Method of producing solid plutonium dioxide solution in uranium dioxide matrix

FIELD: chemistry.

SUBSTANCE: invention relates to the radiochemical industry and nuclear power engineering, is aimed at obtaining a mixed dioxide (U,Pu)O2 and can be used to produce mixed uranium-plutonium MOX fuel for VVER-1000 reactors and fast neutron reactors (BN-600, BN-800) at nuclear power plants. The method of producing a solid plutonium dioxide solution in a uranium dioxide matrix includes reacting nitrate complexes of uranium and plutonium with relative content thereof in the solution of 95-70 wt % and 5-30 wt %, respectively, with hydrazine hydrate to obtain a mixed amorphous compound of uranium and plutonium, holding the mixed amorphous compound of uranium and plutonium in the mother solution at 80-90°C for not less than 3.5 hours to obtain a precipitate of a fine powder of homogeneously mixed hydrated plutonium and uranium dioxide, separating the precipitate from the mother solution and heating to 280-300°C to form the end product.

EFFECT: invention provides a cost-effective, simple and less energy consuming method of producing a solid plutonium dioxide solution in a uranium dioxide matrix.

2 cl, 6 dwg, 2 ex

 

The invention relates to radiochemical industry and nuclear power, aimed at obtaining a mixed oxide (U,Pu)O2and can be used to make nuclear mixed uranium-plutonium MOX fuel of VVER-1000 reactors and fast neutron reactors (bn-600, bn-800) of nuclear power plants.

For the preparation of pelletized MOX fuel is used, the mechanical mixture of powders UO2and PuO2. Most are approved for these purposes MIMAS process (France) ["Advanced MIMAS process. Auteurs: R. DUCROUX; COUTY Y.; J. C. LEROUX Editeur SFEN. Conference: International nuclear conference on recycling, conditioning and disposal, Nice, FRA, 1998-10-25]. It includes two main stages of preparation of powders:

- joint milling of powdered oxides of uranium and plutonium with the formation of a concentrate containing plutonium in the mixture to 25÷30%;

- dry the specified dilution of the concentrate with uranium dioxide to a final content of plutonium.

The main disadvantage of MIMAS process and other methods based on the mixing of dry powders of oxides of uranium and plutonium, is the difficulty of obtaining compositions as homogeneous as possible, which leads to a decrease in the number of burnable fractions and incomplete dissolution of spent nuclear fuel when it is recycled.

This drawback can be avoided if to hold joint osuzhdeni� uranium and plutonium from the solution with subsequent transfer of the compounds into a mixed uranium dioxide and plutonium (U,Pu)O 2.

The known method according to which a mixture of oxides obtained by precipitation from solutions of mixtures of diuranate ammonium and plutonium hydroxide followed by filtration, drying, calcination and reduction with hydrogen [Samoilov A. G. Fuel elements of nuclear reactors. - M.: Energoatomizdat, 1985, p. 64].

The disadvantages of this method include the following:

- the complexity of the process;

- the necessity of using high temperatures;

- the use of hydrogen for recovery.

In another method [EN 2282590 C2, IPC C0G 43/00] the proposed technical solution allows to obtain a mixed uranium dioxide and plutonium with homogeneous distribution of actinides in the granules of the powder by carrying out the following operations:

- the preliminary recovery of the uranium to U(IV) by introducing a solution of a reducing agent ions, hydrazone [N2H5]+;

- stabilization of uranium in the oxidation state IV complexing agents - diethylenetriaminopentaacetic or nitrilosides acids, forming complexes with Pu(IV).

- joint precipitation of uranium and plutonium using a special water-ethanol medium by adding to a solution of 30 (about) % ethanol, and the creation of a pH of 7.5, concentrated ammonia;

- drying and calcination of the precipitate at a temperature above 650°C in an inert atmosphere.

To n�the drawbacks of this method are:

- the complexity of the process;

- the need for pre-reduction of uranium, i.e., the presence of a separate stage of the process;

- the use of special water-ethanol medium for the coprecipitation;

- the need of an inert atmosphere and high temperatures.

A method of producing a solid solution of plutonium dioxide in a matrix of uranium dioxide [RU 2446107 C1, IPC C0G 43/025]. This method includes the interaction of the nitrate solutions of uranium and plutonium with relative content in the solution of 95÷70 and 5÷30 wt. % , respectively, with hydroxylamine, which leads to the recovery of plutonium to the trivalent state and coprecipitation of uranium and plutonium in the form of a homogeneous mixture of hydroxylamine uranyl hydroxide plutonium, and further decomposing the resulting precipitate in air at 200-300°C.

The disadvantages of this method is:

- allocation from the solution of the intermediate razdavlennyh of compounds of uranium and plutonium;

- low thermal stability of the resulting mixture of oxides in the air.

The problem to be solved by the present invention, is to develop economically viable, relatively simple and less energy-intensive method of producing a solid solution of plutonium dioxide in a matrix of uranium dioxide (U, Pu)O2suitable for MOX fuel, at the expense of reducing�Oia number of stages and the temperature of the vote.

To solve this problem a method of producing a solid solution of plutonium dioxide in a matrix of uranium dioxide includes the interaction of the nitrate complexes of uranium and plutonium with relative content in the solution and 95-70 to 5-30 wt. % , respectively, with hydrazine hydrate in a molar ratio of the hydrazine hydrate: uranium, plutonium >2 getting mixed with amorphous compounds of uranium and plutonium, the shutter speed of the mixed amorphous compounds of uranium and plutonium in the mother liquor at a temperature of 80-90°C for at least 3.5 hours to obtain a precipitate of fine powder homogeneously mixed hydrated oxide of uranium and plutonium, the separation of the precipitate from the mother solution and heating it to a temperature of 280-300°C before the formation of the target product.

