RU2603018C1 - Nuclear reactor pebble - Google Patents

Abstract

FIELD: energy.

SUBSTANCE: invention relates to nuclear power, in particular, to nuclear reactor pebbles. Nuclear reactor pebble comprises a fuel microsphere based on oxide fuel and protective coating, including, from first from fuel microsphere, a low-density layer with thickness of 84-110 mcm, second dense layer with thickness of 30-36 mcm, third layer of silicon carbide and fourth high density layer with thickness of 36-42 mcm. All layers are made from silicon carbide, wherein first layer has density 0.9-1.2 g/cm³, second layer has density 2.5-2.9 g/cm³, third layer has density 1.5-2.2 g/cm³ and thickness of 7-13 mcm, and fourth layer has density 3.2-3.3 g/cm³.

EFFECT: technical result is production of a nuclear reactor pebble with longer life (higher fuel burnup depth) due to reduced pressure of gaseous fission products on protective layers.

1 cl, 5 dwg

Description

The invention relates to the field of nuclear energy, in particular to the microfuel of a nuclear reactor.

The use of microspherical fuel with protective coating layers ensures long-term operation of the reactor without rebooting, deep fuel burnup, while ensuring maximum operational safety of the nuclear reactor, since microspherical fuel with a multilayer ceramic coating effectively retains fission products inside the fuel particles, both under normal operating conditions and under the conditions of the maximum design basis accident with loss of coolant, when the fuel temperature can reach 1600 ° C.

Pyrocarbon of various densities and silicon carbides (SiC) or zirconium (ZrC) are currently considered as the main protective coatings. As the first layer (counting from the fuel microsphere), low-density pyrocarbon is usually used, as the second layer is high-density isotropic pyrocarbon, the third layer is made of either SiC or ZrC, the fourth outer layer is used when protection of the third layer (brittle carbide) is required, it is usually made of high-density isotropic pyrocarbon. Three high-density layers in the English literature are called TRISO coating (Wang J., Ballinger RG, MacLean HJ, TIMCOAT: An integrated fuel performance model for coated particle fuel / - Nuclear Technology, vol. 148, Oct. 2004, p. 68-96 )

As part of the microfuel coatings, they perform multi-purpose functions. So, the first layer from the fuel core serves as a free volume for the localization of gaseous and volatile fission products, and is also a compensator for the increase in the volume of the fuel core due to thermal expansion and radiation swelling (swelling). In addition, this layer protects the subsequent dense coating layers from the effects of high-energy fission fragments. The second layer is a diffusion barrier for gaseous and some solid fission products, while it protects the next carbide (SiC or ZrC) layer from the corrosive effects of fission products. The third layer (carbide) is a diffusion barrier for almost all fission products formed during fuel burnup. The fourth outer layer is usually used when protection of the brittle carbide layer from mechanical damage is required.

An analysis of the thermomechanical behavior of microfuel coatings during irradiation makes it possible to construct a scheme of the interaction of layers with each other and with the resulting fission products. Gaseous fission products (mainly krypton and xenon) emerging from the fuel core and the first low-density coating layer, as well as krypton and xenon, as well as carbon monoxide formed during the burning of oxide fuel create internal pressure, which can reach several hundred atmospheres, and dense layers of pyrocarbon and carbide (SiC or ZrC) act as structural layers that counteract this pressure. The layers of pyrocarbon undergo radiation-induced dimensional changes (XRDs) upon irradiation, moreover, at relatively small values of the fast neutron fluence, isotropic shrinkage occurs, but with an increase in fluence, the shrinkage is replaced by anisotropic swelling. Moreover, the speed of radial and tangential radial and tangential directions is significantly different. At the same time, radiation tests have shown good radiation stability of carbides (SiC or ZrC) up to the fluences characteristic of fast neutron reactors.

Post-reactor studies of irradiated microfuel fuel cells show that most often damage occurs in the low-density pyrocarbon layer and at the interface of the dense pyrocarbon and carbide layer (SiC or ZrC). Disruptions in the bond between these layers substantially determine the behavior of the entire coating under irradiation. When irradiated, shrinkage of the inner dense pyrocarbon layer causes a radial tensile stress at the interface between the layers of dense pyrocarbon and carbide (SiC or ZrC). If the voltage exceeds the bond strength between the layers, the inner high-density pyrocarbon layer detaches from the carbide layer.

