RU2382424C2 - Method of preparing uranium-erbium ceramic nuclear fuel - Google Patents

Abstract

FIELD: nuclear physics. ^ SUBSTANCE: invention relates to nuclear engineering, particularly to making fuel elements for power reactors which use nuclear fuel based on uranium dioxide with a burnable absorber in form of erbium oxide. Erbium is added into uranium dioxide powder using a chemical method i.e. when preparing uranium dioxide powder, a given amount of erbium nitrate solution or some other water-soluble erbium salt is added to uranyl nitrate solution before precipitation of ammonium diuranate. Further, uranium-erbium-containing powder of uranium dioxide which is suitable for making fuel pellets is prepared using known techniques. Mechanical mixing of powdered oxides of two components is avoided. There is complete homogeneity of distribution of components in each other and there is improvement of fuel pellet characteristics such as density, average grain size, uniformity of size distribution of grains. ^ EFFECT: improved operational properties and characteristics of nuclear fuel (density, porosity, grain size), possibility of widening range of erbium content. ^ 4 ex

Description

The invention relates to nuclear engineering, in particular to the production of fuel elements for power reactors that use nuclear fuel based on uranium dioxide with the addition of erbium as a burnable absorber.

At present, the active zones of water-cooled nuclear reactors are formed from fuel assemblies containing fuel of various compositions with the addition of a burnable absorber, which makes it possible to compensate for reactivity, equalize energy release over the volume of the active zone and maintain the temperature coefficient of reactivity at a given level. Erbium is used as a burnable absorber in the fuel for RBMK reactors, which is introduced in the form of erbium oxide powder directly in the manufacture of fuel pellets.

The presence of erbium in RBMK fuel and an increase in its content allows the replacement in the core of a part of additional absorbers with working fuel assemblies, which increases the fuel burnup depth.

The use of fuel with the addition of erbium in a sufficiently wide range of its content implies an improvement in the parameters of water-cooled reactors, but does not take into account a slight deterioration in the characteristics of the fuel itself and their effect on the operating conditions of the reactor with an increase in the content of erbium in the fuel. So, when the erbium content in the fuel is more than 1%, the uneven concentration of erbium in the fuel volume increases, which reduces its utilization.

In the production of nuclear fuel, the addition of a burnable absorber is carried out by mechanical mixing of powders of uranium dioxide and erbium oxide (see patent RU No. 2141693. The fuel assembly of a water-cooled nuclear reactor. IPC G21C 3/02, 3/62, 3/64, 1998, RU patent No. 2142170. Nuclear fuel tablet, IPC G21C 3/02, 3/62, 3/64, 1998). Due to the different density, particle size distribution and mass ratio of the mixed components, achieving a uniform distribution of erbium oxide in uranium dioxide is problematic. Nuclear fuel tablets sintered from mechanically mixed powders of uranium and erbium oxides show heterogeneity of the erbium concentration in the volume of the tablet, the degree of which depends on the mixing conditions of the powders and the formation of a solid solution during sintering. During the operation of such fuel, the probability of local destruction of the structure of the material of the tablets during irradiation increases, which can lead to accelerated destruction of the tablets and failure of the fuel rod.

Another problem of uranium-erbium fuel is that the presence of erbium, mechanically mixed with uranium dioxide powder, negatively affects the properties of the fuel itself, this manifests itself in a decrease in density, an increase in open porosity, and a decrease in the average grain size of the fuel pellet.

These reasons limit the possibility of increasing the erbium content in the fuel.

