CS201991B1 - Method of preparing uranium dioxide and/or plutonium dioxide and/or thorium dioxide from aqueous solutios of salts - Google Patents

Description

Process for preparing uranium dioxide or plutonium dioxide or thorium dioxide from aqueous salt solutions

The invention provides a direct treatment of aqueous solutions of uranium or hexavalent cation salts or mixtures thereof with plutonium or thorium formed with volatile anions such as fluorides, chlorides or nitrates, etc., into uranium dioxide and / or plutonium oxide and / or thorium oxide with the parameters of the ceramic nuclear fuel, wherein the treatment of the waste produces as an by-product alkaline fluoride, recyclable back to the intermediate preparation stage.

Ceramic nuclear fuels based on uranium dioxide, containing natural uranium or in isotope-enriched form, or based on a mixture of oxides of UOg / PuOg, are prepared in large scale by three basic processes.

Two of these processes work on the wet road, first by precipitating polyuranates, generally uranyl nitrate solutions, e.g. according to US Patent 3,579,311 or French Patent No. 895,322, and secondly by precipitating some uranyl tricarhonates, eg according to US Patent 3,519,403 with subsequent pyrrolysis and reduction, or treating the uranhexafluoride ha in a dry way by thermally decomposing UFg with steam and reducing it with hydrogen, e.g. according to US Patent 3,547,598, US Patent 3,168,369, or Swedish Patent No. 10516/59. Despite all its apparent simplicity, the dry process is complicated by incomplete reactants.

A very effective variant of the mentioned wet processes is furthermore, for example, the inventions described in MS. No. 173523 and 174298, which introduce into water processes

201 991, moreover, an extraction step into alkylphosphates, however, they greatly increase the quality of the products, the process economy and the waste balance of the polyurethane and carbonate processes.

Treatment of the polyuranates in a wet process also streamlines the process, which always removes the highly difficult and often unrealistic separation of polyuranate precipitates and utilizes direct pyrolytic reduction of aqueous polyuranate suspensions, precipitated on the waterway, usually from nitrate solutions.

Although the latter works practically without waste, it also removes a number of shortcomings in all of the previously mentioned processes, be it the complexity of technology, the complexity of equipment and material handling, and often, especially the high demands of working or waste disposal, this method is not the only way that can advantageously lead to a quality ceramic fuel.

The above process for the preparation of uranium dioxide or plutonium dioxide or thorium dioxide from aqueous solutions of salts of these cations of any valence degree with volatile anions such as fluorides, chlorides or nitrates, or their hydrolyzed forms, which consists in The medium precipitates quantitatively with a mixture of ammonia and hydrogen peroxide, a crystalline, well-separable ammonium peroxide salt which, after simple separation, for example by sedimentation, is subjected to direct pyrolysis in a reducing atmosphere while still wet. the mother liquors are processed into alkaline fluoride of any character, usable in the nuclear fuel production cycle.

An advantage of the process according to the invention is that it is universal, applicable directly to solutions of volatile anions, in particular uranium fluorides, chlorides or even nitrates, or plutonium or thorium in any of their valoric form, whether these salts are hydrolyzed or non-hydrolyzed. A particular advantage of the process is that it is not necessary to convert the halides of said cations into nitrates. A further advantage is that the products pyrolyzed in their own reducing atmosphere, most often in the temperature range of 450 to 1000 ° C, preferably at 500 ° C, exhibit excellent purity and result in high density products.

Most often the process according to the invention is such that, for example, an aqueous uranium hexafluoride hydrolyzate, i.e. a solution of U2F3, precipitates, e.g. at a temperature of about 50 DEG C. by gradually adding a slight stoichiometric excess of an equimolar mixture of aqueous ammonia and hydrogen peroxide solutions. The resulting ammonia and the resulting fluoride are recycled to the production cycle and the still wet product of uranium peroxy salt is subjected to reduction pyrolysis at rapier temperature. 500 ° C to form uranium dioxide.

The procedure is further illustrated by the following non-exhaustive or limiting examples, since the process can also be applied to solutions or mixtures of solutions containing not only uranium but also plutonium or thorium, in the form of different cations of salt, volatile anions of not only fluorides, but also of chlorides or nitrates, or mixtures thereof.

Example 1 ml of 0.6 mol of UO ₂ ^F 2 aqueous hydrolyzate UF 2 ^{in 1000 solution} was mixed with a mixture of 60 ml of 25% ammonium hydroxide solution and 20 ml of 30% hydrogen peroxide ^solution with stirring. The precipitated crystalline substance was filtered off without the presence of unreacted uranium in the filtrate. The pale yellow product obtained was introduced into a crucible and pyrolytically decomposed separated ammonium perborate crystals were heated by heating at 450-730 ° C under a reducing atmosphere of evolving NH 4. A total of 12.9 g of uranium dioxide was obtained, which corresponds in practice to 100% yield.

No fluorides were detected in the product. The specific weight of the product obtained after sintering was

10.6 kg.dm ~ 3, which corresponds to a value of 97 theory. The remaining 130 ml of mother liquor remaining after filtration of the ammonium perborate was concentrated to near dryness with the gradual addition of potassium hydroxide solution (pH 7.0). The consumption of KOH at about 4 g was obtained 4.85 g ^ 2 ^ ^{2 (31ejšího} product, which corresponds to approximately 85% to a yield.

Example 2

In a similar manner, the preparation of UOg from an aqueous solution of UF2 was carried out. The results were practically the same.

Claims (1)

Object of the invention Process for the preparation of uranium dioxide and / or plutonium dioxide and / or thorium dioxide from aqueous solutions of salts of these cations of any valence degree with volatile anions such as fluorides, chlorides or nitrates and / or their hydrolyzed forms, characterized in that ammonium persulphate precipitated by stoichiometric the mixture of ammonia with hydrogen peroxide is subjected to direct pyrolysis in its own ammonia reducing atmosphere.