Preparation of Uranium Carbonitride by Reaction of UC with Ammonia
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Journal of NUCLEARSCIENCE and TECHNOLOGY,5[8], p. 414~418 (August 1968). Preparation of Uranium Carbonitride by Reaction of UC with Ammonia 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 am- monia 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 sinter- ing 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.