UO2 pellet fabrication process

Abstract: There is disclosed a process for fabricating UO₂ pellets from highly active uranium dioxide powders, comprising the steps of (1) uniformly adding at least one pore-former agent in the range of 0.3 to 1.4% by weight to uranium dioxide powders as a starting material, the pore-­ former agent decomposing and sublimating below 600 °C and having an average particle size in the range of 5 to 500 µm, (2) compacting the uranium dioxide powders including the pore-former agent to form green pellets, and (3) sintering the green pellets to form sintered UO₂ pellets having large grain size crystals. According to the present invention, UO₂ pellets having the sintered density in the range of 94 to 97% TD are fabricated from highly active UO₂ powder when the pellets have large crystalline grain sizes exceeding 20 µm. Accordingly, the fission gasses generated during irradiation is restrained to the inside pellet thereof to enhance the safety of the irradiation.

Description: BACKGROUND OF THE INVENTION The present invention relates to a process for fabricating nuclear reactor fuel pellets having large grain sizes from highly active UO 2 powder. In particular, the present invention concerns a method for controlling the sintered density of UO 2 pellets to a predetermined range. When UO 2 pellets are used as the fuel in nuclear reactors, it is important that the fuel density be high within a reasonable limit so that a more compact reactor core can be designed, and that the thermal conductivity of the pellets is sufficiently high. However, when the sintered density of the pellets is too high, swelling of the pellets during irradiation becomes too great, thereby damaging a tube in the reactor. Accordingly, UO 2 pellets commonly used in light-water reactors are usually designed so that the sintered density is in the range of from 94 to 97 % TD (theoretical density). One recent technical innovation is to prolong the useful life of the reactor fuel. This is called the plan for "high burnup", and it is now being studied seriously. In order to execute this plan, it is imperative to restrain the fission gas (FP gas) in the pellets as much as possible. It is well known that producing large crystal grain sizes is effective in confining FP gas in the pellets. However, the conventional technology only produced grain sizes of at most about 10 to 20 µm. In light of the above, the applicants have proposed a process for fabricating UO 2 pellets with large-grain size crystals in JP-A-62-297215, JP-A-63-45127, EP-A-322 480, EP-A-344 395 and EP-A-344 396. These processes have a common effect of producing in that they make crystalline grains of large size by controlling the conditions of ammonium diuranate (ADU) formation. FR-A-2318837 discloses a method for producing UO 2 pellets comprising mixing ammonium oxalate as a pore-former agent to UO 2 power including the pore-former to form a green pellet, and sintering the green pellet to produce a UO 2 pellet. Encyclopedia of Physical Science and Technology, Vol. 9, 1987, Academic Press Inc., "Nuclear Reactor Materials and Fuels", pages 358 to 361 describes conventional manufacturing of UO 2 pellets wherein a granulation step is carried out before the compacting step. BE-A-703-764 mentions the use of a lubricant and pore-former agents in a conventional manufacturing process for nuclear reactor materials. With the processes described in the applications and patents, it is possible to control the crystalline grain size, however at the time these previous patents were filed, the applicants did not consider to control the sintered density of the pellets. In other words, when pellets are fabricated pellets having grain sizes larger than 20 µm by the process described above, the sintered density of the resulting pellets are as high as 98 to 99% TD. In order to reduce the sintered density of the sintered body, if required, it has been common to add a pore-former agent to the raw material powder, which cause the formation of pores when the agent sublimates during sintering. The applicants believe that the method is applicable to the fabrication process for pellets composed of large-grain size crystals. Although a pore-former agent of this kind is effective in reducing the sintered density, the agent is likely to have adverse effects on UO 2 grain growth. In other words, the formation of crystalline grains with large grain size is disrupted. Accordingly, it is desired to develop a method for controlling both the sintered density of the pellets and the crystalline grain sizes. