Fabrication of Uranium-Plutonium Mixed Carbide Pellets

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 18(1), pp. 61-70 (January 1981). 61

TECHNICAL REPORT

Yasufumi SUZUKI, Tatsuo SASAYAMA, Yasuo ARAI and Hitoshi WATANABE

Division of Nuclear Fuel Research, Japan Atomic Energy Research Institute*

Received September 30, 1980 Revised December 2, 1980

Uranium-plutonium mixed monocarbide pellets, which had densities over 90.9 T.D. and near-stoichiometric compositions, were fabricated in the gloveboxes of the atmosphere of purified Ar. The carbide powder was first synthesized by reducing mechanically blended oxides with graphite at 1,510-C in a vacuum of 0.2 Pa. This condition was selected so as to minimize the loss of Pu during the reduction in vacuum. The synthesized carbide was milled and compacted, followed by sintering in flowing Ar at 1,540 to 1,750-C. The experimental conditions covered wide ranges of milling time, compacting pressure, sintering temperature and sintering time. The most suitable condition has been established for fabricating uranium-plutonium mixed carbide pellets by the mechanical-blend carbothermic reduction and sintering method. For the case of Ni addition as a sintering aid, the efficacy of the addition for fabricating dense pellets was confirmed, but it was noticed that the composition tended to hyperstoichiometry and the sesquicarbide phase precipitated especially at the pellet surface.

KEYWORDS: uranium carbides, plutonium carbides, carbide pellets, carbothermic reduction, plutonium loss, sintering, nickel, sintering aid, glovebox, fabrication, mixtures, additives

I. INTRODUCTION Uranium-plutonium mixed carbide has the potential for the advanced fuel for the liquid metal fast breeder reactor because of high fissile atom density and thermal conductivity compared with mixed oxides. However, the carbide is chemically active for oxygen and water and must be handled in gloveboxes of the atmosphere of inert gas. Although many studies have been performed with regard to the fabrication of mixed carbide pellets("-"), some subjects remain unsolved at present. For example, the loss of Pu has been pointed out as one of significant subjects"). It should be inhibited as low as possible in view of economics and safety. Further, the control of the pellet density is an important subject and the effects of fabrication conditions such as milling time, compacting pressure, sintering temperature and sintering time must be systematically studied so as to fabricate the desired pellets. In this work, uranium-plutonium mixed carbide was prepared and dense pellets were fabricated using gloveboxes of the atmosphere of purified Ar gas. Further, the optimum

* Oarai-machi, Ibaraki-ken 311.13,

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condition of carbothermic reduction was determined so as to minimize the loss, of Pu and the sintering characteristics of the carbide pellets were investigated. Dense pellets were also produced by adding Ni powder as a sintering aid to evaluate the effects on the density of carbide pellets.

