[1]3 1 1 7 9 @ 7 2 2 Umted @States Patent Office . @ a t e n t e d A p r . 2 0 , 1 9 6 5 3,179,722 AMTHOD OF PREPAPJNG SPBERICAL NUCLEAR FUEL PARTICLES Howard E. Shoemaker, San Diego, Calff., assignor, by mesne assignments, to the United States of America as 5 represented by the United States Atomic Energy Commission Filed Apr. 2, 1962, Ser. No. 184,612 12 Claims. (Cl. 264-15) 10 The present invention generally relates to nuclear fuel and more particularly relates to a method of preparing nuclo,ar fuel carbides in spheroidized particulate form. Various procedures have been proposed for the preparation of nuclear fuel carbides, some of which procedures 1 involve the preparation of the carbides in particulate form. In this connection, what is meant, for the purposes of the present invention, b'y nuclear fuel carbides are the monocarbides and dicarbides of thorium, uranium and plutonium and mixtures thereof. Procedures ifor the 20 preparation of nuclear fuel carbides, however, are generally relatively costly and time consuming, particularly those where the particles are to be of relatively uniform size and@ shape, such procedures usually requiring shaping operations, before and after formation of the carbides 25 and/or the use of complicated equipment. Nuclear fuel carbides are preferred materials for use in fuel elements of various types of nuclear reactors. In certain of such reactors, it is advantageous from a nuclear standpoint to provide the fuel carbides in dense partic u- 30 late form of uniform-diameter, i.e., in the, form of spheroids or pellets of controlled size. It would therefore be advantageous to provide a low cost simplified procedure for'the production of dense, particulate nuclear fuel carbides of controlled size and shape. Such a low 35 cost, siinplified relatively rapid method utilizing a nlininium of steps has now-been discovere@d. . Accordingly, the principal object of the present invention is to provide a method for ihe manufacture of nuclear fuel carbides in dense, particulate form and of con40 trolled size and shape. It is also an object of the present invention to produce dense particles of carbides of thorium plutonium and uranium and mixtures thereof of controlled diameter in a minimum number of steps over a - relatively short period of time. Further objects and advantages 45 of the present invention will be apparent from a study of the following detailed description and of the accompanying drawings of which: FIGUR-E I is a Vertical s ectioii of one form of appa ratus in which the method of the present invention can 50 be conveniently carried out, portions of internal components of the apparatus being broken away to ill,ustrate the constr-uction thereof; and, FIGURE 2 is an enlarged fragmentary view of fuel 55 pellets -formed by the method of the present invention m place in a portion of the apparatus of FIGURE 1. The method of the present invention generary compnses spheroidizing particles of nuclear fuel carbides without agglomeration thereof. Preferably nuclear fuel C)o particles are utilized which are not initialiy in carbide form, but which during the processing are converted to the carbide form and then are spheroidi,,zed, all within a graphite bed at elevated temperature. The finished spher6idized@nuclear fuel car,bide particles are dense, Of 65 2 controlled size and may contain an adherent protective layer of carbon or graphite on the outer surface thereof. Now referring in more detail to the steps of the method of t@he present invention, nuclear fuel in powdered or fine granular forin is niixed together with carbon or graphite powder. The nuclear fuel may comprise thorium, uranium or plutonitun in unenriched or enriched forins, and mixtures thereof. Preferably, the fuel is in oxide form, as for example, thorium dioxide or uranium dioxide or mixture thereof, although nuclear fuel in metallic form or in carbide form or in a mixture of two or three of the indicated forms is not excluded from the scope of the present invention. Carbon preferably used with the nuclear fuel instead of graphite and may be any suitable carbon powder, preferably reactor grade powder such as is u@ed for critical facility nuclear reactor compacts and the like. A sufficient amount of carbon is mixed with the nuclear fuel to convert the same durin,- subsequent processing to the carbide form, preferably the dicarbide form, and to preferably form an eutectic mixture therewith. It will be understood that where the nuclear fuel is initially in carbide form, the carbon can be eliminated from the mixture. However, nuclear fuel carbide formation during rather than before the processing is