Preparation of Uranium Carbonitride by Reaction of UC with Ammonia

Journal of NUCLEARSCIENCE and TECHNOLOGY,5[8], p. 414~418 (August 1968).

Yumi AKIMOTO* and Koji TANAKA*

Received January 24, 1968 Revised April 11, 1968

With the view to establishing a method of directly converting uranium carbide into uranium carbonitride and hydrocarbons, an attempt has been made to induce reaction between UC and ammonia under various temperatures from 25d to 600dc and pressures from 1 to 1,500 kg/cm2. The reaction aimed at was realized to the extent of practical significance under pressures exceeding 500 kg/cm2 at 450dc and exceeding 250 kg/cm2 at 500dc. The hydrocarbons produced thereby were found to be mainly methane, indicating that the formula of the predominant reaction was UC+NH3 -> UN2-x+CH4+H2. Upon heating the powdery fine black product to 1,800dc in vacuo, unexpectedly marked sintering was found to occur, resulting in dense uranium monocarbonitride without any compaction pre- treatment.

carbon is also reported. I. INTRODUCTION Another commonly known nitriding reac- Uranium mononitride (UN), and uranium tion is that between uranium carbides and carbonitride (UC1-xNx) have attracted much nitrogen, to result in a pulverized mixture of attention as possible fuel materials with quali- higher uranium nitrides and carbon(6)(7): ties surpassing uranium carbide (UC) with 2UC+3/2N2=U2N3+2C. (2) respect to chemical stability and simplicity of stoichiometric control. From the standpoint This reaction also fails to serve the purpose of fuel cost however, UN and UC1-xNx still of producing UN or UC1-xNx because carbon, yield to UC because the process at present once produced, cannot easily be separated available for the preparation of pure UN and from the nitride. The present authors have UC1-xNx requires inherently expensive metallic in fact tried, in vain, to convert the carbon uranium as feed material. into hydrocarbons by treating the nitrided An early paper(1) has reported on the for- products with hydrogen. mation of UN by the reaction of UCl4 with Evolution of hydrocarbons, on the other ammonia. The authors, however, failed to hand, is widely recognized on hydrolytic re- reproduce the reaction after several trials. actions of uranium carbides(8)~(10). The reaction Recent thermodynamical data(2) also predict of uranium carbides with ammonia thus sug- that the reported reaction would be difficult gested itself to the authors from the resem- to bring about. blance of its reaction mechanism to that of More recently, several investigators have water and ammonia. If one assumes the tried to develop a method of converting UO2 hydrocarbon produced to be all methane, the directly into UN or UC1-xNx(3)~(5). The ex- reaction expected would be pected reaction is UC+2NH3=UN2+CH4+H2, (3) UO2+2C+1/2N2=UN+2CO. (1) in analogy of the hydrolytic reaction This process, however, is handicapped by the UC+2H2O=UO2+CH4. (4) difficulty of removing oxygen, which tends In the present paper are described studies to remain in the product as a component of U(C, N, O) solid solution. Occasional conta- * Central Research Laboratory, Mitsubishi Metal Min- mination of the product by UO2 and/or free ing Co., Ltd., Omiya, Saitama-ken.

