Patent Number: 050154222
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a UO.sub.2 fabrication process on the basis of an ADU (ammonium diuranate) process, and more particularly, to an improvement in a method of controlling the grain size of UO.sub.2 pellets made from UO.sub.2 powder produced by the ADU method, by adjustment of the precipitation conditions in the ADU method. RELATED ART As is well known, the ADU method is widely utilized as a process for fabricating UO.sub.2 powder from UF.sub.6. The ADU method is such that an aqueous solution of UO.sub.2 F.sub.2 obtained by reaction of UF.sub.6 gas with water is reacted with NH.sub.4 OH produced by passing NH.sub.3 through the solution to precipitate the ADU. The ADU is then filtered and dried and, thereafter, calcined and reduced to form UO.sub.2 powder. The reaction by which the ADU is formed is represented by the following chemical equations: EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (1) EQU UO.sub.2 F.sub.2 +4HF+7NH.sub.4 OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 O.sub.7 +6NH.sub.4 F+(11/2)H.sub.2 O (2) The UF.sub.6 is first transformed to an aqueous solution of UO.sub.2 F.sub.2 by the hydrolytic reaction represented by chemical equation (1). Then, since four moles of HF exists in the aqueous solution compared to one mole of uranium, a neutralizing reaction of HF takes place simultaneously in the second equation with the formation of ADU precipitate in the form of inactive particles of relatively large size. If the UO.sub.2 powder obtained through the processes of calcination and reduction of the inactive ADU is employed as a raw material to form pellets, the grain size of the pellets is usually made to be approximately 10 .mu.m. In order to burn the UO.sub.2 pellets in a nuclear reactor in a stable manner, it is desirable to decrease the fission-product gas (FP gas) release from the pellets as low as possible. It has been found that, if the grain size of the pellets is increased, the retention of the FP gas is enhanced. However, there is concern that excessively large grain in the pellets may result in a reduction of the mechanical strength thereof. Although an optimum grain size has not yet been determined, it is considered appropriate to aim for an upper limit of 100 .mu.m. In view of the above, the assignee of this invention has previously, in Japanese Patent Application No. 61-190079, proposed a process for fabricating UO.sub.2 pellets having a large grain size. The method disclosed in the above patent application is characterized in that NH.sub.3 is reacted with a UO.sub.2 F.sub.2 aqueous solution containing U but no HF, to form ADU, with the U concentration in the UO.sub.2 F.sub.2 aqueous solution being within the range from 50 to 1000 g/l, and the rate at which the NH.sub.3 is added to the UO.sub.2 F.sub.2 aqueous solution being set to a value equal to or higher than two moles of NH.sub.3 /min for every one mole of U. According to the above method, the lower the U concentration of the UO.sub.2 F.sub.2 aqueous solution, and the higher the rate of adding the NH.sub.3 to the UO.sub.2 F.sub.2 aqueous solution, the smaller the size of the primary particles of ADU formed so that the particles are highly active when burned to form FP gas. As a result of this, the growth of the UO.sub.2 grains of the UO.sub.2 pellet obtained by way of UO.sub.2 powder from ADU is accelerated so that the grain size becomes larger. This is done by appropriately setting the precipitating conditions within the aforesaid range making it possible to fabricate the UO.sub.2 pellets with a grain size within the range from 10 to 100 .mu.m. In the above method, however, the rate of reaction between the U and the NH.sub.3, and the NH.sub.3 /U molar ratio must be brought to a sufficiently high value, because of the necessity to maintain sufficient productivity as the ADU precipitates out. The reason for this is that, if the NH.sub.3 /U ratio is less than 6 (less than pH 10), all the U is not consumed in the reaction to form ADU precipitate so that some U remains in the waste liquid. The reason for the above is also that the low reaction rate results in a reduction in the productivity of ADU. Accordingly, by controlling the U concentration the size of the primary particles of the ADU can be adjusted. If UO.sub.2 pellets are to be fabricated with a relatively small grain size near the low value of the range from 10 to 100 .mu.m, the U concentration in the UO.sub.2 F.sub.2 aqueous solution must be equal to or higher than 500 g/l. This inevitably raises the viscosity of the UO.sub.2 F.sub.2 aqueous solution considerably, affecting the conditions of the precipitate, resulting in the disadvantage that the final UO.sub.2 pellets becomes heterogeneous. SUMMARY OF THE INVENTION The invention has been done in order to solve the above-discussed problems related to optimum grain size, and it is the object of the invention to provide a UO.sub.2 fabrication process which can control the grain size of UO.sub.2 pellets to an optimal value within a range of from 10 to 100 .mu.m, and which can fabricate pellets with homogeneous properties of any particle size.