II. EXPERIMENTAL

1. Materials Uranium dioxide powder was obtained from Spencer Chemical Co. and PuO2 powder was obtained from Harwell laboratory, U.K.AEA. The latter powder was prepared by calcination of plutonium oxalate at 800dc in H2 followed by a further calcination at 800-C in air. Table 1 shows the characteristics of both the uranium and plutonium oxides. Reactor-grade natural Table 1 Characteristics of UO2 graphite powder of 99.994% purity was and Pu02 powders obtained from Graphitwerk Kropfmuhl. The powder has the specific surface area of 10.4 m2/g and the mean particle size of about 10 pm'''. The uranium oxide was reduced to stoichiometric dioxide by heating in flowing Ar -8% H2 mixed gas at 1,000dc. The reduction was continued until no water was detectable in the flowing gas by infrared spectroscopy with a gas cell of 10 rn light path. The plutonium oxide and graphite powders were degassed in a dynamic vacuum of 0.2 Pa at 300~400dc for 2-3 h. 2. Procedure Gloveboxes of the atmosphere of purified Ar were used for the fabrication of uranium-plutonium mixed carbide pellets. The Ar gas was purified through a nickel catalyst and a molecular sieve, and circulated by a special blower at a rate of 1.5 ins/min. The gloveboxes were made to hold an airtight structure with the leak rate of less than 10-s atm cc/s. Thus, the atmosphere impurities of the gloveboxes were maintained below 3 ppm 02 and 5 ppm H20, which protected well the oxidation of carbide powder. The details of the gloveboxes were described in a previous paper'''. Figure 1 shows a flow sheet for the fabrication of mixed carbide pellets . The uranium and plutonium dioxides were mixed with a Pu/(U+Pu) mole ratio of 0.20. Graphite powder was blended with the oxides to make a CAUO2±Pu02) mole ratio of 2.975. The ratio was computed from the assumption that some oxygen would dissolve into the monocarbide and could scarcely be removed in the carbothermic reduction. The oxides and graphite were blended mechanically with a V-blender of hard glass and a ball-mill of alumina . After blending for 24 or 48 h with the min, the powder was compacted under the pressure of 100 MPa for the purpose of increasing the contact surfaces and preventing the powder from the scatter during the reduction. The carbothermic reduction of oxides was performed in a furnace with a tungsten plate heater in a dynamic vacuum. The specimen was heated up to the reduction temperature at the rate of 5-C/min and cooled at the rate of 10-C/min. The carbide powder was synthesized on the conditions mentioned in the Sec. III-1. The temperature of the specimen was measured with a pyrometer , which had

62 — Vol. 18, No. 1 (Jan. 1981) TECHNICAL REPORT (Y. Suzuki et al.) 63

been corrected with a Pt-13% Rh thermo- couple. The fragmentary carbide synthesized by the reduction was ground with a ball-mill of tungsten carbide for 8-.47 h. Polyethylene glycol 6,000 of 0.25 % was added to the carbide powder as a binder by dissolving with trichloroethylene, which was then evaporated for 24 h in vacuum. Further, the powder was compacted to green pellets measuring 7 mm in diameter by 5,-7 mm long (2.1-2.3 g). The green pellets were heated to the sintering temperatures and cooled mainly at the rate of 5-C/min. In order to study the efficacy of Ni as a sintering aid, Ni powder with the Fig. 1 Flow sheet of experiment for particle size of 10 pm was added to the fabrication of uranium-plutonium mixed carbide pellets carbide powder and blended together in an agate mortar for 2 h. The compacted specimen was first heated up to 1,000-C at the rate of 5-C/min and then to sintering temperatures (1,400-1,600-C) at the rate of 10-C/min. After keeping at sintering temperatures for 1 h, it was cooled at the rate of 10-C/min. The green and sintered pellets were weighed with a direct-reading balance with the sensibility of 0.1 mg. Their dimensions were measured with a dial gauge with an accuracy of 0.01 mm. The total amount of carbon in the pellets was determined by a high frequency-heating coulometry 7). The oxygen content was determined by inert gas-fusion coulometrycS). The specimen was ground in a glovebox of the atmosphere of purified Ar and sealed in Sn or Pt capsules in order to protect the specimen from oxidation. X-ray diffraction was performed by the powder technique using a diffractometer with Ni filtered CuKa radiation. The specimen for X-ray analysis was fixed to a holder of Al by mixing with an epoxy resin in the glovebox of the atmosphere of purified Ar. Ceramography was performed in the gloveboxes of air atmosphere. The carbide pellet was mounted in a bakelite resin by a hot press. Coarse polishing was accomplished by using silicon carbide papers (grid No. 320-600) and finish was carried out with diamond paste (7-1 pm). The polished specimen was etched by immersion for 30-N-120 s in the etchant, which consisted of 2 nitric acid, 1 acetic acid and 1 water.

III. RESULTS AND DISCUSSION

1. Synthesis of Mixed Carbide The uranium-plutonium mixed carbide was synthesized by the carbothermic reduction of oxides with graphite mainly in a dynamic vacuum (0.2 Pa). The oxides were reduced at 1,320 to 1,570dc in order to determine the optimum condition for the synthesis of the stoichiometric monocarbide with less than 0.2 oxygen. The results are summarized in Table 2. Weight losses between 17 and 20% were obtained from this experiment. Assuming that only CO gas is generated during the reduction, the weight loss is theoretically evaluated at 18.5%. In practice, only a slight amount of CO2 gas was identified except CO gas.

