RU2382423C2 - Coated fuel particle for fast neutron reactor - Google Patents

Abstract

FIELD: nuclear physics. ^ SUBSTANCE: invention relates to nuclear power engineering, particularly to microspherical fuel with ceramic protective coatings for fast neutron reactors. The coated fuel particle for fast neutron reactors consists of fuel microspheres based on PuO2 and a four-layered protective coating. The third layer from the microsphere is made from silicon carbide or zirconium carbide, the second and fourth layers are made from titanium-silicon carbide with formula Ti3SiC2. The first layer is deposited on the microsphere and is made from a carbon-silicon carbide composition with content of the silicon phase within defined limits. Content of the silicon phase in the surface zone of the outer boundaries of this layer is 35-45 wt % with depth of the zone equal to 0.03-0.05 of the thickness of the layer. ^ EFFECT: longer operation due to high radiation-chemical stability of the Ti3SiC2 coating. ^ 1 tbl

Description

The invention relates to the field of nuclear energy, in particular to microspherical fuel with ceramic protective coatings for fast neutron reactors.

Microtel (MT) of a nuclear reactor is a fuel microsphere (TM) made of fissile material (UO ₂ , (U, Pu) O ₂ , PuO ₂ , etc.) with layers of protective coatings.

The main functions of protective coatings are the retention of gaseous (GPA) and solid fission products (TPD) inside the MT. At the same time, the protective coatings protect the MT power and diffusion barriers, for example, SiC or ZrC layers, as well as the TM material from the corrosive effects of impurities in the fuel element (ball fuel rod or compact) or in the coolant (Minato K., Ogawa T., Kashimura S ., Fukuda, K., Metallic impurities - silicon carbide interaction in HTGR fuel particles. - J. of Nucl. Mater., 175, 1990, p.14-19).

The main protective coatings of MT are currently considered: pyrocarbon of various densities (Rus), silicon carbide (SiC) or zirconium carbide (ZrC). As the first layer on the TM, low-density (buffer) pyrocarbon (RuSbuf) is used, as the second layer is high-density isotropic pyrocarbon (RuSv), the third layer is made of either SiC or ZrC, the fourth is the outer layer is made of high-density isotropic pyrocarbon ( RusSn). In cases where RusCb and RusCn are precipitated from a methane-argon mixture at pyrolysis temperatures of more than 1900 ° C, the MT between the layers of RusCoof and RusCoV contains an additional CoC layer with a density of about 1.60 g / cm ³ , which is a diffusion barrier preventing the reaction, for example, UO ₂ + 3C↔UC + 2CO, flowing at high speed under conditions of equilibrium CO pressure (Chernikov A.S. Fuel and VTGR fuel rods, Atomic energy, vol. 65, issue 1, July 1988, p. 32-38).

Microfuel based on ТМ, for example, from UO ₂ and Rus, SiC or ZrC coatings are four-layer (low-temperature Rus) or five-layer (high-temperature Rus):

Layer number Material Thickness, microns Density, g / cm ³ one Rus 90 1,0 2 Rus thirty 1.4-1.6 3 Rus thirty 1.8-1.9 four SiC fifty 3.2 5 Rus fifty 1.8-1.9

As part of MT, pyrocarbon coatings perform multi-purpose functions: RuSbuf - volume for the localization of GPA and a compensator for mismatch in the coefficients of linear thermal expansion between the TM material and subsequent high-density layers; RusSv is a diffusion barrier for GPA and some TPD, while protecting the carbide (SiC or ZrC) layer from the corrosive effects of TPD; RusSn - protects brittle carbide layers from mechanical damage; carbide (SiC or ZrC) layer - the main force layer and diffusion barrier for TPD.

