RU2396610C2 - Ceramic nuclear fuel - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: ceramic nuclear fuel contains fissile material in form of uranium dioxide and/or nitride and nano-structure carbon modifier corresponding to fullerenes and/or fullerene-like structures at following ratio of elements in vol %: fullerenes and/or fullerene-like structures - 2-10, fissile material in form of uranium dioxide and/or nitride - the rest. ^ EFFECT: raised radiation resistance of ceramic nuclear fuel. ^ 1 cl, 2 tbl

Description

The invention relates to the field of powder metallurgy, in particular to the production of ceramic nuclear fuel used in fuel elements of nuclear reactors.

Known ceramic nuclear fuel based on uranium dioxide, modified with ultrafine (nanostructured) powder of uranium dioxide (patent RU No. 2186431, G21C 3/00, 2002). Modification of ceramic nuclear fuel based on uranium dioxide with an ultrafine powder of the same material leads to a decrease in the grain size of the fuel material, an increase in the mechanical properties of crack resistance and a decrease in sintering temperature in the manufacture of fuel pellets.

A disadvantage of the known ceramic fuel based on uranium dioxide is the high swelling under the influence of reactor irradiation, comprising 3-6% per 1% burnout at a temperature of 2000 ° K for 5000 hours.

This drawback is due to the fact that gaseous and solid fission fragments resulting from the fission of nuclear fuel have a mechanical effect on the fuel element (fuel rod), which leads to its swelling, cracking and, in some cases, violation of the integrity of the fuel cladding.

Known ceramic dispersive nuclear fuel, which is a core (sphere) of a fuel-containing compound (UO ₂ , UC ₂ , ThC ₂ , PuO ₂ ), placed in a continuous matrix of non-fissile material (AG Samoilov. Fuel elements of nuclear reactors. M .: Energoatomizdat. 1985, p. 79-87). Dispersion type nuclear fuel combines the strength of a ceramic core with ductility and good nuclear-physical and corrosive matrix properties.

The most common matrix material is pyrocarbon (Rus) in the form of a coating applied to fuel cores (spheres) by the thermal decomposition of hydrocarbons (methane, ethane, propane, acetylene, benzene, toluene) in fluidized bed reactors.

A disadvantage of the dispersion-type ceramic ceramic fuel is the poor radiation resistance of coatings during reactor irradiation associated with the destruction of fuel spheres and coatings due to the formation of gaseous and solid fission products. To retain fission fragments, a three-layer coating is used: a layer of porous pyrocarbon, which makes it possible to compensate for the thermal expansion of fuel, a layer of dense silicon carbide and a strong layer of dense pyrocarbon. The SiC coating should retain fission products at high temperatures, and the Rus layer should contain gaseous fission products. However, the problem of the destruction of coatings under the action of fuel swelling and pressure created by fission fragments and the pressure of carbon oxides due to the interaction of Rus with fuel remains unresolved.

In order to reduce the damage (swelling) of the fuel cladding between the fuel core and the cladding, a radial clearance of up to 0.3 mm and a compensation volume above the fuel are created to reduce the pressure of the gaseous fission fragments under the cladding.

The closest in technical essence and achieved effect to the proposed ceramic nuclear fuel - the prototype - is a ceramic nuclear fuel containing a chemical compound of fissile material based on dioxide, carbide and / or nitride of uranium and a nanostructured carbon modifier - nanodiamond (E.K.Dyakov, V. D. Blank. Non-ferrous metals. No. 11, 2007, pp. 62-66).

A disadvantage of the known ceramic nuclear fuel is the poor radiation resistance of the fuel associated with the formation of solid and gaseous fission products during operation and burning of fissile material.

Modification of ceramic nuclear fuel with a nanostructured carbon material - nanodiamond leads to an increase in strength and Young's modulus, decrease in brittleness and sintering temperature of the material, as well as grain size of the fuel material, however, the problem of radiation resistance of the known ceramic nuclear fuel remains unsolved.

The aim of this invention is to increase the radiation resistance of ceramic nuclear fuel.

This goal is achieved in that the ceramic nuclear fuel containing a chemical compound of fissile material and a nanostructured carbon modifier, according to the invention as a nanostructured carbon modifier, it contains fullerenes and / or fullerene-like structures with the following ratio of components, in vol.%:

fullerenes and / or fullerene-like structures 2-10 chemical compound of fissile material rest

The essence of the claimed ceramic nuclear fuel is as follows. Fullerene is a spherical carbon molecule. The C ₆₀ fullerene molecule consists of 20 hexagons and 12 pentagons with carbon atoms at their vertices. The radius of the fullerene C ₆₀ is 0.3512 nm, the thickness of the carbon shell is 0.0529 nm, an almost electron-free cavity of radius 0.3 nm is formed in the center of the fullerene, so that the C ₆₀ molecule is like an empty cell with a volume of 0.113 nm ³ , where atoms of other elements can enter if the atoms of the latter have a size that allows them to fit into the existing cavity. This cavity can be filled with atoms, molecules, ions, radicals and other types of particles. The molecules of fullerenes, which contain inside themselves the molecules or atoms of other substances, are called endrohedral compounds - endofullerenes. At present, dozens of endofullerenes have been synthesized.

