Patent Number: 049851837
Section: summary

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.sub.2 powder. In particular, the present invention concerns a method for controlling the sintered density of UO.sub.2 pellets to a predetermined range. When UO.sub.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.sub.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 .mu.m. In light of the above, the applicants have proposed a process for fabricating UO.sub.2 pellets with large-grain size crystals in JP-A-62-297215, JP-A-63-45127, U.S. patent application Ser. Nos. 139447, 296802 and 296808. 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. 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 .mu.m 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.sub.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.sub.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 .mu.m are fabricated from highly active UO.sub.2 powder, and thereby restrain the fission gasses generated during the irradiation in the pellets thereof to enhance the safety of irradiation. According to an aspect of the present invention, there is provided a process for fabricating UO.sub.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 to form a starting material, the pore-former agent decomposing and sublimating below 600.degree. C. and having average grain size in the range of 5 to 500 .mu.m, (2) compacting the uranium dioxide powders, including the pore-former agent, to form green pellets, and (3) sintering the green pellets to form sintered UO.sub.2 pellets having large grain-size crystals. According to the present invention, Accordingly, 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.sub.2 pellets, according to the present invention, will be specifically described below. First, a pore-former agent is added to highly active UO.sub.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 .mu.m, and preferably between 10 and 100 .mu.m. When the particle size is less than 5 .mu.m, 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 .mu.m, 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.degree. C., more preferably at 500.degree. C. When the decomposition temperature is higher than 600.degree. 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.sub.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.sub.2 powder, to which a pore-former agent has been added, is fabricated into pellets by any of the methods described below. (a) UO.sub.2 powder, to which a pore-former agent has been added, is filled into a mold and pressed to make a pressed body. (b) A lubricant is added uniformly to UO.sub.2 powder to which a pore-former agent has been added. Thus prepared, the powder is filled into a mold and subjected to compacting. (c) UO.sub.2 powder, to which a pore-former agent has been added, is filled into a mold coated with a lubricant. Subsequently, the compacting step is carried out on the filled mold. (d) UO.sub.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 any of the processes described in (a) to (c) above. 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. When the granules are made by the method described in (d), the size of the granules should be less than 2000 .mu.m, and more preferably less than 1000 .mu.m. When the size exceeds 2000 .mu.m, it is difficult to fill a designated amount of the granule into a mold and perform the compacting step.