A study of the radiation resistance of silicon carbide coatings as a structural material when irradiated with neutrons in the temperature range 300-1400 ° C and a fluence (4.0-6.0) 10 ²² n / cm ² (E> 0.1 MeV) shows:

- at relatively low temperatures (<800 ° C), point defects and small dislocation loops form in SiC;

- at higher temperatures (1400 ° С), point defects grow in Frank loops and, finally, develop in network dislocations.

The formation of such defects, however, does not lead to a significant change in the physicomechanical and thermophysical characteristics of SiC coatings as a structural material. However, in the composition of MT, the silicon carbide coating, during the destruction of RusSv, is subjected to intense corrosion by TPD and CO. Together with increasing internal GPA pressure, this leads to an increase in the probability of destruction of the entire multilayer MT coating.

It was experimentally established that the destruction of RusSv and RusSn begins at fast neutron fluences of more than 2.0 · 10 ²¹ n / cm ² .

The microtelle of a nuclear reactor based on TM from UO ₂ and a five-layer coating is known, the first layer of which is made of pyrocarbon of low (1.0-1.1 g / cm ³ ) density, the second layer is made of medium pyrocarbon (1.40-1.60 g / cm ³ ) density, the third layer is made of high density isotropic pyrocarbon (1.80-1.90 g / cm ³ ), the fourth layer is made of silicon carbide with a density of 3.20 ± 0.02 g / cm ³ , the fifth layer made of isotropic pyrocarbon of high (1.80-1.90 g / cm ³ ) density (Chernikov AS, Mikhaylichenko LI, Orlov GV, Kurbakov SD, Mikrotvely VTGR. Mother properties of coatings and the results of pre-reactor tests (Atomic Energy, vol. 68, vol. 3, March 1990, p. 181-186).

The disadvantage of this microfuel of a nuclear reactor is the inability to use it in a fast neutron medium. A similar MT design was developed and successfully applied in the conditions of a moderate (thermal) neutron spectrum. In the hard neutron spectrum (fast neutron reactor), radiation-chemical damage to coatings has a decisive influence on the integrity of MTs. Moreover, the effect of these damages will already affect low levels of fuel burnup, i.e. at relatively low GPA pressures inside the MT. The most significant changes in the early stages of irradiation will occur in the pyrocarbon layers and especially in the low-density and medium-density Rus. It should be noted that the ultimate resistance of Rus is limited by the fast neutron fluence (2-4) · 10 ²¹ n / cm ² , while in fast gas-cooled reactors, radiation damage above 200 dpa can be expected (fluence greater than 2.0 · 10 ²³ n / cm ² ). At this level of damage, the formation of He (according to the n-alpha reaction), the formation of pores and swelling of SiC should be taken into account, which will ultimately affect the change in the strength of the SiC layer. Based on the experience of operating fast breeder reactors for the fuel structure of MT fast gas-cooled reactors, the following generalized requirements can be formulated:

- the need for a high density of heavy atoms;

- enrichment of Pu 15-20%;

- the requirement for coating materials to withstand fast neutron fluences (E> 0.1 MeV) up to ~ 10 ²³ n / cm ² ;

- MT's ability to retain fission products is also effective (R / B ratio for normal, nonequilibrium and emergency situations, where R is the exit velocity and

B-rate of formation of fission products), as in thermal reactors.

Thus, mixed monocarbide (U, Pu) C, nitride fuel (U, Pu) N, or plutonium (PuO ₂ ) fuel can be considered as nuclear material for fast gas-cooled reactors.

Requirements for protective coatings (layer thickness, density, ratio of open and closed porosity, crystal lattice parameters, etc.) are specified at the stage of experimental design work for a specific type of reactor.

Known microtel fast gas-cooled reactor based on TM from (U _0.8 Pu _0.2 ) O _{2 with a} diameter of 850 μm with a three-layer coating, the first layer of which is made of low-density pyrocarbon 45 μm thick, the second layer is made of high-density pyrocarbon 5 μm thick, the third layer is made of silicon carbide about 50 μm thick (M. Dalle and G. Schumacher. Considerations on PyC and SiC coated oxide particles for gas cooled fast reactor application. - J. of Nucl. Mater., 40, 1971, p. 27 -40).

