a plurality of vertical columns of fuel elements in accordance with Claim 21 stacked one atop another.