RU2459289C1 - Method for obtaining nuclear fuel pellets based on uranium dioxide - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: method consists in addition to initial fine uranium dioxide of nanodisperse uranium hydride, thorough mixing of components, drying of mixture in vacuum at 300-330°C, at which decomposition reaction of uranium hydride to metal takes place, pressing of pellets of dried product and their sintering in dynamic vacuum at 1500-1550°C.

EFFECT: increasing the content of fissile material, improving strength and heat-conducting properties of material.

Description

The invention relates to the field of technology for producing nuclear fuel based on uranium dioxide having a high density and an increased content of fissile material.

Uranium dioxide is currently one of the most common materials used as nuclear fuel. But, when combining a number of valuable properties, such a fuel also has some significant drawbacks. The disadvantages of a pure uranium dioxide fuel include a low coefficient of thermal conductivity, especially at elevated temperatures (~ 2.0 W / m · K at 2000 K), low fissile element content compared with uranium-composite fuel, as well as low strength properties that appear after pressing, especially if plutonium oxide must be added to uranium dioxide. // Ma B.M. Materials of nuclear power plants. M .: Energoatomizdat, 1978 // Kotelnikov RB, Bashlykov S.N., Kashtanov A.I., Menshikova T.S. High temperature nuclear fuel. M., Atomizdat, 1978 //.

Kermet and carbonitride compositions (W-UO ₂ , U-Zr-CN, UN) have improved properties compared to pure uranium oxide: they have a higher fissile element content, their thermal conductivity is several times higher than that of uranium dioxide, due to more elastic, they are less prone to cracking. // Gavrilin S.S., Deniskin V.P., Nalivaev V.I., Fedik I.I. Microfuel-based fuel rods for AFMMs with enhanced performance and safety. Sat doc. conf. "Small Energy 2006", Moscow, 2006 // Fedik I.I., Dyakov E.K., Deniskin V.P., Tukhvatulin Sh.T., Kenzhin E.A., Gagarin A.E. Joint development and production of promising nuclear fuel in Kazakhstan. On Sat doc. Conference "Nuclear Energy of the Republic of Kazakhstan", Kurchatov, East Kazakhstan region, Republic of Kazakhstan, 3-5 September, 2007 //. But the production of such a fuel material in comparison with purely oxide fuel is more expensive and time-consuming both in the manufacturing process and in the processing; it is prone to swelling and relatively poor compatibility with the material of the shell during irradiation. In addition, nitride fuel can be used in reactors at a temperature not exceeding 1650 ° C.

Fuel from uranium dioxide and metallic uranium should have a higher density and increased fissile element content compared to oxide fuel. But in the manufacture of the fuel composition by ceramic technology, which involves mixing powders of metal and oxide of uranium, after all operations of mixing, vibration compaction, pressing and sintering at the output, a material with insufficiently strong adhesion of particles of different phases is obtained. Due to the reduced strength, such fuel pellets have increased fragility and the ability to local destruction both from the application of small efforts, and due to the desire for spontaneous scattering.

It is known to use uranium hydride fuel in a reactor // Patent RU No. 2379773, IPC G21C 003/42, (2008.06) //, but the essence of producing such fuel consisted in treating metallic uranium with a mixture of deuterium and tritium directly in the core of a nuclear reactor with simultaneous flow nuclear fission and fusion reactions. Thus, the uranium hydride in the above example was the final monophasic product, the temperature of use of which is limited to 250 ° C.

As a prototype, the method of obtaining a fuel tablet from uranium oxide with alloying additives using ceramic technology was selected // Patent RU No. 2193242 G21C 3/62, 12/18/2000 //. In the prototype, uranium dioxide powder was mixed with calcined aluminosilicate powder (up to 0.2 wt.%), Giving a fusible eutectic along the grain boundaries with uranium dioxide. Depending on the tasks, alloying additives in the form of rare earth or other metal oxides could be added to the tablet. Tablets suitable for use as nuclear fuel were prepared using conventional ceramic technology. Since all additives to uranium oxide used in the prototype are lighter than uranium dioxide itself, the density of tablets obtained from such material could not be higher than the density of the oxide matrix.