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Table 2 Results of carbothermic reduction of oxides with graphite in vacuum or in flowing He

Therefore, it is expected that the weight loss larger than 18.5% may be caused from the vaporization of Pu. Besmann et al.") stated that mechanically blended oxides could not dissolve together, but each oxide powder was reduced independently. If plutonium oxide is reduced without dissolving into uranium oxide, a considerable loss of Pu will be caused because the equilibrium partial pressure of Pu is nearly equal to that of CO over oxy- carbide with carbon or sesquicarbide. In this work, the reduction of oxides was conducted in a high vacuum of 10-s Pa (by an oil diffusion pump and a rotary pump) or in a low vacuum of 0.2 Pa (by a rotary pump only). In the latter condition, the weight loss observed during the reduction at 1,510-C was 18.4 ,~18 .6% and consistent with that calculated theoretically. This result indicates that the Pu loss may be negligibly small in spite of Besmann's suggestion. However, the weight loss more than 18.5% was observed in the former condition. The loss of Pu by vaporization can be estimated from the amount of the residual oxide in the products and the weight loss during the reduction. Figure 2 shows an estimation on the loss of Pu caused by the reduction in the higher vacuum. Obviously it increases with the reduction temperature. The values estimated have relatively large errors because the amount of the residual oxide was determined by X-ray diffraction. However, it is considered from Fig. 2 that the loss of Pu is above 10% at 1,570-C and considerably large as pointed out by Besmann et al. The experimental results indicate that the extent of the Pu loss is influenced by the vacuum conditions as shown in the following. The solution of Pu with U will take place in the form of carbide rather than oxide. In the course of the reduction of PuO2 to carbides, Pu partial pressures over carbides become great Fig. 2 Calculated Pu loss caused by re- and cannot be ignored, although partial duction of mechanically blended pressures of Pu over oxides with carbon oxides in high vacuum of 10-3 Pa

64 — Vol. 18, No, 1 (Jan. 1981) TECHNICAL REPORT (Y. Suzuki et al.) 65 are much lower than that of CO('). Therefore, a considerable loss of Pu will occur if the plutonium carbides are placed in a high vacuum before the solution. Since the vaporization rate of Pu is reduced in a low vacuum, a sufficient reaction time will be given to form a solid solution without Pu loss. Thus, in the present experiment blended oxides were reduced to form a homogeneous monocarbide in the lower vacuum of 0.2 Pa at 1,510dc. The lattice parameters of monocarbides in the final products were in a range of 0.4965 '-0.4966 nm. In contrast, the values of 0.4963-,0.4966 nm were found for the monocarbide present at intermediate stages of reduction. These values are lower than that reported previously'". As dissolved oxygen lowers the lattice parameter of the monocarbide, it is suggested that some amount of oxygen remained in the monocarbide phase at the intermediate stages. The sesquicarbide was found in quantity at the intermediate stages of the reduction by X-ray diffraction. In the case of the synthesis of uranium monocarbide by the reduction of oxide, dicarbide forms instead of sesquicarbide. The present result indicates that the sesquicarbide may be stabilized as an intermediate product by the addition of 20% Pu. 2. Sintering Characteristics The carbide synthesized by reduction was milled with a ball-mill made of tungsten carbide for 8, 16, 24 and 47 h. After adding a binder, the carbide powder was compacted at 300 MPa into green pellets, which were sintered at 1,700-C for 3 h in flowing Ar. The densities of the green and sintered pellets are plotted against milling time in Fig. 3. The density of 12.4 g/cm' (91%T.D.) was achieved for the pellets fabricated after milling for 47 h. The densities of the green and sintered pellets have tendencies to increase with the milling time. The oxidized powder will release CO gas during sintering and the densification of the pellets will be influenced by the release of CO gas. In the present work, since the carbide powder was milled in the atmosphere of purified Ar, the oxygen content increased slightly from 0.18 to 0.21 N during the milling for 24 h. It is confirmed from these results that an inert atmosphere and a long-time milling may result in the densification of pellets. In order to examine the effect of compacting pressure on the pellet density, two kinds of powders were compacted into green pellets at pressures of 130~500 MPa. One was ground with a mortar grinder of agate so as to pass a 400 mesh screen and then sintered at 1,700dc for 3 h. The other was ground with a ball-mill of tungsten carbide for 47 h and sintered at 1,540dc for 3 h. The results obtained are shown in Fig. 4. The density of the green pellets increases with the compacting pressure to 500 MPa, while the density of the sintered pellets Fig. 3 Effect of milling time on densities remains constant for pressures above 300 of green and sintered pellets