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a process for fabricating UO 2 pellets having the sintered density of pellets within the range of from 94 to 97% TD, when pellets having large crystalline grain sizes in excess of 20 µm are fabricated from highly active UO 2 powder, and thereby restrain the fission gasses generated during the irradiation in the pellets thereof to enhance the safety of irradiation. According to the present invention, there is provided a process for fabricating UO 2 pellets comprising (1) uniformly adding at least one pore-former agent in the range of 0.3 to 1.4% by weight to uranium dioxide powders as a starting material, the pore-former agent decomposing and sublimating below 600°C and having an average particle size in the range of 5 to 500 µm, (2) subjecting the uranium dioxide powders including the pore-former agent to rough compacting and pulverization to form granules having particle sizes less than 2000 µm, (3) adding at least one lubricant to the granules, (4) compacting the granules and the lubricant to form green pellets, and (5) sintering the green pellets to form sintered UO 2 pellets having an average crystalline grain size larger than 20 µm and having a sintered density in the range between 94 to 97% of the theoretical density. Thus, according to the present invention, both the sintered density of the pellets and the crystalline grain sizes can be easily controlled to the desired range, which was not previously possible. DETAILED DESCRIPTION A process for fabricating UO 2 pellets, according to the present invention, will be specifically described below. First, a pore-former agent is added to highly active UO 2 powder. From experimental results, the inventors have determined that the following conditions must be met in order to maintain a sintered density in the range between 94 to 97% TD. (1) It is necessary that the particle size of the pore-former agent be between 5 and 500 µm, and preferably between 10 and 100 µm. When the particle size is less than 5 µm, the pores left behind after the sublimation of the agent during sintering disrupted the growth of the crystals, and thus, the crystalline grain sizes in the pellets is small. On the other hand, when the particle size exceeds 500 µm, large pores are formed on the surface of pellets. These large pores must be avoided because they accelerate the absorption of the water into the pellets. (2) It is necessary to use a pore-former agent that decomposes and sublimates below 600 °C, more preferably at 500 °C. When the decomposition temperature is higher than 600 °C, the pore-former agent is confined to the inside of the pellets during the sintering process and cracks or holes appear in the pellets. Ammonium acetate, ammonium carbonate, ammonium bicarbonate, ammonium oxalate, ammonium alginate, stearic acid, and the like, are pore-former agents that meet the above requirements. These compounds are used either alone or in a mixture. (3) The appropriate amount of these pore-former agent is in the range of 0.3 to 1.4% by weight of the UO 2 powders. The sintered density of the pellet will not be in the range of 94 to 97% TD, when the added amount falls outside this range. Subsequently, UO 2 powder, to which a pore-former agent has been added, is fabricated into pellets by the method described below. UO 2 powder that includes a pore-former agent is roughly molded into a lump which is pulverized to obtain granules. The granules are compacted by adding at least one lubricant to the granules and compacting the granules and the lubricant to form green pellets. Stearic acid, zinc stearate, lithium stearate, stearic amide, ethylene-bis-stearic amide, methylene-bis-stearic amide, polyethylene glycol, and the like, are suitable compounds for the lubricant. These compounds are used either alone or in mixtures. When a lubricant of this kind is added to the raw material powder, or when a lubricant is applied to the mold, the press-compacting is easily carried out. The size of said granules is less than 2000 µm, and more preferably less than 1000 µm. When the size exceeds 2000 µm, it is difficult to fill a designated amount of the granule into a mold and perform the compacting step. EXAMPLES The following examples are cited to prove the usefulness of the invention. Example 1 ADU (ammonium diuranate), produced by the reaction between 300gU/l of UO 2 F 2 aqueous solution and aqueous ammonia, was calcined and reduced to form UO 2 powder. After 0 to 1.5% by weight of pore-former agents, whose particle size had been adjusted to several different particle sizes, was uniformly added to the UO 2 powder, the pressed bodies were formed under the pressure of 3 t/cm 2 by the use of various compacting processes. Ammonium oxalate and zinc stearate were selected as the