Currently, the production of fuels with an erbium content from 0.46 to 0.64 wt.% Erbium has been mastered on an industrial scale with a conditional mass fraction of U-235 from 2.6 to 2.8 wt.% (Patent RU No. 2157568. Nuclear fuel tablet, MPC G21C 3/62, 3/02, 1999). Uranium dioxide powder for the production of fuel pellets in this method can be obtained both by "wet" technology through the hydrolysis of uranium hexafluoride, extraction-re-extraction of uranium, precipitation of ammonium diuranate from a solution of uranyl nitrate (re-extract), drying - thermal decomposition and recovery, and by the method dry conversion of uranium hexafluoride. The open porosity of a compressed and sintered mixture of uranium dioxide with the addition of erbium is not more than 1% and grain size from 10 to 20 microns. A method of manufacturing nuclear fuel is that in a mixer, in particular a blade type, a two-component mixture of uranium dioxide and erbium oxide powders of the required composition is prepared, from which a press powder is prepared by known technology, tablets are pressed and sintered. Some of the technical difficulties of the powder mixing operation are related to the fact that the amount of erbium oxide is relatively small. To improve the quality of mixing the powders, a two-stage mixing technique is used with the preparation of the first stage of the master alloy mixture with a high content of erbium (“rich” mixture).

The disadvantages of this method of producing nuclear uranium-erbium fuel are:

- deterioration of the characteristics of nuclear fuel with an increase in its erbium content;

- non-uniformity (heterogeneity) of the distribution of erbium in the tablet at both the macro and micro levels (even with perfect mixing of the powders, the uniform distribution of erbium in uranium is determined by the particle size of the powders, the size of which varies from a few to hundreds of microns);

- a decrease in the density and grain size of sintered tablets with an increase in the erbium content in the fuel;

- multi-stage, the complexity of the process of preparing the mixture;

- a large number of revolutions of the production of tablets.

This method is chosen for the prototype.

These disadvantages are eliminated by the proposed method.

The objective of the present invention is to increase the uniformity of the distribution of erbium in the fuel tablet at the macro and micro levels.

The technical result consists in improving the operational properties and characteristics of nuclear fuel (density, porosity, grain size), the possibility of expanding the range of erbium content in it.

The essence of the invention lies in the fact that, in contrast to the known method for producing nuclear ceramic uranium-erbium fuel, including hydrolysis of uranium hexafluoride, extraction-re-extraction of uranium, precipitation of ammonium diuranate from a solution of uranyl nitrate, drying - thermal decomposition and reduction to obtain uranium dioxide powder , preparation of a two-component mixture of uranium and erbium, pressing and sintering the resulting mixture, in the proposed method, the preparation of a two-component mixture of uranium and erbium is carried out by mixing of uranyl nitrate and erbium nitrate or other water-soluble salts thereof, before the precipitation of ammonium diuranate, an erbium-containing uranium dioxide powder is pressed and sintered.

The proposed method of mixing uranium and erbium allows to obtain a highly homogeneous mixture of two components. Considering the similarity of the properties of uranyl and erbium ions (close ionic radii and densities of solutions, complete ionization and solubility at pH <3), the homogenization of erbium and uranyl ions in a solution occurs rather quickly when the solution is mixed or pumped. After mixing under the indicated conditions, gravitational or other separation of erbium and uranyl ions is in principle impossible.

Since the pH of the precipitation of ammonium diuranate and erbium hydroxide practically coincides, when an aqueous solution of ammonia is added, they simultaneously co-precipitate and quantitatively precipitate, which ensures complete homogeneity of their distribution relative to each other in a solid precipitate, which is maintained during all further fuel production operations.

The homogeneous distribution of erbium in uranium dioxide does not block the contact of powder particles, so the usual course of sintering a tablet from a powder of uranium dioxide is not violated, all the characteristics of a fuel tablet obtained by technology without the addition of erbium are preserved. The inventive method allows to obtain tablets of nuclear fuel having a density, average grain size, uniformity in the distribution of grain size is at least the same as a tablet without the addition of erbium.

Considering that the addition of erbium with respect to uranium is small, a solution of any erbium salt can be used to introduce erbium into a solution of uranyl nitrate, the anions of which decompose without heat and do not pollute uranium dioxide with limited impurities. These salts may be, for example, nitrates, citrates, acetates, etc.