- 32 - Vol.5, No.8 (Aug. 1968) 415 for the production of UC1-xNx utilizing reac- of 30 ml designed to stand 2,000 kg/cm2 of tion (3) for the nitriding of carbide. In order pressure at 600dc. Ten grams of UC powder to prevent excessive thermal decomposition of were placed in a quartz crucible in an Ar dry ammonia prior to the reaction, as well as to box, and was transferred into the pressure favor hydrocarbon formation, relatively high vessel after being covered by dried hexane. pressures and low temperatures were chosen The vessel was then evacuated to eliminate for reaction conditions, i.e., 1~1,500 kg/cm2 and the hexane, cooled to dry ice temperature, 25d~600dc. and charged with a large excess of sodium The reaction products were then heated dried ammonia for condensation onto the car- in vacuum at a high temperature to remove bide. The amount of ammonia was adjusted nitrogen remaining combined in excess of the between 7 and 25 g so as to produce the ratio 1 : 1. The ultimate aim held in view desired pressures at a given reaction tempera- was the establishment of an economically prac- ture. The pressure vessel was then gradually tical flowsheet for the production of uranium heated in an electric furnace. monocarbonitrides. After the reaction had been completed, the vessel was cooled, its cock opened to let II. EXPERIMENTAL the reaction gas out, and then filled with 1. Materials hexane in the same manner as when intro- Three kinds of uranium carbide specimens ducting ammonia. The reaction product was have been used for the reaction with am- thus removed from the vessel without contact monia: with air. (1) Arc fused UC (kindly furnished by Mr. In preliminary experiments conducted at R.F. Dickerson, Battelle Memorial Institute) not very stringent conditions, the trouble of was ground to -150 meshes in an Ar dry performing the hexane substitution was box. The specimen contained 4.7 w/0 of avoided, by the use of sealed pyrex tubes as carbon and showed a sharp X-ray diffrac- reaction vessels. A tube containing about 2 g tion pattern of pure UC. Oxygen and of carbides and excess liquid ammonia was free carbon contents were analyzed to be sealed at the neck and was brought to reac- 0.08 and 0.04 w/0, respectively. tion temperature. A carefully sealed pyrex (2) Carbothermic UC was prepared by the tube 8 mm in diameter and 1 mm in wall thick- reduction of reactor-grade UO2 with pure ness had been proved to stand more than 100 graphite. The source materials were kg/cm2 of pressure difference. Moreover, for homogeneously mixed in a ball mill, cold security, the sealed tube was in most cases pressed and heated at 1,800dc for 1 hr in heated in a pressure vessel under pressurized . The contents of total carbon, freevacuo Ar. carbon and oxygen were chemically ana- 3. Heat Treatment lyzed to be 5.15, 0.01 and 0.12w/0, respec- With the dual aim of thermally decompos- tively; powder diffraction photography ing the higher nitride produced, and promot- showed the existence of a slight amount ing the homogenization of the UC1-xNx solid of UC2 besides UC. solution, some of the products of nitriding (3) Hypostoichiometric UC was prepared by were heated in a vacuum induction furnace the action of methane on metallic urani- to 1,800dc. Upon heating, the specimens were um powder at 800dc. Chemical analysis observed to release nitrogen especially near which gave as result 3.77 w/0of carbon and 300d and 1,200dc. In order to prevent scat- 0.21w/0 of oxygen, combined with X-ray tering of the samples, the temperature was diffraction, indicated the product to be a raised gradually to 1,800dc taking about 30 min. mixture of UC and metallic uranium. Evolution of gas ceased at about 1,600dc, and 2. Nitriding heat treatment at 1,800dc was performed at Most of the nitriding experiments have 2x10-5 torr. for 30 min. The specimens were been carried out in a stellite pressure vessel then cooled down to 900dc taking 30 min, after

- 33 - 416 J. Nucl. Sci. Technol.,

which they were left to cool down to room by means of the previously described sealed temperature in vacuo. The specimens were tube technique. In all the runs performed transferred to and from the furnace immersed below 300-C with ammonia pressures up to in hexane. 300 kg/cm2, both carbothermic and arc fused 4. X-ray Diffraction UC were found to remain unaffected. At A Guinier de Wolf focusing camera was 400-C, weak X-ray diffraction patterns of UN2 appeared after treatment with ca. 300 kg/cm2 used throughout for identification purposes. of ammonia' vapor for 4 hr. The nitriding Precise measurements of UC1-xNx lattice reaction was considerably enhanced at 450- parameters were made by means of Debye- and 500-C. It might be added that the tubes Scherrer camera of 114.6 mm diam. with Cu were invariably found broken during the re- Ka radiation. Correction was made against action despite efforts to reduce the stress in film shrinkage, and the lattice parameters the way already described. This may have were extrapolated to t=90- through the cos2t/ been due to rapid increase of pressure caused sint+ cos2 t/t function. In all the cases the specimens were coated with dry silicon grease possibly by the cracking of ammonia. The specimens were found turned into swollen and in an Ar dry box to avoid oxidation. fluffy black powder, that showed very broad 5. Chemical Analysis diffraction patterns, and whose lattice param- Oxygen and nitrogen was analyzed by eter roughly corresponded to UN2. Typical fusion in He gas flow. The carbon content values from analysis are: 2.49W/Ototal carbon, was obtained by measuring the electro-con- 0.04W/Ofree carbon, 5.6W/Onitrogen and 0.4W/O% ductivity of the KOH solution after being oxygen for a product from arc fused UC treat- placed in contact with the CO2 gas produced ed with ca. 250 kg/cm2 of ammonia at 450-C by the combustion of the specimens in oxygen for 4.5 hr. flow at 1,300-C. The foregoing preliminary trials have been followed by systematic experiments using III. RESULTS AND DISCUSSION hypostoichiometric UC as source material. In order to roughly determine the reaction The runs in this case were scheduled to let conditions, several dozen runs were performed the reaction continue for 3 hr after taking 1 Table 1 Phases observed in nitriding products