pore-former agent and the lubricant, respectively. The pressed body was heated to 1750 °C for four hours in hydrogen atmosphere to produce sintered pellets. Table 1 shows the relationship between pellets fabrication conditions and physical properties of the pellets. As is clearly seen from Table 1, the pellets fabricated by the processes according to the present invention have a sintered density between 94 and 97% TD. Furthermore, significant reduction of the crystalline grain size, and reduced probability of having large pores, both due to the presence of a pore-former agent, is apparent. EXAMPLE 2 One or more compounds were selected as pore-former agents from the group consisting of ammonium acetate (C 1), ammonium carbonate (C 2), ammonium bicarbonate (C 3), ammonium oxalate (C 4), ammonium alginate (C 5) and stearic acid (C 6). 1.0% by weight of the selected compound or mixture thereof was added to the same UO 2 powder as in Example 1. The resulting UO 2 powder was subjected to rough compacting, pulverization and granulation to form granules. Subsequently, one or more compounds were selected as a lubricant from the group consisting of stearic acid (D 1), zinc stearate (D 2), lithium stearate (D 3), stearic amide (D 4), ethylene-bis-stearic amide (D 5), methylene-bis-stearic amide (D 6) and polyethylene glycol (D 7), and 0.2% by weight of the selected compound or compounds was added to the granules to make the mixture. Furthermore, the mixture was subjected to the same compacting and sintering as in Example 1 to form pellets. Table 2 shows the relationships between the fabrication conditions of pellets and the physical properties of the pellets. As is clearly seen from Table 2, the pellets having the excellent physical properties are fabricated by using the above-described pore-former agents and lubricants in the process according to the present invention. Pore-Former Agent Lubricant Grain Size of Pellets (µm) C 1 D 2 50.2 C 2 " 48.6 C 3 " 49.3 C 4 " 52.9 C 5 " 49.4 C 6 " 51.1 C 1 + C 4 " 49.5 C 1 + C 2 + C 4 " 48.3 C 1 + C 2 + C 3 + C 4 " 49.9 C 1 + C 2 + C 3 + C 4 + C 5 " 49.7 C 4 D 1 51.3 " D 3 50.4 " D 4 48.5 " D 5 51.1 " D 6 49.7 " D 7 50.3 " D 1 + D 2 50.5 " D 1 + D 2 + D 3 49.8 " D 1 + D 2 + D 3 + D 4 50.9 " D 1 + D 2 + D 3 + D 4 + D 5 52.2 " D 1 + D 2 + D 3 + D 4 + D 5 + D 6 50.5 " D1 + D2 + D3 + D4 + D5 + D6 + D7 49.8

Claims: A process for fabricating uranium dioxide (UO 2 ) pellets from highly active uranium dioxide powders, comprising: (1) uniformly adding at least one pore-former agent in the range of 0.3 to 1.4% by weight to uranium dioxide powders as a starting material, the pore-former agent decomposing and sublimating below 600°C and having an average particle size in the range of 5 to 500 µm, (2) subjecting the uranium dioxide powders including the pore-former agent to rough compacting and pulverization to form granules having particle sizes less than 2000 µm, (3) adding at least one lubricant to the granules, (4) compacting the granules and the lubricant to form green pellets, and (5) sintering the green pellets to form sintered UO 2 pellets having an average crystalline grain size larger than 20 µm and having a sintered density in the range between 94 to 97% of the theoretical density. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein the compacting step is performed after at least one lubricant is added to the uranium dioxide powders including the pore-former agent. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein the compacting step is performed after inner surfaces of a mold are coated with at least one lubricant. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein the compacting is performed after the granules are filled into a mold whose inner surfaces is coated with at least one lubricant. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein at least one pore-former agent is selected from the group consisting of ammonium acetate, ammonium carbonate, ammonium bicarbonate, ammonium oxalate, ammonium alginate and stearic acid. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein at least one of the lubricants is selected from the group consisting of stearic acid, zinc stearate, lithium stearate, stearic amide, ethylene-bis-stearic amide and polyethylene glycol. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein said pore-former agent has a particle size between 10 and 100 µm. A process for fabricating UO 2 pellets as claimed in Claim 1, wherein said granules have particle sizes less than 1000 µm.

Inventors: TANAKA HIROSHI, YATO TADAO, TANAKA, HIROSHI, YATO, TADAO
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1989
Main classification: G21C3
IPC codes: C01G43, G21C3