In the private embodiment, the exposure of amorphous compounds of uranium and plutonium in the mother solution, and heating the fine powder is carried out by convective heat supply.

The presence of plutonium in the matrix of uranium dioxide so stabilizes the crystal structure, even after heating in the air of a solid solution of plutonium dioxide in a matrix of uranium dioxide up to 800°C is identified by only one homogeneous mixture of two materials (U, Pu)O2.

The choice of the relative abundances of uranium and plutonium in the solution is due to the fact that the content of plutonium in MOX is from 5 to 30 wt. %.

The essence of the claimed invention is illustrated by the following illustrations.

Fig. 1 shows the spectrum of the initial solution U(VI) and Pu(VI) in 0.1 mol/l HNO3diluted 50 times.

Fig. 2 shows the spectrum of a solution of the mixed compounds U and Pu are separated from the suspension, kept at Τ=90°C for 3.5 hours, and dissolved in a mixture of 6 mol/l ΗΝΟ3and 0.01 mol/l HF.

Fig. 3 shows the spectrum of the solution U obtained by dissolving the compounds isolated from the suspension, kept at Τ=90°C for 2 days, in a mixture of 6 mol/l ΗΝΟ3with 0.1 mol/l HF.

Fig. 4 shows the spectrum of the Pu solution obtained by dissolving the compounds isolated from the suspension, kept at Τ=90°C for 3.5 hours, in a mixture of 6 mol/l ΗΝΟ3with 0.1 mol/l HF.

Fig. 5 shows the data synchronous thermal analysis of hydrated dioxides of uranium and plutonium.

Fig. 6 shows the radiograph of a solid solution of plutonium dioxide in a matrix of uranium dioxide (U, Pu)O2after they were heated in air to 800°C: UO2(1), PuO2(2).

Examples of the method

Example 1

To the original aqueous solution of the nitrates of uranium and plutonium in 0.1 mol/l ΗΝΟ3(Fig. 1) was added hydrazine hydrate Ν2Η5ΟΗ in a molar ratio of N2H5OH:(U, Pu)>2. In a solution formed of amorphous suspension yellow-grey. On�Le 3.5 hours of heating at 90°C suspension of bulk amorphous poorly ustawowego state quickly becomes a black precipitate deposited.

The resulting precipitate uranium and plutonium are separated from the mother liquor. To establish the modes of occurrence of uranium and plutonium in the sediment part of the precipitate dissolved in nitric acid. As can be seen from the data in Fig. 2, after dissolving in a solution there are only ions U(VI) and Pu(III). Their appearance can be explained by the occurrence in acid solution redox reactions of Pu(IV)+U(IV)=Pu(III)+U(VI). This clearly proves that in the solid phase formed after heating the suspension and make it well otchaivaysya black precipitate, uranium and plutonium are in the 4+ oxidation state, because only in this case, when dissolved in acid solution can flow the said reaction. Specially performed experiments separately with uranium and plutonium showed that in similar conditions in solutions of uranium (Fig. 3), and plutonium (Fig. 4) are in the 4+ oxidation state. Thus, a black precipitate of mixed compounds of uranium and plutonium is a mixture of hydrated dioxides.

Was held simultaneous thermal analysis separated from the mother liquor of the precipitate (Fig. 5). After conducting a simultaneous thermal analysis by heating the sample to 800°C was charged radiograph obtained product (Fig. 6). According to simultaneous thermal analysis weight reduction analizaremos� of the sample occurs in the temperature range from ~80°C ~280°C.

Further heating link up to 800°C leads to a slight change in his weight. Curve DTA (differential thermal analysis) shows that the crystallization of a solid solution of uranium dioxide and plutonium after loss of water of hydration occurs in two stages with the maximum exothermic effects at 175 and 225°C.

The result of x-ray diffraction, are shown in Fig. 6, clearly shows that in the sample of the mixture of oxides is heated to 800°C, identified by only one phase dioxide actinides (U, Pu)O2or homogeneous mixture of two materials.

Example 2

A method of producing a solid solution of plutonium dioxide in a matrix of uranium dioxide is carried out as in example 1, the process is conducted at 80°C for 5 hours under convective heat supply to the formation of rapidly deposited sediment black. The precipitate was separated from mother liquor and calcined in air at 300°C.

Thus, the developed a simple and effective method of obtaining from nitric acid solution of uranium and plutonium solid solution of plutonium dioxide in a matrix of uranium dioxide, which can be applied in the production of MOX fuel.

1. A method of producing a solid solution of plutonium dioxide in a matrix of uranium dioxide, including the interaction of nitrate complexes of uranium and plutonium with relative ODS�the neighing them in a solution and 95-70 to 5-30 wt. % , respectively, with hydrazine hydrate to obtain a mixed amorphous compounds of uranium and plutonium, the shutter speed of the mixed amorphous compounds of uranium and plutonium in the mother liquor at a temperature of 80-90°C for at least 3.5 hours to obtain a precipitate of fine powder homogeneously mixed hydrated oxide of uranium and plutonium, the separation of the precipitate from the mother solution and heating it to a temperature of 280-300°C before the formation of the target product.

2. A method according to claim 1, characterized in that the holding amorphous compounds of uranium and plutonium in the mother solution, and heating the fine powder is carried out by convective heat supply.



 

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