65 — 66 TECIEN1CALREPORT (Y. Suzuki et al.) J. Nucl. Sci. Technol.,

MPa. This tendency is consistent with the result of uranium carbide(". It was noticed that the variation of holding-time from 2 to 5 s for compacting did not influence the densities of the green and sintered pellets. Figure 5 shows the effect of sintering temperature on the density of carbide pellets. The density increases with the sintering temperature up to 1,700dc and is constant over 1,700dc. The sintering temperature of 1,700dc is considered to be suitable for fabricating dense carbide pellets (over 90-T.D.) in flowing Ar, because the sintering at higher temperatures will increase the Pu loss by vaporization. The concentra- tion of Pu, Pu/(U+Pu), was 0.196 (mole ratio) for the pellet sintered at 1,700dc for 16 h510>. This shows that the loss of Pu is negligibly small for the pellets sintered Fig. 4 Effect of compacting pressure on den- below 1,700dc. sities of green and sintered pellets The effect of sintering time on the sintered density is shown in Fig. 6. In this work, three types of carbide powders were used. One of them was ball-milled for 47 h and sintered at 1,700dc (type (A) in Fig. 6). Two others were provided by grinding with an agate mortar to pass a 200 mesh or 325 mesh screen and denoted by (B) and (C) in Fig. 6, respectively. They were sintered by heat-

Fig. 5 Effect of sintering temperature Fig. 6 Effect of sintering time on on density of sintered pellets density of sintered pellets

66 — Vol. 18, No. 1 (Jan. 1981) TECHNICAL REPORT (Y . Suzuki et al.) 67 mg up to 1,740dc and cooling from this temperature at the rate of 10-C/min. The results show that the sintered density of the carbide is strongly dependent on the properties of the powder. In the case of the fine powder (type (C)) , the sintered density rapidly reaches a constant value (12.4-.42 .6 g/cm3) after sintering for 3 h. On the other hand, the density for the coarse powder (type (B)) gradually increases with sintering time. The powder ground for 47 h with the ball-mill (type (A)) shows the intermediate characteristic. The influence of sintering atmosphere on the density or chemical composition were examined by sintering the carbide milled for 47 h either in vacuum or in flowing Ar at 1,700 and 1,750dc. No difference was detectable in the density and the composition of the sintered pellets. Photograph 1 shows a microstructure of the mixed carbide pellet which was sintered at 1,700-C for 3 h in flowing Ar. This pellet had a near-stoichiometric composition (carbon 4.60 %, oxygen 0.26 % and carbon equivalent 4.79 %) and a bulk density of 12.3 g/ cm'. The dicarbide, which precipitates in the shape of the Widmanstaten structure, can be seen in the grains of the monocarbide. It is considered that small precipitates observed as white spots form sesquicarbide

which may precipitate during cooling. But Photo. 1 Microstructure of uranium-putonium they were not detected by X-ray diffraction mixed carbide pellet sintered at due to their small amount. 1,700-C in flowing Ar ( x 300) 3. Effects of Ni Addition on Carbide Pellets Nickel powder is generally used as a sintering aid to fabricate dense carbide pellets at lower temperature(')(2). In this work, sintering characteristics of the pellets with 1.6 % Ni were studied at 1,400, 1,500 and 1,600- C. The results obtained from sintering for 1 h are summarized in Table 3. Evidently, the Ni addition is very effective for the fabrication of dense pellets. The bulk density of 97%T.D. was achieved by sintering at 1,400-C for 1 h in flowing Ar.