Due to the chemical method of introducing the additive, the range of erbium content in nuclear fuel can be expanded to be from 0.02 to 3 wt.% Without deterioration of fuel characteristics, while in the prototype the range is limited from 0.46 to 0.64 wt.% For erbium. When the erbium content is less than 0.2 wt.% According to the prototype, the labor costs for the implementation of the method significantly increase, namely, the number of stages of intermediate mixing increases. It is known from practice that in order to achieve uniform distribution of components in an industrial batch of powders in one stage, it is necessary to mix the powders in a ratio of 1: (4 + 6) and then in the same order using the rich mixture obtained in the previous mixing stage, until until you reach the desired ratio, which for the erbium content, for example, 0.1% is 1: 1000.

In the case of values of the erbium content above 0.64% upon receipt of nuclear fuel according to the prototype, the effects affecting the quality of operation of the nuclear reactor and fuel characteristics are fully affected. This is a decrease in the density and grain size of the grain, an increase in the uneven distribution of grain size and open porosity of nuclear fuel, the heterogeneity of the distribution of erbium both in a single tablet and between them, which negatively affects the neutron-physical parameters and technological properties of a nuclear reactor / 1 /.

EXAMPLES OF IMPLEMENTATION

Example 1

It is necessary to obtain uranium-erbium nuclear fuel with an erbium content of 0.02 ± 0.002 wt.%.

Erbium was introduced into a uranyl nitrate solution (industrial re-extract) with a uranium concentration of 74.5 g / l as an erbium nitrate solution with an erbium concentration of 9.79 g / l (20.8 g / l by erbium nitrate). The calculation of the volume of the erbium solution was carried out to obtain the erbium content in uranium dioxide of 0.02 ± 0.002 wt.%. The composition of the resulting solution: U - 74.5 g / l, ER - 0.017 g / l.

Precipitation of uranium and erbium from the resulting solution was carried out at pH 7.0 with aqueous ammonia. The erbium-containing ammonium polyuranate was calcined at a temperature of 650 ° C. The uranium dioxide powder after reduction with hydrogen of uranium oxide-uranium contained 0.019 wt.% Erbium.

The resulting erbium-containing uranium dioxide powder was compacted in a vibratory mill, mixed therewith with a plasticizer in an amount of 6 wt.% And granulated to obtain a press powder. Spent pressing and sintering of the compacts. Sintered tablets were ground and their characteristics determined: density, open porosity, grain size and erbium content. The results are presented in the table.

Example 2

It is necessary to obtain uranium-erbium nuclear fuel with an erbium content of 0.60 ± 0.03 wt.%.

Erbium was introduced into an industrial re-extract with a uranium concentration of 70 g / l in the form of a solution of erbium acetate with a concentration of 10 g / l (20.5 g / l for erbium acetate). The calculation of the volume of the erbium solution was carried out to obtain the erbium content in uranium dioxide - 0.60 ± 0.03 wt.%. The composition of the resulting solution: U - 67 g / l, Er - 0.46 g / l.

The precipitation of ammonium polyuranate and its calcination was carried out analogously to example 1. The uranium dioxide powder after reduction with hydrogen of uranium oxide-uranium contained 0.58 wt.% Erbium.

The resulting uranium dioxide powder containing erbium was compacted in a vibration mill together with a modifying additive: uranium oxide oxide powder containing 0.60 wt.% Erbium, in terms of uranium dioxide in an amount of 5%. Plasticizer was not administered. The characteristics of the obtained fuel pellets and the erbium content in them are shown in the table.

Example 3

It is necessary to obtain uranium-erbium nuclear fuel with an erbium content of 2.8 ± 0.2 wt.%.