In addition to the above disadvantages associated with the significant damage to pyrocarbon layers by fast neutrons, this MT design has additional limitations due to the high corrosion damage to the SiC layer from the inside by solid fission products and from the outside by metallic impurities in the coolant. All these factors significantly limit the life of the MT.

The microtelle of a nuclear reactor based on TM from UO ₂ with a four-layer coating is known, the first layer of which is made of pyrocarbon of low (1.11 g / cm ³ ) density and 64 microns thick, the second layer is made of high-density isotropic (1.84 g / cm ³ ) pyrocarbon with a thickness of 26 μm, the third layer is made of zirconium carbide with a density of 6.6 g / cm ³ and a thickness of 31 μm, the fourth layer is made of isotropic high-density (1.95 g / cm ³ ) pyrocarbon with a thickness of 55 μm (Minato K., Fukuda K. , Sekino I., Ishikawa A., Oeda E. Deterioration of ZrC - coated fuell partikle caused by failure of pyrolytic carbon layer .-- J. of Nucl. Mater., 252, 1988, p.13-21).

Despite the higher thermal stability of ZrC compared to SiC (especially under conditions of possible MT overheating to temperatures above 2000 ° C), the high damage to the pyrocarbon layers ultimately limits the integrity of the entire four-layer coating ensemble, which limits the MT operating life.

The closest analogue to the prototype of the proposed technical solution is a microtel of a nuclear reactor based on a fuel microsphere made of PuO _{2 with a} diameter of 200 ± 25 μm with a four-layer coating, in which the first layer is made of low-density pyrocarbon with a thickness of 100 ± 15 μm and a density of 1.0 g / cm ³ the second layer is made of high-density isotropic pyrocarbon with a thickness of 35 ± 8 μm and a density of 1.80 g / cm ³ , the third layer is made of silicon carbide with a thickness of 35 ± 8 μm and a density of 3.2 g / cm ³ , the fourth layer is made of high-density isotropic pirog hydrogen chloride with a thickness of 40 ± 8 μm and a density of 1.80 g / cm ³ (Chernikov A.S., Permyakov L.N., Kurbakov S.D. et al. Nuclear fuel for HTGR based on plutonium oxide microspheres. - Atomic energy , vol. 88, issue 1, January 2000, p. 35-38).

The disadvantage of this microfuel, as well as the previous ones, is the high damage to the pyrocarbon layers, limited by fast neutron fluences (2-4) · 10 ²¹ n / cm ² , and the damage to the second high-density isotropic Rus is additionally activated by the corrosive action of PD and an increased oxygen output from for the decay of plutonium fuel. The released oxygen in the interaction with Rus leads to the intense formation of CO, which leads to corrosion of the silicon carbide layer by the reaction SiC + 2CO → SiO ₂ + 3C. In turn, this leads to an increase in the probability of destruction of the SiC power layer and complete depressurization of the four-layer coating. All these factors limit the MT operating life under conditions of their irradiation in the fast neutron flux.

The authors of the proposed technical solution had the task of increasing the service life (fuel burnup depth) of the microfuel due to the increase in the radiation resistance of the protective coating under the conditions of a hard spectrum of fast neutrons.

The problem is solved in that in the microtelement of a fast neutron nuclear reactor, consisting of a fuel microsphere based on PuO ₂ and a four-layer protective coating, in which the third layer from the fuel microsphere is made of silicon carbide or zirconium carbide, the second and fourth layers are made of titanium silicon carbide composition of Ti ₃ SiC ₂ and the first layer applied to the microsphere composition is made of carbon - silicon carbide phase containing silicon in the range 1,0-20,0% by weight, wherein the content of silicon in pripoverh phase. ostnoy zone outer boundary of this layer is 35-45 wt.% at a depth of 0.03-0.05 zone thickness.