Known microtel of a nuclear reactor containing a fuel microsphere and a multilayer protective coating consisting of layers of low-density pyrocarbon, high-density isotropic pyrocarbon, zirconium carbide, a layer of a pyrocarbon-silicon carbide composition with a silicon phase content of 20-45 wt.%, Silicon carbide and the outer layer of high-density isotropic pyrocarbon (patent RU No. 232325711, IPC G21C 3/28, 3/62, publ. 27.05.2008).

The disadvantage of this design is the use as a volume for localization of gaseous fission products of a layer of low-density pyrocarbon, which is destroyed during the long-term operation of the microfuel and forms carbon monoxide during the reaction with oxygen, which creates high pressure and lacks an oxygen getter. The use of alternating dense layers of pyrocarbon and carbide (SiC or ZrC) leads to a high probability of destruction of these layers at their boundaries due to different coefficients of linear thermal expansion and radiation size changes when exposed to a neutron flux.

Known microtel of a nuclear reactor containing a fuel microsphere made of plutonium dioxide and a multilayer protective coating consisting of successively deposited layers of low-density pyrocarbon, high-density isotropic pyrocarbon, aluminum nitride, silicon carbide and the outer layer of high-density isotropic pyrocarbon (patent RU No. 2326457 / 28, 3/62, publ. 10.06.2008).

The disadvantage of this design is the lack of an oxygen getter, and as a result, a gradual increase in the pressure of carbon monoxide and gaseous fission products, the destruction of the low-density layer of pyrocarbon, the migration of the core from the center of the microfuel and the subsequent destruction of the microfuel by the mechanism of the vessel under pressure.

Known microfuel with a two-layer protective coating of the fuel microsphere, in which the inner layer is made of a pyrocarbon-silicon carbide composition, the outer layer is made of a composition Ti ₃ SiC ₂ -C-TiM (patent RU No. 2393558, IPC G21C 3/28, 3/62 published on June 27, 2010).

The disadvantage of this design is the use of chlorine derivatives when applying the first layer to an unprotected core, as a result of which the resulting hydrogen chloride will actively interact with the fuel microsphere, causing it to corrode and contaminating the gas treatment equipment and system with radioactive compounds. Joint deposition of silicon carbide and pyrocarbon is a problematic process, because uncontrolled low-density porous structures are formed.

The microtel of a nuclear reactor is known in which the protective coating consists of a layer of zirconium carbide deposited directly on the fuel microsphere and subsequent successively deposited coating layers: low-density pyrocarbon, high-density isotropic pyrocarbon, a power layer of silicon carbide and an outer layer of high-density isotropic pyrocarbon (RU patent No. 1212212 IPC G21C 3/28, 3/62, publ. 05.27.2008).

The disadvantage of this design is the technical complexity of the process of applying a layer of zirconium carbide on the fuel core. The problem is related to the need to protect the fuel core with a thin layer of metal zirconium, the application of which involves the use of separate (additional) equipment. Further, zirconium carbide is deposited from the chlorine derivatives on the fuel core protected from hydrogen chloride. In this case, zirconium carbide, used as an oxygen getter, is isolated metal zirconium. Contact with oxygen released during fuel burning will be possible only when the zirconium carbide layer is destroyed, and the destruction of a fairly strong layer can cause a shock wave, which provokes the destruction of subsequent layers. In addition, such an oxygen getter will have a very small contact surface, which will significantly affect its effectiveness. Zirconium carbide has a large coefficient of linear thermal expansion, 7.01 × 10 ^-6 deg ^-1 , this can lead to damage, destruction and loss of the brittle layer of zirconium carbide already during heating to apply the following coating layers.