In the method proposed in the application, the task was to obtain tablets of nuclear fuel based on uranium dioxide, but with a higher density and with a higher content of fissile element than in pure uranium dioxide. This task cannot be performed by adding to the uranium oxide alloying additives that were used in the prototype.

The problem is solved in a way, the essence of which is to add to the uranium dioxide a certain amount of metallic uranium introduced into the fuel composition in the form of uranium hydride.

The method involves the implementation of the following processes in the specified sequence:

1. The crushed (~ up to 50-30 microns) uranium dioxide is mixed with uranium hydride having a particle size of not more than 80 nm, the amount of hydride in terms of metallic uranium does not exceed 50% (varies from 5 to 50%).

2. The mixture is homogenized by co-grinding and mixing.

3. The homogenized mixture was slowly heated (at a rate of 1 degree per minute), to 300-330 ° C in vacuum (10 ^-4 -10 ^-5 mm Hg).

4. From the dried material, in which the uranium hydride is almost completely decomposed to form elemental uranium, tablets are pressed using an organic binder (PVA).

5. The tablets are calcined at a temperature of 1500-1550 ° C in a dynamic vacuum (with a vacuum of at least 10 ^-4 mm Hg).

After cooling in the oven and removing the tablets, they remain stable in air while maintaining the tetravalent state of uranium in the oxide composition. As a result of all the described actions, a tablet is obtained consisting of uranium dioxide and metallic uranium in an amount of up to 50 wt.%. Uranium metal particles are evenly distributed in the volume of the oxide matrix with the formation of continuous structural networks, which helps to increase the strength of such tablets. The value of the density of the tablets ranged from 11.6 to 11.9 g / cm ³ . The introduction of metal into uranium oxide by the proposed method contributed to an increase in the density of the oxide tablet.

The following examples illustrate the application of the proposed method.

Example 1, illustrating that the introduction of uranium into uranium dioxide in the form of metallic uranium powder does not give the desired result.

To finely divided uranium oxide was added 10% of uranium powder. After homogenization of the mixture, the mixture was heated in vacuum (or hydrogen) to 350 ° C, pressed into tablets, which were heated again, re-crushed and pressed. These tablets were sintered at 1650 ° C. According to the results of probe microanalysis in tablets, significant porosity was observed, uranium particles unevenly distributed at the phase boundaries from uranium oxide formed conglomerates in places. Even repeated grinding of powders heated several times did not lead to a uniform distribution of uranium metal particles in the oxide matrix.

Example 2

Fine uranium dioxide was mixed with nanosized uranium hydride in a ratio of 1: 1 (in terms of metal). The mixture was ground, dried with pumping to 10 ^-5 mm RT.article at 330 ° C, in a vacuum, cooled to room temperature, then pressed into tablets. The tablets were sintered at 1500 ° C with pumping up to 10 ^-4 -10 ^-5 mm Hg. and exposure at this temperature for 1 hour. A probe microanalysis of such a tablet showed that in the sintered material, uranium microparticles are evenly embedded in the phase regions of uranium oxide, but the formation of a small amount of conglomerates is noted at the boundaries of the structural blocks. The measured density of such a sample was 11.6 g / cm ³ .

Example 3

Fine-grained uranium dioxide was mixed with nanosized uranium hydride in the ratio of 18% uranium + 82% uranium dioxide. Further, the synthesis was carried out according to the method described in example 2. The measured density of the calcined tablets was 11.9 g / cm ³ , thermal conductivity ~ 12 W / m · K (at 300 K). A probe microanalysis confirmed the uniform distribution of uranium microparticles over the entire phase region of uranium oxide.

Examples 2 and 3 indicate that in the proposed method, the introduction of metallic uranium into uranium dioxide in the form of a hydride has quantitative limitations: an excess of 50% of the amount of uranium in the oxide leads to a softening of the structure, i.e. to lower density and strength.

Claims (1)

A method of producing tablets of nuclear fuel based on uranium dioxide and a metal-containing additive, comprising obtaining a fuel composition by grinding and mixing the components of the charge, compressing the tablets from it and subsequent high-temperature sintering of the tablets, characterized in that nanodispersed uranium hydride powder is introduced into the crushed uranium dioxide in an amount not exceeding 50%, the resulting mixture is heated in vacuum at 300-330 ° C, after which the tablets pressed from the resulting mixture are sintered in vacuum at 1500-1550 ° C.