preferred, as previously indicated. A small concentration, for example about 2 percent or more, of a suitable binder material, is added to the mixture, preferably ethyl cellulose of suitable viscosity, but a .0us other resinous binder such as polyvinyl alcohol, v r' shellac, Lucite, Bakelite, furfural alcohol resin, paraffin, etc. can be successfully used in small concentrations. It @Rl be understood that the relative proportions of the nuclear fuel, carbon and binder can be somewhat varied, depending upon the results desired. Preferred compositions for various nuclear purposes, utilizing preferred constituents are set forth in the following table: Table I ConcenConstituents trations, Produced Carbides gms. ------------ 85.0 (I)--- C.] ------------ 15. I UC2- lethyl Cellul6se ----- 2 2) ITho2 --------------- 85 Carbor -------------- 15@,5 iThc2. Ethyl Cellulose ----- 2 Th O2 ---- ----------- 80. (3)- uo @ ---- ----- Th C2-UC2 (Th: U=10: 1). Carbon ------ - 1 1'6: oo lethyl Cellulose ----- 2 ThO2 -------- ------- '7'-.'O ThC2-UC2-C eutectic 1 U02 ---------------- (4)- Carbon ------------- 16.7 (Th:TJ=10:1). l ethyl Cellulose ----- 2 ThO2 -- ------------- 5 6 ( 6) U02 -- --------------- 2 5: 0, ThC2-UC2-C cutectic -- - Carbo -- ----------- 1 7.5 E thyln ( Th:TJ=4.5:2). Cellulose ----- 2 In accordance with the m ethod of the present invention, the indicated constitlient-s are thorougbly mixed t6gether and particulated to suitable particle size. Thus, the nuclear fuel, carbon (when used) and binder can be mixed together dry and then a suitable volatilizable Solvent for the binder, preferably trichloroethylene in the case of ethyl cellulose, can be added to dissolve the binder and to form a slurry of the constituents. The
[2]3,179,722 3 'solvent can be evaporated, as mixing is carried out, so that -thorough mixing of the constituents together with agglomeration thereof into particles can th@-reby be effected. The described, preferred mixing and agglomeratin.- protedure can be carried out in any suitable apparatus. One particularly suitable type of apparatus for relatively large batches containing more than about three thousand grams 'of mix is known as a PK Twin Shell blender. Such @apparatus can be advantageously used, as for example, as follows: A 3 kilogram batch of di-y powders (nuclear fuel oxide and carbon, plus about 60 grams ethyl cellulose binder) is loaded into the blender and blonding is affected for about 30 minutes at low spe6d, after whir-h approximately 850 ml. bf trichloroethylene can be added slowly to the blender as, for example, over a 45 minute period. Blnding is continued until the desired particle size for t'iie mix is obtained. Some control of particle size can be achieved by regulating the concentration of th6 s6lvent add@d to the blender. Where stnaller ba-tches of particles are to be prepared from the ind icated dry powder, a small6r size mixer, as for example a Hobart mixdr, can be used. An example of such use is as follows: Powder batches of less than 3 kilo,@rams are added in dry powder forrri first to the PK blender and blended dry for 30 minutes. The mixed po@vders are'then transferred to the Hobart mixer and @n amount of tricbloroethylene proportional to that indicated with respect to the PK blender is added to produce a slurry while the Hobart mixer is operated at a low speed. When the rnix c6ases to stick to the walls of the mixer th6 speed is increased and mixing is conducted until small balls of the mix are obtained and no dust is evideiit i,n the mix. Thereafter, the balled mix is transferred to the PK blender and blended until the d6sired particle size is obtaiiied or, alternatively, it can be left in the Hobart mixture and mixing carried on until the desired particle size is obtaiiied. The particles obtairied from the d-@scribed rriixing and agglomerating operation are preferably of from about 300 tb 500 micfon size. They @re'then removed from th6 mixer and oven dried, for example, at 140, F. then sieved to obtain the desired particle size for further processing, for example from ab6ut 295 to 495 microns (+48-38 mesh) size, i.e., aboiit 75 micr-ons largor th@n the desired size of the finished spheroidized fuel pellets. Particles outside the desired siie limit Tange can be recycled through the described mixing operations by adding more trichlorethylene or other suitable solvent for the binder. In t,he event that the PK blender'is used for the recycling, it is advantageous to first reduce the particl-,s to powder foirm before adding the trichlorethylene. The pre-sized ag.