Patent Number: 050154222
Section: description

DETAILED DESCRIPTION A process for fabricating UO.sub.2 pellets, according to the invention, will specifically be described below. The process is characterized in that when ADU is precipitated, the U concentration in UO.sub.2 F.sub.2 aqueous solution is brought to a value within the range from 50 to 500 g/l, that the reaction of the UO.sub.2 F.sub.2 aqueous solution with NH.sub.3 is divided into two stages, and that the NH.sub.3 /U molar ratio is set in the first step to a value within the range from 3 to 6, and in the second stage to a value within the range from 6 to 12. According to the above-described process, the properties of the ADU formed are substantially determined by the precipitating reaction in the first stage. In this connection, if conditions are such that the NH.sub.3 /U molar ratio is equal to or less than 6, it is possible to form ADU as primary particles which are relatively large in size, even if the U concentration in the UO.sub.2 F.sub.2 aqueous solution is equal to or less than 500 g/l. It is considered that the reason for this is that in the case where the NH.sub.3 /U molar ratio is equal to or less than 6, as the NH.sub.4 F concentration increases on the basis of the reaction represented by the following chemical equation (3) in which the ADU is precipitated out of the UO.sub.2 F.sub.2 aqueous solution, the NH.sub.4 F causes a reaction to occur whereby the ADU is formed by way of ammonium uranyl fluoride (AUF), as represented by the equations (4) and (5): EQU UO.sub.2 F+3NH.sub.4 OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 O.sub.7 +2NH.sub.4 F+(3/2)H.sub.2 O (3) EQU UO.sub.2 F.sub.2 +3NH.sub.4 F.fwdarw.(NH.sub.4).sub.3 UO.sub.2 F.sub.5 (4) EQU (NH.sub.4).sub.3 UO.sub.2 F.sub.5 +3NH.sub.4 OH.fwdarw.(1/2)(NH.sub.4).sub.2 U.sub.2 O.sub.7 +5NH.sub.4 F+(3/2)H.sub.2 O (5) Since the AUF is a crystalline material inert under normal conditions, the ADU formed by way of the AUF is also inert and has relatively large primary particles. The lower the NH.sub.3 /U molar ratio set in the first stage reaction, the greater the tendency for the ADU to be formed by way of the AUF, so that ADU which is inert and has larger primary particles is obtained. It is not desirable for the NH.sub.3 /U molar ratio to be lower than 3 in the first stage precipitating reaction, because this will lower the ratio at which the U is precipitated. On the other hand, if the NH.sub.3 /U molar ratio is equal to or higher than 6, the conventional problems cannot be solved. In this connection, although the U remains in the aqueous solution even if the NH.sub.3 /U molar ratio is within the range of from 3 to 6, it is possible to react the U sufficiently if the NH.sub.3 /U molar ratio in the second stage reaction is brought to a value within the range of from 6 to 12. Further, if the NH.sub.3 /U molar ratio is lower than 6 in the second stage reaction, the U is not sufficiently precipitated. On the other hand, there is no value in having a NH.sub.3 /U molar ratio above 12, because this merely increases the amount of water used and an amount of waste liquid. Also in the case where the NH.sub.3 /U molar ratio is brought to a value within the range from 6 to 12 in the second stage in order to precipitate the ADU sufficiently in the manner mentioned above, the properties of the resulting ADU are no different than those of the ADU formed in the standard method equation (2). Thus, according to the process of this invention, it is possible to easily fabricate pellets with an optimal particle size within the range from 10 to 100 um and which are homogeneous in properties, without wasting the U even under for the condition where the U concentration in the UO.sub.2 F.sub.2 aqueous solution is equal to or less than 500 g/l. The advantages of the invention will next be expounded with reference to an embodiment. The UO.sub.2 F.sub.2 powder was dissolved in demineralized water to form an aqueous solution whose U concentration within the range from 40 to 600 g/l. The aqueous solution and NH.sub.3 water were first fed continuously to a first-stage settling chamber, with a 2.5 to 6.5 NH.sub.3 /U molar ratio, to carry out the first stage ADU precipitation. Subsequently, the ADU slurry formed in the first-stage settling chamber, and the aqueous NH.sub.3 were fed continuously to a second stage settling chamber, and the NH.sub.3 /U molar ratio brought to a value within the range from 5 to 15. The resulting second-stage ADU slurry was filtered and dried and, thereafter, calcined and reduced at 650.degree. C. under a H.sub.2 atmosphere, to transform the slurry into UO.sub.2 powder. The UO.sub.2 powder was compacted at a pressure of 5 t/cm.sup.2, and then sintered for four hours at 1750.degree. C. in an H.sub.2 atmosphere, to form pellets. The following table indicates the relationship between the pellet grain size and the ADU precipitating conditions at each of the first-stage and second-stage settling chambers. TABLE ______________________________________ U CONCENTRATION FIRST SECOND PELLET UO.sub.2 F.sub.2 STAGE STAGE GRAIN AQUEOUS SOLUTION NH.sub.3 /U NH.sub.3 /U SIZE (g/l) RATIO RATIO (.mu.m) ______________________________________ 50 2.5 9.0 7 50 3.0 9.0 10 50 4.3 9.0 46 50 6.0 9.0 98 50 6.5 9.0 110 40 6.0 9.0 105 50 6.0 9.0 96 300 6.0 9.0 42 500 6.0 9.0 23 600 6.0 9.0 9 100 5.0 5.0 34 100 5.0 6.0 36 100 5.0 9.0 33 100 5.0 12.0 35 100 5.0 15.0 34 ______________________________________ As will be clear from the above table, in case where the U concentration in the UO.sub.2 F.sub.2 aqueous solution was 50 g/l, pellets having a grain size within the range from 10 to 100 .mu.m were obtained when the NH.sub.3 /U molar ratio in the first stage was within the range from 3 to 6. Further, if the NH.sub.3 /U molar ratio in the first stage was brought to 6, pellets having a grain size within the range from 10 to 100 .mu.m were obtained when the U concentration in the UO.sub.2 F.sub.2 aqueous solution was within the range from 50 to 500 g/l. Moreover, if the U concentration was 100 g/l and the NH.sub.3 /U molar ratio in the first stage was brought to 5, the grain size of the pellets remained practically unchanged, even if the NH.sub.3 /U molar ratio in the second stage varied within the range from 5 to 15. If, however, the NH.sub.3 /U molar ratio in the second stage was 5, the loss of the U was so great that approximately 20% of the U remained in the waste liquid. On the other hand, even if the NH.sub.3 /U molar ratio in the second stage was 15, the U loss remained the same as for when the ratio ranged from 6 to 12, and a sufficiently high collecting ratio was obtained even if the NH.sub.3 /U molar ratio in the second stage was within the range from 6 to 12. As described above, according to the UO.sub.2 pellet fabrication process of the invention, it is possible to easily fabricate pellets which have their optional grain size within the range from 10 to 100 .mu.m and which are homogeneous in properties, without wasting the U even under conditions where the U concentration in the UO.sub.2 F.sub.2 is equal to or less than 500 g/l. Thus, the amount of the pellets restrict the rate of release of fission product gas can be set to a desired value, making it possible to enhance the combustion stability of the pellets.