Each of the layers of the proposed microtel of a fast fast neutron nuclear reactor performs the following functions:

- the first layer of the carbon - silicon carbide composition contains a “free" volume for the localization of the GPA and compensates for the mismatch in the coefficients of linear thermal expansion between the TM and subsequent high-density layers; at the stages of deep burn-up of the fuel, it compensates for its swelling;

- the second layer of titanium-silicon carbide is a diffusion barrier for GPA and most TPD, as well as a getter for CO;

- the third layer of SiC or ZrC is the main power layer that resists the high internal pressure of the GPA and the diffusion barrier for TPD;

- the fourth layer of titanium-silicon carbide is a getter for metal impurities from the fuel element matrix or coolant, and also protects the fragile SiC or ZrC force layer from mechanical damage.

The essence of the proposed technical solution is as follows. Titanium-silicon carbide Ti ₃ SiC ₂ is a material in the hexagonal crystal lattice of which every three packed layers of titanium atoms alternate with one layer of silicon atoms, and carbon atoms occupy all the octahedral pores between the titanium atoms. As a result, the unit cell acquires a layered (nanolaminar) structure (a = 0.30665 nm and c = 1.777 nm). A characteristic feature of this crystal lattice is the very large axial ratio c / a = 5.76, two types of interatomic bonds: tightly directed covalent Ti-C and metallic Ti-Si. In addition, the bonding forces of Si atoms with each other and with Ti atoms are much smaller than the Ti – C bonding forces. The structural features of Ti ₃ SiC ₂ determine, in comparison with carbides and silicides of group IIIA and IVA metals, the nature of its radiation-dimensional changes under the influence of fast neutrons. At relatively low (~ 200 ° C) irradiation temperatures, as for most ceramic materials, a slight swelling of the material is observed. As the radiation dose is set and the temperature rises, the swelling is replaced by shrinkage, which stabilizes when the irradiation temperature reaches 1100–1300 ° С. Annealing of radiation defects in this temperature range, as well as radiation-thermal creep of Ti ₃ SiC ₂ contribute to the relaxation of tensile stresses in the second and fourth layers of MT. Stable compressive stresses in the SiC or ZrC layer reduce the likelihood of fracture of the multilayer MT structure due to the increasing internal pressure of the GPA.

In addition, it should be borne in mind that Ti ₃ SiC ₂ is a corrosion-resistant material with respect to TPDs such as Cs, Rb, Sr, Ag, Pd, Eu, etc. With metals such as Ag, Pd, Fe, Co, Cr, titanium-silicon carbide does not form fusible eutectics, which contribute to a decrease in the diffusion characteristics of the material and a decrease in its strength. Ti ₃ SiC ₂ is an excellent getter with respect to oxygen and carbon monoxide, which form Ti-O and Si-O bonds in the interlayer space. Due to the fact that the ionic radius of oxygen is less than the interplanar distance (c / a = 5.76 at c = 1.767 nm), the oxidation process is not accompanied by bulk changes in the material.

In the process of MT irradiation, in which low density pyrocarbon (analogues and prototype of the present application) is used as the first layer, intense anisotropic shrinkage processes occur under the influence of fast neutrons. This, as a rule, is expressed in the formation of tangential and radial cracks in the layer and its detachment from the second — Ti ₃ SiC ₂ layer. Radial cracks can spread over the entire thickness of the second layer, opening direct channels for transporting TPD to carbide force layers. It was experimentally established that the introduction of silicon in the form of a silicon carbide phase into a pyrocarbon layer can significantly reduce the size and nature of radiation shrinkage of coatings. The quality of the silicon carbide phase introduced into the first layer should allow controlling the density and structure of the pyrocarbon – silicon – silicon composition, providing the required “free” volume for GPA and compensating for the mismatch in the coefficients of linear thermal expansion between the TM material and high-density layers.