-lomerated fuel-containing particles so produced are then mixed with or uniformly dispersed in a sufficient amount of graphite flour, preferably in a particleto-graphite ratio of abbut 8: 1, so that the particles are out of physical contact with one anbthcr. Other ratios with relatively more graphite can be utilized but it has been found for most purposes it is desirable to have a@, l,east one part, by weight, of graphite present to eight parts, by weight, of the fuel-containiiig particles. In accordance with the method of the present invention, the mixtlire or dispersion of the fuel-cohtairiing particles and graphite flour is placed in a reaction zone and heated, in an at least substantially oxygen-free environment, preferably in a vacrum, to convert the fiiel to carbide form, if not already in that for@n, and to spheroidize the fiiel particles. This treat-ment can be conveniently carried out in a grapbite crijcible@ disposed within a suitable hi,-h temperature apparatus. However, othercomparable high temr)eratiire adparattis can also be einployed. FIGURE I of the accompanying drawin,@s illustrates one form of suitable apparatus for carrying out the car I burizing and spheroidizing steps of the p resent n-iethod. Now referrin.@ to FIOURE 11 a reaction a aratus 9 is , pp , illustrated which includes a- graphite crucible 11 100@elY 4 disposed witwn a graphite susceptor 13 fitted with a carbon cap 15. The susceptor is in turn disposed within a carbon black insi.,Iator bed 17 in the bottom portion of a quartz reaction tube 19. Tube 19 is fitted with a centrally disPosed line 20, to which are connected a vacuum line 2 with valve 23, and a third line 25 with valve 27 and sight glass 29, as shown in FIGURE 1. A rubber gasket 31 seals the cover 33 of tube 19 to the flanged upper end of the subwall 35 thereof. An induction heating coil 37 is 10 disposed around the lower portion of tube 19 to bring the crticible to reaction temr).,rature. The grapnite crucible 11 is generally cylindrical and includes a bottom portion 39 with an integral centrauy disposed vertically extending gra@hite heat distribution 15 core 49. Sidewall 41 of crucible 11 is integrally connected to bottom 39. To the upper e.nd of sidewall 41 is releasably secured, as by threads 43, a -rapbite cap 45. Cap 45 is provided with an up-,vardly extending, hollow chimney 47, th-. cavity 49 thp-rein int6rconnecting with a horizontally extending cavity 51 in the c@p 45, as shown in FIGURE 1. Chimney 47 extends up through thb carbon cap 15 and terminates above the level of the carbon black insulator 17 in quartz tube t9. Adja@,ent its uppdr end, cwmney 47 is provided with a plurality of horizontal 25 vent holes 53 interconnectin.- with cavity 49- and with 'a vertical si,@ht hole 55 aligned' with lini,- 20, line 25, -valve' 27 and sight glass 29. a.0 With such an arrangement, the chimney 47 serves two purposes. It conducts reactioii gases out of reaction zone, and it provides means wh6reby pyrometer readiiigs can be made to determine th6 teniperature in ctucible 11. Thus, reactioii gases (such as caibon monoxide, etc., resulting from reduction of fuel oxides with carbon), migrate out of crucibl6 11: through the walls thereof into @, 5 the space 57 betwe6n cru6ible 11 and susceptor 13, then 'Lhrough cavity 51 into ca@ity 49 of cfiimney 41. Such gasses pass up through cavity 48, o-ut of thd chimney through holes 53 and 55 into the s'pace 59 above the level of the carbon black insulator bed in tube 10. Such I gases 40 are removed from spa6e 59 tlirough line 20, exh@ust line 21 and valve 23. It is, of course, important to have accurat6 detdrmiria-tions of crucible 11 temperatuie during processing in a - ccordance with the present method. Pyromeirie measurements of crucible 11 can be periodically made o'n a direct 45 line through sight glass 29, valve 27, line 25, line 20, sight hole 55, ca-@ity 49 and the in-@line portion of cavity 51, a shown in FIGUP@E 1. Such measurements may be carried o-n optically or other@vise, in accordance tith 50 known principles based upon the high temperature- char. acteristics of black bodies, crucible 'II acting as a black body. In utilizin.a the describ6d apparat@s, the mixt'ure of graphite ffour and agolome@ated nuclear fuel-containing 5,5 particlesisplaced,withincru6iblelitofill'thesarhe. Cap 45 is then screwed tightly in place. The crucible is theri positioned within suscdptor 13 and'the susceptor c@p@15 is fitted into place. The susciptor is then positioiidd within the carbon black insulatof bed 17 in tube 19, as shown in FIGURE 1, with thd upper@ end of -chimriey 47above the level of, bed 17. Gasket 31 is piit iii place and cover 33 is disposed therearound. Valvo 23 is then opened and a vacuum is drawn through line 21 to remove oxygen from the system. If desired, the system can be flushed wi . th inert gas or reducin- gas and vacuum can then beapplied. When substantiaffy all oxidizing gas has thus been removed from the system, crucible 11 is gradually heated to sintering and carburizing temperature. Preferably, 70 high vacuum is applied for example, below 200-300 microns pressure) throughout the heating procedure so as to remove any evolved gase from the system. in most cases, the sintering and carburizing temp,@r-atures can be fr6rii ab6iit 2000 to about 2300' C., such tempera75 tures being r-each6d over a heating period 6f"for example,