Table 3 Characteristics of mixed carbide pellets sintered with Ni for 1 h

67 — 68 TECHNICAL REPORT (Y. Suzuki et al.) J. Nucl. Sci. Technol.,

As given in the chemical analysis in Table 3, the oxygen of 0.3-0.5 % was found in the pellets with Ni. The oxygen content seems to increase with oxygen and water ab- sorbed at the surface of the fine Ni powder. X-ray diffraction gave the result that no oxide phase was found, but a considerable amount of the sesquicarbide was formed. Further, the lattice parameter of the monocarbide was lowered to 0.4963-0.4966 nm. In general, the excess oxygen in pellets will react with carbide and be released as CO gas during sintering. However, as the pellet with Ni is rapidly sintered at lower temperature, much oxygen will remain in the monocarbide. The solution of oxygen into the monocarbide lowers the lattice parameter's' and shifts the composition to hyperstoichiometry, as seen from the result in Table 3 that the carbon equivalent is about 5.00 %. The lattice parameter of the sesquicarbide precipitated was 0.8097 +0.0001 nm. Leary"" suggested the relation between the lattice parameter and the Pu mole ratio, Pu/(U+Pu), in uranium-plutonium mixed sesquicarbide. Using this relation, the Pu mole ratio in the sesquicarbide was estimated to be about 0.4. Assuming that the amount of sesquicarbide in the pellet is 10-15 %, the Pu mole ratio in the monocarbide phase will be 0.16-4.18. This value is consistent with Potter's calculation on the segregation of Pu between mono- and sesqui-carbide phases of the mixed carbided12'. A typical microstructure of the pellet with Ni of 0.1 % is shown in Photo. 2. The pellet was sintered at 1,500dc for 1 h in flowing Ar. A considerable amount of sesquicarbide precipitate is seen as white phases. In some pellets an unidentified phase is observed in grain boundaries as shown in Photo. 3, where gray and narrow precipitates exist in the grain boundaries. This phase could not be detected by X-ray diffraction. Matzke" stated that the grain boundary diffusion was accelerated by the formation of a molten phase and the densification was easily caused at lower temperature. In the case of uranium carbide, the molten phase may be U-Ni alloy at 1,500dc". Further, Photo. 2 Microstructure of mixed carbide Anselin et al.") stated that the molten phase pellet sintered at 1.600dc with in the mixed carbide with Ni was (U, Pu)Ni, 0.1 76 Ni ( x 300)

Photo. 4 Microstructure near surface of mixed Photo. 3 Microstructure of mixed carbide carbide pellet with Ni shows pre- pellet with 0.1 76 Ni ( x 480) cipitation of sesquicarbide (x 150)

68 — Vol. 18, No. 1 (Jan. 1981) TECHNICAL REPORT (Y. Suzuki et al.) 69 but no pseudo-binary compound was identified actually in their experiments. Photograph 4 shows the microstructure near the surface of the pellet with 0.1 % Ni, which was sintered at 1,600dc for 1 h. The white phase precipitated at the surface was identified as sesquicarbide by X-ray diffraction. Since the thickness of the phase increased with the amount of Ni added and Ni was hardly detected within the pellets sintered above 1,500dc"), it is considered that the vaporization of Ni will result in the precipitation of the sesquicarbide at the surface. In contrast, the precipitation of the sesquicarbide in the interior of pellets is related to the solution of oxygen as described before.