— 34 — Vol.5 , No. 8 (Aug. 1968) 417 hr to attain reaction temperature. Table 1 face centered cubic diffraction patterns, whose presents the characteristics of the product of lattice parameters depended on the preceding nitriding as determined by X-ray diffraction nitriding conditions. Their values and C/(C + and chemical analysis. It is notable that the N) ratios assuming Vegard's relation are free carbon content is always low regardless plotted in Fig. 1. The oxygen content has of the amount present of UN2 or unreacted been analyzed on several specimens and found UC. Each of the products of nitriding was to be 0.1 to 0.2W/O.. then subjected to the heat treatment describ- Overall consideration of the results pre- ed in the previous chapter. The fluffy black sented in Fig. 1 and Table 1, leads to the con- product was found to have changed into a clusion that higher values of reaction tem- dense hard lump with a greyish metallic perature and pressure are both conducive luster. This marked progress of sintering at to higher nitrogen content product. It is fur- the relatively low temperature can be con- ther to be noted that the carbon content of sidered one of the merits of this procedure. the final product does not necessarily depend In some of the runs, the reaction gas was on the amount of unreacted UC remaining sampled from the nitriding vessel for gas in the product of nitriding. Upon heating chromatographic analysis. It revealed the UC and ammonia under conditions such that existence of methane and a trace amount of ammonia cracking cannot be disregarded, we ethane, together with a large amount of ni- must well expect the reaction trogen, hydrogen and ammonia. Since the UC+ N2-> U (N, C)2-x ( 5 ) nitrogen and hydrogen gases are produced by the thermal cracking of ammonia the result- to occur in parallel with the reaction ing increase of pressure, that is, the difference UC+ NH3 -> UN2-x+ CH4+H2. ( 3 ) of pressure within the vessel before and after the nitriding reaction was measured — both at The consistently low free carbon content, ob- room temperature — to determine the degree served indicates that U(N, C)2-x rather than a of cracking, that had taken place: In all mixture of UN2-s and carbon has more likely cases, the ammonia cracking had attained been formed under the present nitriding con- equilibrium stage during the nitriding reac- ditions. The unusual diffuseness of the dif- tion. fraction patterns may be due to size effect The heat treated products clearly showed but may also indicate inhomogeneity of the nitriding product in C/N as well as in U/(C + N) ratios. Despite its resemblance in form to hydrolytic reaction, the ammonolytic reaction mechanism does not appear attributable solely to the present nitriding reaction. That the nitriding reaction requires a relatively high temperature where thermal cracking of ammonia is dominant, and that the hydrocarbons produced are quite different from those of hydrolytic reaction in their composition, strongly suggest that elementary hydrogen may be taking part in this reaction. . CONCLUSION IV Reaction of UC and ammonia has been found to occur to the extent of potential prac- Fig. 1 Lattice parameter and C/(C+N) ratio of heat treated products of tical utilization under pressures above 500kg/ nitridation cm2 at 450-C and above 250kg/cm2 at 500-~-

— 35 — 418 J. Nucl. Sci. Technol.,

600-C. While complete removal of carbon was (2) AKIMOTO,Y.: Newer Metal Ind., (in Japanese), found to be difficult because reaction between 10, 41 (1965). UC and nitrogen proceeded in parallel with (3) STOCKER,H. J., NICKEL,H., IMOTO,S. , et al.: Ber. the main reaction, UC1-xNx was successfully Deut. Keram. Ges., 43, 150 (1966). obtained in various compositions upon subse- (4) HYDE,K . R. , LANDSMAN,D. A. , MoRRis,J. B. , et al.: Special Ceramics 1964 Proc. Symp. Brit. Ceram. quent heat treatment. Res. Assoc., p. 1~16 (1965), Academic Press. Further improvement of the apparatus for (5) NISHIYAMA,A. , KASAMATSU,T.,ITANI,K., et al.: the reaction is now being undertaken. A On the preparation of uranium nitride fuel (II), Pre- carbonitride specimen prepared from 8 % print 1965 Ann. Meeting At. Energy Soc. Japan, enriched UO2 is planned to be subjected to Apr. 5~7, Kyoto, C31, (1965). irradiation test. (6) HANSON,L.A. : Reprocessing of uranium carbide by a nitride-carbide cycle, (I), Kinetics of nitride ACKNOWLEDGMENTS formation, NAA-SR-8388, (1963). (7) SUDO,K., TANNO,T.: Bull. Res. Inst. Mineral The authors wish to thank Messrs. M. Dressing and Met. Tohoku Univ., 22, 121 (1966). Suzuki, Y. Kuroda, M. Kojima and K. Obinata (8) FROST,B.R. T. : J. Nucl. Mater., 10, 265 (1963). for their assistance, and Messrs. I. Namiki (9) ROUGH,F. A., CHUBB,W.: An evaluation of data and A. Kizawa for chemical analysis. Thanks on nuclear carbides, BMI-1441, (1960). are also due to Mr. R.F. Dickerson of the MURBACH,E. W.: The chemical reactivity of urani- Battelle Memorial Institute for supply of arc um carbide, NAA-SR-11235, (1965). fused uranium carbide.

—REFERENCES-

(1) KATZ, J. J., RABINOWITCH, E.: "The Chemistry of Uranium", Part 1, p. 232 (1951), McGraw Hill.

— 36 —