[3]5 2 to 5 hours. The paiticular temperatures selected wiR depend on the particular constituents utilized as the nuclear fuel components. Generally, the higher the concentration of thorium in a thorium-uranium mixture (oxide form) the hi.-her the carburizing temperature required. A temperature range of 2000 t6 2300' C., is suitable, for example, for nucle r fuel particles containing an atom ratio of thorium-to-uranium of about 4@5:2. Reduction of the nuclear fuel oxides to the dicarbides ig accomplished at the indicated sintering and carburizing temperature, i.e., carbide formation is effected, accompanied by evolution of reaction gases (CO, C02, etC-)Carburizing and sintering temp6rature is maintained in the crucible until carbide formation is completed. The desired carbide formation can be detected by a reduction in the pressure in the system ' since reaction gases no longer are evolved. It will beu-nderstood that the carburizing step does not take place where the nuclear fuel of the particles being treated i initially in the dicarbide form. At any rate, during heating of the particles, PyrOlysis of the binder in the particles occurs with some evolution of gases, usually well- below the indicated carburizing temperatures. These gases are &awn off through the vacuum line, as described with respect to the carbide reaction gases. Whether the nuclear fuel carbides in the fuel particles are initially present or.whether they are formed in situ at carburizing teinpetature' inaccordance with the present method the temperature in@ the crucible is ultimately raised to abov6 the melting point of the highest meltidg point carbides or eutectic mixture, when present, in the particles, prefefably to above 50' C., above such melting point. Usually, such tem@erature will be around 2500' C., but this will depend on the particularcomposition of the fuel particles. @The melting point of the fuel iparticles can be detected during the heating operation since, at such melting point, gas is suddenly substantially evolved ther6from (voids between the sub particles of the sintered particles are filled with molten carbides, entrained gases are expelled, etc.). There also is an accompanying arrest in the rate of temperature rise in the system, due to utilization of heat for fusion or transition of the particles fr6m @olid to liquid form. Vacuum is applied to the system during suchfurther heating. After such teihperature is reached, it need only be maintained for a relatively short period of time, for example, 15 to 30 minutes, that is, only long enough to assure complete melting of the carbides of all fuel particles in the crucible. Thus, eath of the fuel T)articles, while being maintained separate from afl other luel particles in the crucible by the graphite Hour, is melted. The melting results in an increase in the density of each fuel particle over that of the same particle in the sintered carburized form. Moreover, each melted fuel particle 61 assumes a spherical shape, as shown in FIGURE 2 since it is suspended in the graph;te flbur 63 and dbes not agglomerate with other melted fuel Varticles 61 in the crucible 11 due to the presence of the graphite flour 63 physically separating it from all other nuclear fuel particles 61. The densified, spheroidized ftiel particles ar6 then gradually cooled to ambient temperature, preferably with the aid of a cooling gas, for example in an atmosphere of methane or other hydrocarbon gas. Thereafter, the apparatus is disassembled and the sealed crucible is transferred to an inert dry atmosphere, wherein the crucible is disassembled and the particles are removed and sieved or otherwise suitably separated (as by blowing, etc.) from the graphit-, flour. For exaniple, the particles can be sieved through 35 and 100 mesh screens. Material retained on the 100 mesh screen is of about 150-420 microns in diameter@ The small porcentage of oversized material greater than 35 mesh may be stored for fuel reprocessing, while the small percentage of undersized material