For this, the process of deposition of the first layer is carried out so that 95-97% of the coating thickness contains 1.0-20.0 wt.% Of the silicon phase. This is achieved by selecting the ratio of reagents (C ₂ H ₂ , SiH ₄ ) in the gas mixture supplied to the fluidized bed apparatus and the pyrolysis temperature. The deposition of the remaining part of the layer (3-5% of the required thickness) is carried out in such a way as to ensure the content of the silicon phase in the composition at the level of 35-45 wt.% In the near-surface zone.

An example of the implementation of the proposed technical solution. A four-layer protective coating on TM with a diameter of 500 μm is sequentially deposited in a fluidized bed:

- the first layer of the carbon-silicon carbide composition is precipitated by a mixture of C ₂ H ₂ -SiH ₄ -Ar at a pyrolysis temperature of 1200-1400 ° C, and the ratio of C ₂ H ₂ / SiH _{4 is} chosen equal to 0.5-0.9 for the temperature range of 1200-1300 ° C and 0.3-0.6 for the temperature range of 1300-1400 ° C, the total consumption of the gas mixture of 1500 l / h;

- the second layer of titanium-silicon carbide is precipitated at a temperature of 1350 ° C, TiCl ₄ concentration is 3.0 vol.% (carrier SiCl ₄ argon), SiCl ₄ concentration is 1.0 vol.%, CH ₄ concentration is 2.0 vol.%, argon consumption - 600 l / h, hydrogen - 900 l / h;

- the third layer of SiC is precipitated at a temperature of 1550 ° C, the concentration of CH ₃ SiCl ₃ - 1.0 vol.%, the flow rate of hydrogen - 1500 l / h; the third ZrC layer is precipitated at a temperature of 1450 ° C, ZrCl ₄ concentration is 0.5 vol.%, CH ₄ concentration is 0.3 vol.%, hydrogen consumption is 1600 l / h;

- the fourth layer of titanium-silicon carbide is deposited according to the regime of the previous (second) MT layer.

The table shows a comparison of the operational characteristics of the known design of the microfuel with microfuel according to the proposed technical solution.

Table
Comparison of the operating life of the well-known microfuel with the proposed microfuel. Parameter Mikrotvel famous Mikrotvel offered The limitation on the magnitude of the fluence of fast neutrons, n / cm ² (2.0-4.0) 10 ²¹ ~ 1.0 · 10 ²³ Maximum permissible temperature of fuel operation before depressurization of coatings, ° C 900-1100 ¹⁾
1250 ²⁾ 1350-1400 ³⁾ Depth of fuel burnout at the time of depressurization of coatings,% t 10-15 more than 20 Note:
1) the maximum temperature when reaching the fuel burnup depth of 15% t;
2) the maximum temperature with a burnup depth of less than 10% tons;
3) the maximum temperature of 1350-1400 ° C can be maintained up to a fuel burnup depth of more than 20%

As follows from the data in the table, the proposed microfuel of a fast fast neutron nuclear reactor provides an increased service life due to the higher radiation-chemical stability of Ti ₃ SiC ₂ coatings. It should be noted one more important advantage of the proposed microfuel - this is the possibility of high energy release (as evidenced by the high level of operating temperatures) in a wide range of fuel burnup depths (more than 20% tons).

Claims (1)

A microfuel of a fast neutron reactor consisting of a fuel microsphere based on PuO ₂ and a four-layer protective coating, in which the third layer from the fuel microsphere is made of silicon carbide or zirconium carbide, characterized in that the second and fourth layers are made of titanium-silicon carbide of the composition Ti ₃ SiC ₂ , and the first layer deposited on the microsphere is made of a carbon-silicon carbide composition with a silicon phase content in the range of 1.0-20.0 wt.%, And the silicon phase content in the surface area of the outer g the boundary of this layer is 35-45 wt.% with a zone depth of 0.03-0.05 of the layer thickness.