Erbium was introduced into a uranyl nitrate solution (industrial re-extract) with a uranium concentration of 74.5 g / l as an erbium nitrate solution with an erbium concentration of 45 g / l (95.7 g / l by erbium nitrate). The calculation of the volume of the erbium solution was carried out to obtain the erbium content in uranium dioxide - 2.8 ± 0.2 wt.%. The composition of the resulting solution: U - 70.8 g / l, Er - 2.26 g / l.

The precipitation of ammonium polyuranate, its calcination and reduction was carried out analogously to example 1. The uranium dioxide powder after hydrogen reduction with uranium oxide contained 2.75 wt.% Erbium.

Further operations as in example 1. Characteristics of fuel pellets and the erbium content in them are shown in the table.

Example 4 (prototype)

It is necessary to obtain uranium-erbium nuclear fuel with an erbium content of 0.60 ± 0.03 wt.%.

A two-component mixture of uranium dioxide and erbium oxide powders was prepared in a laboratory drum mixer of 5 kg of uranium dioxide and 34.31 g of erbium oxide. Averaging time 30 min. Before mixing, a “rich” mixture was prepared manually, containing all erbium oxide and 1.67 kg of uranium dioxide. Then the press powder was prepared by compacting the mixture in a vibratory mill, mixing with a plasticizer and granulation. Spent pressing and sintering of the compacts. Sintered tablets were ground and characteristics were determined: density, open porosity, grain size and erbium content. The results are presented in the table.

As a comparison sample, a press powder was prepared from uranium dioxide powder without the addition of erbium. Spent pressing and sintering of the compacts. Sintered tablets were ground and their characteristics were determined: density, open porosity, grain size. The results are presented in the table.

From the data presented in the table, it is seen that the fuel pellets obtained by the present method have a more uniform grain structure, lower porosity and higher density compared to the prototype. The introduction of erbium into uranium dioxide powder in the form of Er ₂ O ₃ powder by mechanical means led to the appearance of an uneven grain structure. Accumulations of small grains 1-3 microns in size are clearly distinguished in the samples.

The microstructure and density of sintered tablets of uranium dioxide with erbium, introduced by the present method and prototype

The inventive method - example 1 Er - 0.021%, density - 10.69 g / cm ³ , average grain size - 13 μm, the volume fraction of open pores - 0.41% The inventive method - example 2 Er - 0.60%, density - 10.68 g / cm ³ , average grain size - 15 μm, the volume fraction of open pores - 0.43%

The inventive method - example 3 Er - 2.79%, density - 10.69 g / cm ³ , average grain size - 16 μm, the volume fraction of open pores - 0.44% Prototype method Er - 0.61%, density - 10.54 g / cm ³ , average grain size - 10 μm (accumulation of areas with grain size 1-3 μm), volume fraction of open pores - 0.76%

Comparison tablet - without erbium additive, density - 10.66 g / cm ³ , average grain size - 8 microns, volume fraction of open pores - 0.67%

Obtaining uranium-erbium fuel in accordance with the claimed method allows to reduce by 10-15% the cost of the acquisition and operation of equipment for the preparation of press powders and tablets, to reduce energy and labor costs. By improving the uniform distribution of erbium in batches of uranium dioxide powders, the number of rejects and production turns will decrease.

The characteristics of the obtained uranium-erbium fuel make it possible to achieve better performance (lower gas evolution, greater burnup depth, etc.) during their operation in nuclear reactors, for example, RBMK.

Claims (1)

A method for producing nuclear ceramic uranium-erbium fuel, including hydrolysis of uranium hexafluoride, extraction-re-extraction of uranium, precipitation of ammonium diuranate from a solution of uranyl nitrate, drying-thermal decomposition and reduction to obtain uranium dioxide powder, preparation of a two-component mixture of uranium and erbium, pressing and sintering mixtures, characterized in that the preparation of a two-component mixture of uranium and erbium is carried out by mixing solutions of uronil nitrate and erbium nitrate or its other water-soluble and before the precipitation of ammonium diuranate, pressing and sintering erbiysoderzhaschy subjected to uranium dioxide powder.