SU1684263A1 - Method of preparation of solid solution of uranium and plutonium dioxides - Google Patents

Abstract

The invention relates to methods for producing a solid solution of uranium dioxide and plutonium and allows increasing the yield of the final product and reducing the duration of the process. The mechanical mixture of uranium dioxide and plutonium is mixed with a eutectic mixture of lithium and sodium fluorides containing 25-35 mol,% aluminum fluoride. The resulting mixture is placed in a nickel crucible, heated under argon to 750-800 ° C, and the salt melt is stirred for 40-60 minutes. The melt is then transferred to a sintered metal filter and the bulk of the molten salt is separated from the oxide precipitate. The yield of the U02-Pu02 solid solution is 97.9-98.4%. 1 z.p f-crystals, 1 tab., 1 ill.

Description

This invention relates to methods for producing a solid solution of uranium and plutonium dioxides, which can be used as fuel materials in the manufacture of heat generating elements.

The purpose of the invention is to increase the yield of the final product and reduce the duration of the process.

Example 1: 90.7 g of UO2 and 22.7 g of PuOz are mixed 47.1 g of LiF, 84.7 g and 101.2 g (the content of fluoride in the mixture is 35 mol%). The mixture is placed in a nickel crucible, heated under argon to 800 ° C, and the salt melt is stirred for 40 minutes. The melt is then transferred to a nickel-metal-ceramic filter and the bulk of the salt melt is separated from the oxide precipitate by filtration. The remaining amount of molten salt is removed by vacuum sublimation at

1000 ° C and a residual pressure of 0.1 mm Hg As a result, 111.2 g of a black powder was obtained (yield: 98.4%). It was determined by X-ray diffraction analysis that the substance obtained was a solid solution of uranium and plutonium dioxides with a lattice parameter of 5.456 A and a crystal size of 10-40 µm. Spectral analysis showed that the total content of technological impurities does not exceed the value of May.% O

Examples 2-5. Carried out as example 1, but with a different content of aluminum fluoride in the melt.

The conditions of the process and the results obtained are tabulated.

In example 5, strong salting of the salt bath due to the sublimation of aluminum fluoride was noted.

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4 SO

It can be seen from the table that the addition of 25-35 mol.% Of eutectic mixture LIF-NaF of aluminum fluoride provides an increase in the yield of the finished product and reduces the process time. Moreover, the process time depends on the content of A1Pz in the salt mixture: with a higher content, less time is required for complete recrystallization of the initial uranium and plutonium dioxides to the solid solution (U, Pu) 0. and vice versa.

The specified process temperature limits are determined as follows. The lower limit (750 ° C) is determined by the melting point of the eutectic mixture of lithium and sodium fluorides, the value of which is 650 ° C. Therefore, in order to operate without risk of freezing the bath, the lower limit of the process temperature, equal to 750 ° C, is adopted. Increasing the temperature above 800 ° C is not rational, since the sublimation of aluminum fluoride from the salt bath becomes noticeable above this temperature, which can lead to deterioration of the process conditions.

It was established that during the formation of a solid solution, the limiting stage of the process is diffusion due to the insignificant value of the diffusion coefficient.

To increase the diffusion coefficient, the recrystallization of the drawdown t in nozzle melts. The increase in the coefficient is achieved due to the solubility of the dioxides in salt melts. The particles of uranium dioxide and plutonium dioxide dissolved in the molten salt collide to form a solid solution, which precipitates. Therefore, the more the solubility of the dioxides in the salt melt, the faster the recrystallization process and, accordingly, the process of formation of the solid solution (U, Pu) 02.

The solubility of uranium dioxide and plutonium in the melt of a eutectic mixture of lithium and sodium fluorides is small and does not exceed 0.04 wt.%. It is known that in a melt of sodium cryolite (2NaF. A1Pz) the solubility of oxides of some metals reaches a significant value (1-35 wt.% ). Relative solubility of oxides in lithium cryolite (3ZIF) is not available.

The sodium cryolite has a melting point of 1000 ° C, and the eutectic mixture of lithium fluoride and sodium melts at 650 ° C. Therefore, experimentally need

determine how much aluminum fluoride needs to be added to the LiF-NaF eutectic mixture in order to significantly increase the solubility of uranium and plutonium dioxides and not to change the melting point of the salt mixture too much.

The drawing shows a graph of the solubility of uranium dioxide and plutonium in the melt of a eutectic mixture of lithium and sodium fluorides at 800 ° C, depending on the added amount of aluminum fluoride.

As can be seen from the drawing, when the content of aluminum fluoride in a mixture of more than 25 mol.%, There is a sharp increase in the solubility of uranium dioxide and plutonium dioxide

The invention allows, in comparison with the prototype, to increase the yield of the final product from 62-89 to 97.9-98.4% and to reduce the process time by 50 times due to the reduction of the dissolution time (from 5-10 h to

40-60 min.) And isolation of the final product.

Claims (2)

1. A method of obtaining a solid solution of uranium dioxide and plutonium dioxide, including dissolving the mechanical mixture of uranium and plutonium dioxides in a salt melt and their interaction, crystallization and isolation of the final product, is also distinguished by the fact that, in order to increase the yield of the final product and shorten the process, a eutectic mixture of lithium fluorides is used as a salt melt and sodium containing 2535 mol.% aluminum fluoride. 2. Method pop. 1, characterized in that the dissolution is carried out at 750-800 ° C in an inert atmosphere with stirring. 1020 30W The content of A1P3) mol%