which passes through the 100 mesh screen along with graphite powder can be reused, for example, as 3,179,722 graphite insulation, etc. In most casds, th@ yi@ld of'150420 micron size spheroidized nuclear fuel carbide particles exceeds 99 percent of the particles treated by the present method, so thdt the method is very efficient. The finished nuclear fuel carbide particles thus produced are dense, substantially spherical and usually cdntain a thin covering of adherent graphite which has the beneficial function of acting as a barrier against migration of fission products from the nuclear fuel during use thereof in a nuclear re10 actor at elevated temperatures. The finished particles are ready for immediate use in nuclear fuel elements and the like, but can, if desired, be further treated as by coating the surfaces thereof with pyrolytic carbon or the like. Inasmuch as enriched nucleai fuel, containing, for ex15 ample, uranium 235, is more expensive than unenriched nuclear fuel and inasmuch as some loss of nuclear fuel from the nuclear fuel particles to the surrounding graphite may be encountered during the described high temperature processing, it is perferred to "break in" the crucible and 20 graphite floiir prior to their use with enriched nuclear fuel particles by first using them in the described method with unenriched nuclear fuel particles. This has the effect of outgassing thl- graphite flour aiid crucible graphite ahd of saturating the same with unenriched nuclear fuel 25 (thorium 232, uranium 238, etc.) so that during subsequent processing with enriched nuclear fuel, such enriched fuel will n--t be lost to the' graphite. TI-ic following examples further illustrate certain features of the present iiivention: 30 EXAMPLE I A 1200 gm. batch of nuclear fuel particle n-iix was prepared utilizing the constituents specified in Table 11 below. 35 Table 11 Constituents: Parts by weight Thorium dioxide ----------------- ------- 56 Uranium dioxide ------------------------ 25 Carbon ------------------------ -------- 17.5 40 Ethyl cellulose --------- ----------------- 2 The constituents were mixed together dry for 30 minutes and then trichlorethylen-- was slowly added to a total amount of about 340 ml. Mixing was continued 45 until small particles were obtained. The particles were then oven dried at 140' F. and sieved to obtain 295-495 micron size particles. The particles were then mixed with grapfiite flour in an 8-to-1 particle-to-graphite weight ratio (about 150 gm@ of graphite flour), placed in a 50 graphite crucible of a reaction apparatus, substantially as shown in FIGURE I of the accompanying dranving, and heated therein to 2300' C. over a 3 hour period after evacuation of the apparatus to below 200 microns. Such low pressure was maintained until carbide formation in 55 the particles was complete. The temperature was then raised to about 2500' C. while maintaining pressure below 300 microns, and held for 15 minutes, after which the system was cooled to ambient temperature with methane. The crucible was then removed to an inert atmosphere 6o and therein opened. The pqrticles were sieved using 35 and 100 mesh scrcens and those particles (over 99 percenl) 150-420 microns in size were retained. The retained particles were exaniined and found to be dense, hard, graphite coated, generally spherical and p,,irticularly 6,5 suitable for use in high temperature nuclear reactors. The nuclear fuel of the particles was found to be essentially completely in the dicarbide form. EXAMPLE 11 70 Sph-,roidized, hard, dense nuclear fuel carbide particles ar6 prepared substantially as described in Example I utilizing the same constituents, concentrations, etc., except that the nuclear fuel is thorium dicarbide and uraniu,m dicarbide and no free carbon is present in the mix. 75 During the heat treatment, the temperature of the parti-
[4]3,179)722 7 cle@ is graduall@ raised without interruption to 2550' C. aiid that t&inperature is maintained for 20 minutes. Following th6 co6ling and sieving steps, the finished particles are examin6d and found to have substantiauy thb same characteristics as the finished particles of Exaihple 1. 5 Accordingly, an improved niethod for the m'anufacture of dense liird spher6idiz6d n-dclear fuel carbide patticle in unag.-loffierated form from nuclear fu6l is prdvided, which niethod is efficient, relatively simple and relativel rapid-. The fini@hed particles are ready for use in high 10 temeprature nuclear reactors without further treatment. Other advantages are as set forth in th@ foregoing. Va'riou of the features of the present iilvention are set forth in the appended claims. What is