IV. CONCLUSIONS

Uranium-plutonium mixed carbide powder was first obtained by the carbothermic reduction, and the carbide was ball-milled, compacted and sintered in purified Ar gas in order to obtain the mixed carbide pellets. It was possible to fabricate the pellets with the oxygen content less than 0.3 % and the density larger than 90%T.D. In the course of the fabrication, the behavior of the carbothermic reduction and the sintering characteristics with or without sintering aid were studied. Concerning the carbide synthesis, the near-stoichiometric carbide (carbon 4.6-4.7 ';v6, oxygen 0.1-0.2 ;70) was synthesized at 1,510-C in a dynamic vacuum of 0.2 Pa with a C/ (UO2+Pu02) mole ratio of 2.975. Also, it was made clear that the carbothermic reduction in a high vacuum may result in the increase of Pu loss. The monocarbide containing much oxygen and the sesquicarbide were identified in the intermediate stages of the reduction. As to the pellet fabrication without addition of Ni powder as a sintering aid, it was observed that the densities of green and sintered pellets increased with milling time and that no oxidation of the carbide took place during milling in a purified Ar gas atmosphere. Furthermore, both the compacting at pressures over 300 MPa and the sintering at very high temperature or for very long time were found not to increase the sintered density. Thus, the most suitable condition for the pellet fabrication with the density higher than 90%T.D. was given by the milling for 47 h, the compacting at the pressure of 300 MPa and the sintering at 1,700-C for 3 h in flowing Ar. Dense carbide pellets of 95'97%T.D. were fabricated by adding Ni powder as a sintering aid. In this case, the chemical composition of the pellets was slightly hyper- stoichiometric accompanied with formation of sesquicarbide. Further, an unidentified molten phase formation at the grain boundary and the sesquicarbide precipitation especially at the surface of pellet were noticed.

ACKNOWLEDGMENTS

The authors wish to express their thanks to Dr. J. Shimokawa and Dr. K. Iwamoto for their interest in this work, and also to Dr. M. Handa and Mr. A. Maeda for the determina- tion of carbon and oxygen contents. The generous helps of Dr. T. Omichi, Mr. S. Fuku- shima, Mr. I. Takahashi and Mr. J. Abe are also gratefully acknowledged.

REFERENCES

(1) STRASSER, A., STAHL, D.: UNC-5134, (1965). (2) HORSPOOL, J. M., ROSE, N. F., FINLAYSON, M. B. : Proc. Nuclear and Engineering Ceramics, Brit. Ceram. Soc., No. 7, p. 23 (1967).

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(3) ANSELIN, F., DEAN, G., LORENZELLI, R., PASCARD, R.: Proc. Symp. Carbides in Nuclear Energy, 1963, (eds. RUSSELL, L. E. et al.), p. 113 (1964), Macmillan, London. (4) WEDEMEYER, H., GUNTHER, E.: KFK-2238, (1976). (5) SUZUKI, Y., SASAYAMA, T., ABE, J., ARAI, Y., MAEDA, A., WATANABE, H.: JAERI-M 7601, (in Japanese), (1978). (6) NOMURA, S., SHIMOKAWA,J., UEMATSU, K., NORO, K.: Proc. Advanced LMFBR Fuels, Tucson, 1977, (eds. LEARY, J. et al.), p. 61 (1977), ANS. (7) HANDA, M MAEDA, A., SHIOZAWA, K.: JAERI-M 8414, (in Japanese), (1979). (8) HANDA, M., MAEDA, A., YAHATA, T.: JAERI-M 8406, (in Japanese), (1979). (9) BESMANN, T. M., LINDEMER, T. B.: J. Nucl. Mater., 67, 77 (1977). YAHATA, T., MAEDA, A.: Private communication. (10) (11) LEARY, J. A.: Proc. Ceramic Nuclear Fuels, (Amer. Ceram. Soc. Spec. Publ. 2), p. 38 (1969). (12) POTTER, P. E. : J. Nucl. Mater., 42, 1 (1972). (13) MATZKE, Hj. : ibid., 52, 85 (1974). (14) DUTTA, S. K., WHITE, J.: Ref. (2), p. 177.

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