Fullerene-like structures include carbon nanotubes (single-walled and multi-walled) and graphene nanofibres.

Single-walled nanotubes are a monoatomic thickness rolled graphene sheet rolled into a cylinder. The diameters and lengths of nanotubes are respectively in the range of 0.8-5 nm and 1-500 nm.

Multilayer nanotubes are composed of coaxial single-walled single-walled nanotubes. Graphene nanofibres mainly consist of plane-parallel graphene plates with an interplanar spacing of about 3.4 nm.

Carbon nanotubes and nanofibers have an abnormally high specific surface (up to 3000 m ² / g), which determines their sorption characteristics. Carbon nanotubes and nanofibers should be considered as a unique container for storing substances in a gaseous, liquid or solid state. In this case, the graphene shell provides a fairly good protection of the material contained in it from external chemical or mechanical effects.

Fullerenes and fullerene-like structures (carbon nanotubes and graphene nanofibres) should be considered as a unique means of storing materials for a long time.

As a result of a nuclear reaction and fission of nuclear fuel, more than 60 types of fission fragments are released, the main of which are:

Kr, Xe - forming gas bubbles and pores;

Rb, Cs, Te, Br, I - easily mobile;

Rn, Rh, PoL - released in the metal phase;

Mo, Te - in the metal or oxide phase;

Sr, Ba — passing into the oxide phase;

Y, Zr, Nb - form oxides partially soluble in fuel;

O ₂ is an oxidizing agent of metals.

The atomic radius of these radioactive fission fragments ranges from 0.13 nm for Sr to 0.29 nm for Cs, which allows them to freely penetrate and fill the free cavities of fullerenes and fullerene-like structures.

The free volume of fullerenes, capable of absorbing molecules and atoms of the resulting fission fragments, is 70% of its total volume.

The dependence of nuclear fuel reactor swelling on the irradiation temperature, fast neutron fluence, and test time (effective hours) is given in the table.

Nuclear fuel Irradiation temperature, ° С Fluence b / n, n / cm ² Test time, eff.hour Swelling% UO ₂ 800-1000 (2-4) · 10 ²¹ 300 3.0-4.0 1000-1350 4,5 UN 1000-1400 (8-10) · 10 ²¹ 300 1.5-2.0

To completely compensate for the swelling of nuclear fuel in the range from 1.5 to 4.5%, it is sufficient to introduce 2.15-6.43 vol.% Fullerenes or fullerene-like structures into the chemical compound of the fissile material.

Since reactor swelling can exceed the values indicated in the table and in more stringent reactor tests it can reach 6%, therefore, it is advisable to introduce the amount of fullerenes and fullerene-like structures into the chemical compound of fissile material in the range from 2 to 10 vol.%.

Example:

Fullerenes and fullerene-like structures were obtained by C ₃ H ₆ pyrolysis on carbon particles with catalysts deposited on their surface from Fe ₂ O ₃ -NiO ₃ oxides at a temperature of 1000 ° C-1250 ° C, followed by extraction of fullerenes and fullerene-like structures with the separation of carbon particles. The obtained extraction product contained about 60 wt.% Fullerenes and the rest was fullerene-like structures — single-layer and multilayer nanotubes and graphene nanofibers.

Fullerenes and fullerene-like structures were mixed in a ball mill with ceramic fuel - uranium dioxide, mononitride and uranium monocarbide, pressed fuel pellets and sintered in an inert gas atmosphere (argon) at a temperature of 1800 ° C.

The table shows the data on the reactor swelling of the proposed ceramic nuclear fuel containing fullerenes and fullerene-like structures in comparison with the reactor swelling of the known ceramic nuclear fuel.

As follows from the data in the table, the proposed ceramic nuclear fuel (examples 1-3, 5-7, 9-11) provides, in comparison with the known nuclear fuel (examples 4, 8, 12) an increase in radiation resistance, and in some cases completely eliminates reactor swelling.

A decrease in the content of fullerenes and fullerene-like structures in the fuel of less than 2 vol.% Becomes ineffective, and an increase in the content of more than 10 vol.% Is impractical because this reduces the fuel load and, consequently, the efficiency of the reactor.

Claims (1)

Ceramic nuclear fuel containing fissile material in the form of uranium dioxide and / or nitride and a nanostructured carbon modifier, characterized in that it contains fullerenes and / or fullerene-like structures in the following ratio of components, vol.%:
fullerenes and / or fullerene-like structures 2-10 fissile material in the form of uranium dioxide and / or nitride rest