RU2690155C1 - Method of producing pelleted porous uranium dioxide - Google Patents

Abstract

FIELD: physics.SUBSTANCE: invention relates to nuclear power engineering and can be used to obtain pellets of uranium dioxide fuel cores of high-temperature ventilated fuel elements primarily thermionic reactors-converters of built-in version. Method of producing pelleted porous uranium dioxide involves preparing a charge from uranium dioxide starting powder with addition of binder and volatile pore-forming substance in form of a mixture of granules with a fractional size in range of 20 to 60 mcm in amount of 10–20 % of the weight of the uranium dioxide powder, pellets pressing and sintering in high-temperature vacuum furnace at 1900 ± 20 °C, at residual medium pressure 5⋅10mm Hg. for four hours.EFFECT: invention enables to ensure optimum microstructure formation in the fuel cores of high-temperature ventilated fuel elements in the initial period of operation of thermionic reactor-converters.1 cl, 3 dwg

Description

The invention relates to the field of nuclear energy and can be used to obtain tablets of uranium dioxide fuel cores of high-temperature ventilated fuel elements (TVELOV) mainly thermal emission transducer reactors (TRP) built-in.

Resource TRP significantly depends on the characteristics of the microstructure of fuel cores of fuel elements from uranium dioxide, which is quickly formed relative to the resource of the converter in the initial period of operation of a nuclear power plant (≤300 hours).

When using typical uranium dioxide in the form of tablets with an equiaxial structure, a stable microstructure in the form of radially oriented columnar grains with a maximum width (transverse size) of 300-400 μm is formed in the fuel core of the TVELA TRP. This circumstance leads to the appearance of local zones in the fuel core of a low-creep TVELA, which can lead to shell damage due to swelling fuel [Gontar AS, Gridnev AA, Rakitskaya EM and others. "Optimization of the structure of uranium dioxide in relation to TVELU thermionic reactor converter". Atomic Energy, 2005, vol. 99, no. 4, s. 264-268].

The optimal microstructure of the fuel core of a fuel rod of uranium dioxide is characterized by a columnar grain width <200 μm and is formed in the fuel core of TVELA using uranium dioxide tablets with a predominant pore size of 20-60 μm with a total porosity of 10-20% [RF Patent №2260862, IPC ⁷ G21C 3/58 "Method of forming the microstructure of the core of the fuel element" A.S. Gontar, A.A. Gridnev and others, claimed 01/20/2004].

The implementation of this method of forming the optimal structure in the fuel core of a fuel element from uranium dioxide at the initial moment of operation of a TRP requires the development of an appropriate cost-effective and technologically advanced method of producing tablets with the above-mentioned initial characteristics.

A method of obtaining tablets of nuclear fuel based on uranium dioxide, including the preparation of the mixture of raw materials, pressing and sintering blanks [see. Kotelnikov, R. B., Bashlykov, S.N., Kashtanov, A.I., Melnikova, T.S. "High-temperature nuclear fuel", M .: Atomizdat, 1978, 432 p.].

This method allows to obtain tablets from the original powder of uranium dioxide with a density of 84-93% of the theoretical density value. When using granules or microspheres as starting materials, the density of tablets reached 95% of the theoretical density value. In this case, the predominant spectrum of pores was pores with a size of 3-5 μm. Therefore, the known method does not meet the requirements for the spectrum of pores for uranium dioxide tablets, necessary for the formation of the optimal structure in the fuel core of the fuel element TRP.

There is also known a method of producing uranium dioxide tablets for nuclear reactor fuel reactors [RF Patent №2253913, IPC G21C 3/62 "Method for producing fuel pellets for uranium dioxide fuel elements (variants) N.Z. Lyakhov, G.R. Karagedov et al. 23.05.2003].

This method, which includes preparation of the charge (press powder) from uranium dioxide of a ceramic grade, pressing and sintering tablets at a temperature of ≤1900 ° C, is technically closest to the one proposed and selected as a prototype. In the mixture, formed from the crushed particles of rejected finished tablets, add 2-4% of the mass of volatile pore-forming substances.

The disadvantage of this method is that the possibility of forming the required pore spectrum of 20-60 microns in tablets of uranium dioxide is eliminated due to the characteristic for the prototype of the prevailing spectrum of small pores (3-5 microns) and porosity ≤5% by mass. Pores with a transverse size of 5–10 μm do not allow the formation of a microstructure in the fuel core of the TVELA TRP due to the lack of the ability to form a grid of radial microchannels that provide column widths <200 μm due to the fact that pores ≤ 10 μm in size cannot change their shape when motion in a temperature gradient field. Consequently, uranium dioxide tablets made with the help of the prototype do not allow forming the optimal microstructure in the fuel rod of the fuel element at the initial moment of operation of the EPR.

In ensuring the formation of an optimal microstructure in the fuel core of the TVELA in the initial period of operation of the EPR, the authors were faced with the task of obtaining uranium dioxide tablets with a predominant pore size of 20-60 μm at a total porosity level of 10–20%.

The range of the particle size distribution of the pore-forming volatile substance and its percentage was selected based on the results of research work, which showed that pores of 20-60 microns in size make it possible to form columnar grains with a width of less than 200 microns in the core. This ensures minimal swelling of the cores of the fuel elements in the reactor. The cores offered do not require a strong shell.

