URANIUM DIOXIDE PELLET PRODUCTION PROCESS

Abstract: A process for fabricating UO₂ pellets comprising steps of adjusting a U concentration of said UO₂F₂ aqueous solution to a value between 50 and 500 g/l, reacting the UO₂F₂ aqueous solution with the NH₃ in first and second stages, setting the NH₃/U ratio in the first stage to a value between 3 and 6, and setting in the second stage to a value between 6 and 12, thereby forming UO₂ pellets having their grain size within the range from 10 to 100 µm.

Description: The present invention relates to a UO₂ 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 UO₂ pellets made from UO₂ powder produced by the ADU method, by adjustment of the precipitation conditions in the ADU method. As is well known, the ADU method is widely utilized as a process for fabricating UO₂ powder from UF₆. The ADU method is such that an aqueous solution of UO₂F₂ obtained by reaction of UF₆ gas with water is reacted with NH₄OH produced by passing NH₃ through the solution to precipitate the ADU. The ADU is then filtered and dried and, thereafter, calcined and reduced to form UO₂ powder. The reaction by which the ADU is formed is represented by the following chemical equations: UF₆ + 2 H₂O → UO₂F₂ + 4 HF (1) UO₂F₂ + 4 HF + 7 NH₄OH → (1/2)(NH₄)₂U₂O₇ + 6 NH₄F + (11/2)H₂O (2) The UF₆ is first transformed to an aqueous solution of UO₂F₂ 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 UO₂ 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 µm. In order to burn the UO₂ 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 µm. In view of the above, the assignee of this invention has previously, in Japanese Patent Application No. 61-190079, proposed a process for fabricating UO₂ pellets having a large grain size. The method disclosed in the above patent application is characterized in that NH₃ is reacted with a UO₂F₂ aqueous solution containing U but no HF, to form ADU, with the U concentration in the UO₂F₂ aqueous solution being within the range from 50 to 1000 g/l, and the rate at which the NH₃ is added to the UO₂F₂ aqueous solution being set to a value equal to or higher than two moles of NH₃/min for every one mole of U. According to the above method, the lower the U concentration of the UO₂F₂ aqueous solution, and the higher the rate of adding the NH₃ to the UO₂F₂ 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 UO₂ grains of the UO₂ pellet obtained by way of UO₂ 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 UO₂ pellets with a grain size within the range from 10 to 100 µm. In the above method, however, the rate of reaction between the U and the NH₃, and the NH₃/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 NH₃/U ratio is less than 6 (less than pH 10), all the U is not consumed in the reaction for 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 UO₂ pellets are to be fabricated with a relatively small grain size near the low value of the range from 10 to 100 µm, the U concentration in the UO₂F₂ aqueous solution must be equal to or higher than 500 g/l. This inevitably raises the viscosity of the UO₂F₂ aqueous solution considerably, affecting the conditions of the precipitate, resulting in the disadvantage that the final UO₂ 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 UO₂ fabrication process which can control the grain size of UO₂ pellets to an optimal value within a range of from 10 to 100 µm, and which can fabricate pellets with homogeneous properties of any particle size. DETAILED DESCRIPTION A process for fabricating UO₂ pellets, according to the invention, will specifically be described below. The process is characterized in that when ADU is precipitated, the U concentration in UO₂F₂ aqueous solution is brought to a value within the range from 50 to 500 g/l, that the reaction of the UO₂F₂ aqueous solution with NH₃ is divided into two stages, and that the NH₃/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 NH₃/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 UO₂F₂ 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 NH₃/U molar ratio is equal to or less than 6, as the NH₄F concentration increases on the basis of the reaction represented by the following chemical equation (3) in which the ADU is precipitated out of the UO₂F₂ aqueous solution, the NH₄F 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): UO₂F + 3 NH₄OH→ (1/2)(NH₄)₂U₂O₇ + 2 NH₄F + (3/2)H₂O (3) UO₂F₂ + 3 NH₄F→(NH₄)₃UO₂F₅ (4) (NH₄)₃UO₂F₅ + 3NH₄OH→ (1/2)(NH₄)₂U₂O₇ + 5 NH₄F + (3/2)H₂O (5) Since the AUF is 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 NH₃/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 NH₃/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 NH₃/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 NH₃/U molar ratio is within the range of from 3 to 6, it is possible to react the U sufficiently if the NH₃/U molar ratio in the second stage reaction is brought to a value within the range of from 6 to 12. Further, if the NH₃/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 NH₃/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 NH₃/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 UO₂F₂ 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 UO₂F₂ 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 NH₃ water were first fed continuously to a first-stage settling chamber, with a 2.5 to 6.5 NH₃/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 NH₃ were fed continuously to a second-stage settling chamber, and the NH₃/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 °C under a H₂ atmosphere, to transform the slurry into UO₂ powder. The UO₂ powder was compacted at a pressure of 5 t/cm², and then sintered for four hours at 1750 °C in an H₂ 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. U CONCENTRATION UO₂F₂ AQUEOUS SOLUTION (g/l) FIRST STAGE NH₃/U RATIO SECOND STAGE NH₃/U RATIO PELLET GRAIN SIZE (µm) 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 UO₂F₂ aqueous solution was 50 g/l, pellets having a grain size within the range from 10 to 100 µm were obtained when the NH₃/U molar ratio in the first stage was within the range from 3 to 6. Further, if the NH₃/U molar ratio in the first stage was brought to 6, pellets having a grain size within the range from 10 to 100 um were obtained when the U concentration in the UO₂F₂ aqueous solution was within the range from 50 to 500 g/l. Moreover, if the U concentration was 100 g/l and the NH₃/U molar ratio in the first stage was brought to 5, the grain size of the pellets remained practically unchanged, even if the NH₃/U molar ratio in the second stage varied within the range from 5 to 15. If, however, the NH₃/U molar ratio in the second stage was 5, the less of the U was so great that approximately 20% of the U remained in the waste liquid. On the other hand, even if the NH₃/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 NH₃/U molar ratio in the second stage was within the range from 6 to 12. As described above, according to the UO₂ 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 µm and which are homogeneous in properties, without wasting the U even under conditions where the U cencentration in the UO₂F₂ 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.