To solve this task, an economical single-stage technological method for producing preformed porous uranium dioxide of fuel cores of high-temperature ventilated fuel elements of thermal emission reagent reactors of the built-in version has been developed. pore-forming substance is added as a mixture of granules with fractional m size in the range from 20 to 60 .mu.m in an amount of 10-20% by weight of uranium dioxide powder and sintering the pellets is carried out in a high vacuum furnace at the sintering temperature 1900 ± 20 ° C at a residual pressure medium 5⋅10 ^-5 mm.rt .st. for four hours.

Preferably, paraffin or zinc stearate is used as a volatile blowing agent.

As follows from the above, the essence of the invention lies in the fact that the addition of a volatile pore-forming substance in the form of a mixture of granules with a fractional size in the range from 20 to 60 μm in an amount of 10-20% by weight of uranium dioxide powder allows to obtain uranium dioxide tablets in one stage with a predominant pore size of 20-60 μm due to evaporation of the volatile pore-forming substance and its removal at the sintering stage of compressed uranium dioxide tablets. The formation of a uniformly distributed pore volume from 20 to 60 microns in tablets of uranium dioxide increases the uniformity of the columnar grains formed in the field of the temperature gradient in the fuel rod of the fuel element. The fractional size of the volatile pore-forming substance and its percentage are determined according to the results of test experiments conducted by the authors with different granulometric composition of the volatile pore-forming substance and its percentage in the charge, taking into account the volumetric shrinkage of the tablet during sintering.

FIG. 1 shows a photograph of the typical microstructure of the finished tablets of uranium dioxide.

FIG. 2 shows a photograph of a typical microstructure of uranium dioxide tablets after testing in the field of temperature gradient as applied to the temperature conditions of operation of a high-temperature TVEL TRP.

An example of a specific implementation. Example 1

From paraffin, as a volatile pore-forming substance, formed granules with a fractional size in the range from 20 to 60 microns. To prepare the mixture, a mixture of paraffin granules with a fractional size in the range from 20 to 60 μm in an amount of 19% by weight of uranium dioxide powder and a binder were sequentially added to the initial uranium dioxide powder. Then mixed until a homogeneous composition of the mixture. A mixture of ethanol and glycerin was used as a binder in a volume ratio of 3: 1 in an amount of 4% by weight of uranium dioxide powder.

The tablets were pressed, and after they were sintering the tablets in a high-temperature vacuum furnace at a known sintering temperature of 1900 ± 20 ° C at a pressure of residual medium of 5⋅10 ^-5 mm Hg. (6.76⋅10 ^-3 Pa) for four hours. Then the furnace was cooled.

FIG. 3 shows a photograph of the microstructure of the finished tablets with a pore size of from 20 to 60 microns,

Example 2

Granules with a fractional size ranging from 20 to 60 microns were formed from zinc stearate. To prepare the mixture, a mixture of zinc stearate granules with a fractional size in the range from 20 to 60 microns in an amount of 11% by weight of the powder of uranium dioxide and a binder was sequentially added to the initial uranium dioxide powder. Then mixed until a homogeneous composition of the mixture. A mixture of ethanol and glycerol in the ratio of 3: 1 in the amount of 5% by weight of uranium dioxide powder was used as a binder.

The tablets were pressed, and after they were sintering the tablets in a high-temperature vacuum furnace at a sintering temperature of 1900 ± 20 ° C at a pressure of residual medium of 5⋅10 ^-5 mm Hg. (6.76⋅10 ^-3 Pa) for four hours. Then the furnace was cooled.

Tablets made in accordance with the claimed invention, had the following characteristics:

- geometric characteristics: outer diameter of 17.1 mm; internal diameter 8.0 mm; height is 8,4 mm;

- matrix density: 80-90% of theoretical density;

- total porosity: 10-20%;

- pore size: 20-60 microns.

Consequently, studies show that in tablets of uranium dioxide made in accordance with this invention, the formation of a microstructure with a pore size of 20-60 microns with a total porosity of 10-20%. The resulting microstructure ensures the formation of an optimal microstructure in the fuel core of the fuel element in the initial period of operation of thermal emission reactors-converters. As a result, minimum swelling of the fuel element cores in the reactor is achieved, the load on the shell is minimized, which allows the use of less durable and cheaper shells. The developed method of obtaining preformed uranium dioxide significantly reduces the time of obtaining preformed uranium dioxide relative to the method of obtaining a typical preformed uranium dioxide with equiaxial structure, since is one-step, therefore, more economical.

Claims (2)

1. A method of producing preformed porous uranium dioxide of fuel cores of high-temperature ventilated heat-generating elements of thermionic reactors built-in converters, including the preparation of the charge from the original uranium dioxide powder with the addition of ligament and volatile pore-forming substance, pressing and sintering of the tablet, which is volatile in the formament of pellets, is prepared by the pellet. in the form of a mixture of granules with a fractional size in the range from 20 to 60 microns in an amount of 10-20% by weight of powder uranium dioxide, and sintering of the tablets is carried out in a high-temperature vacuum furnace at a sintering temperature of 1900 ± 20 ° C, at a pressure of the residual medium of 5⋅10 ^-5 mm Hg. for four hours. 2. The method according to p. 1, characterized in that paraffin or zinc stearate is used as a volatile pore-forming substance.

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