Claims: A process for fabricating UO₂ pellets of controlled grain size comprising: (a) treating an aqueous solution of 50 to 500 g/l of UO₂F₂ containing substantially no HF with NH₃ at a molar ratio of NH₃/UO₂F₂ of 3 to 6; (b) adding more NH₃ to said solution to obtain an ADU(NH₄)₂U₂O₇ precipitate at a NH₃/U molar ratio of from 6 to 12; (c) filtering and drying said (NH₄)₂U₂O₇ precipitate; (d) calcinating and reducing said (NH₄)₂U₂O₇ under a hydrogen atmosphere to form UO₂ powder; (e) compacting said UO₂ powder; and (f) sintering said compacted UO₂ powder in a H₂ atmosphere, to form pellets of UO₂ of a grain size of from 10 to 100 µm. A process for fabricating UO₂ pellets according to claim 1 wherein UO₂F₂ powder is dissolved in demineralized water to form an aqueous solution which contains substantially no HF. A process for fabricating UO₂ pellets according to claim 1 wherein NH₃ water and an aqueous solution of UO₂F₂ are first fed continuously to a first-stage settling chamber, so that the NH₃/UO₂F₂ molar ratio is between 3 to 6, to carry out the first-stage ADU precipitation. A process for fabricating UO₂ pellets according to claim 3 wherein ADU slurry is formed in the first-stage settling chamber, so that aqueous NH₃ is fed continuously to a second-stage settling chamber, and so that the NH₃/UO₂F₂ molar ratio becomes within a range of from 6 to 12. A process for fabricating UO₂ pellets according to claim 4 wherein second-stage ADU slurry is filtered, dried, calcined and reduced under an H₂ atmosphere, to transform the slurry into UO₂ powder. A process for fabricating UO₂ pellets according to claim 1 wherein UO₂ powder is compacted at a pressure of 5 t/cm², and then sintered for 4 hours at 1750 °C in an H₂ atmosphere, to form pellets.

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