Known microfuel of a nuclear reactor with a three-layer protective coating of the fuel microsphere, in which the first layer from the fuel microsphere is made of a carbon-silicon carbide composition with a silicon phase content of 30-35 wt.% In the surface area of the outer boundary of the layer with a depth of 0.05-0.10 from the thickness of the layer and the silicon phase content in the rest of the first layer is 1-15 wt.%, the second layer is made of silicon carbide, the third layer from the fuel microsphere is made of a carbon-silicon carbide composition, the third layer is made with silicon phase 5-10 wt.% in the surface zone of the outer boundary of the layer with a depth of 0.1-0.2 of the thickness of the layer and the silicon phase content in the rest of the third layer is 15-30 wt.% (RU patent No. 2333552, IPC G21C 3/28 3/62, published September 10, 2008).

The disadvantage of this design is the simultaneous deposition of carbon and silicon carbide, by pyrolysis of acetylene and methylsilane. As experiments have shown, at temperatures above 1200 ° C, as a result of the deposition of the protective layer from the specified gas mixture, loose carbon-silicon structures with uncontrolled sizes and other technological parameters are formed. The physical characteristics of alternating coating layers are significantly different, as a result, during the long-term operation of such a microfuel, we should expect destruction of coatings at the boundaries of the layers. Also a disadvantage of this microfuel of a nuclear reactor is the low resistance of the pyrocarbon coating layers to a hard neutron spectrum.

Closest to the proposed technical solution is a microfuel of a nuclear reactor containing a fuel microsphere of uranium dioxide and a four-layer protective coating, the first layer of which is deposited on the fuel microsphere, made of highly porous pyrocarbon with a density of 1.10 g / cm ³ and a thickness of 84-110 μm. The second layer is made of dense isotropic pyrocarbon with a density of 1.85 g / cm ³ and a thickness of 30-36 microns. The third layer with a density of 3.2 g / cm ³ and a thickness of 32-36 μm is made of silicon carbide and the fourth, outer layer with a density of 1.853 g / cm ^{3 is} made of high-density isotropic pyrocarbon and a thickness of 36-42 μm (see, Minato K. , Sawa K., Koua T. et al. Fission product real ease behavior of individual coated fuel particles for high temperature gas-cooled reactors, Nuclear Technology, vol. 131, July 2000, p. 36-47).

Due to the lack of a substance in the microfuel structure that binds oxygen released during the operation of oxide fuel, a gradual increase in the pressure of carbon monoxide and gaseous fission products will lead to the destruction of the microfuel by the mechanism of the pressure vessel. It should be noted that the ultimate stability of pyrocarbon layers is limited by the fast neutron fluence (2-4) · 10 ²¹ n / cm ² , while in fast reactors a fluence greater than 2.0 · 10 ²³ n / cm ² can be expected. The use of pyrocarbon coating layers, especially low-density ones, will significantly affect the operating time of a microfuel before its destruction due to the insufficient resistance of pyrocarbon under the influence of a fast neutron flux. The alternation of layers of dense pyrocarbon and dense silicon carbide leads to destruction at the boundary of these layers due to their different physical properties.

EFFECT: obtaining a microfuel of a nuclear reactor with an increased service life (increasing the fuel burnup depth) by reducing the pressure of gaseous fission products into protective layers.

The technical result is achieved due to the fact that the microfuel of a nuclear reactor containing a fuel microsphere based on oxide fuel and a protective coating comprising a first low-density layer from a fuel microsphere 84-110 μm thick, a second dense layer 30-36 μm thick, a third layer of silicon carbide and the fourth high-density layer 36-42 μm thick, characterized in that all the layers are made of silicon carbide, while the first layer has a density of 0.9-1.2 g / cm ³ , the second layer has a density of 2.5-2.9 g / cm ^3, the third layer has a density of 1.5-2.2 g / cm ³ and then schinu 7-13 microns, and the fourth layer has a density of 3.2-3.3 g / cm ^3.

Each of the layers of the proposed microfuel (MT) of a nuclear reactor performs the following functions:

- the first layer of porous silicon carbide contains about 70% of the pores, which is the volume for the localization of gaseous fission products and compensates for the swelling of the fuel microsphere at the stages of deep fuel burnout, this layer is also a getter for oxygen and palladium;

- the second layer of dense silicon carbide, obtained without the use of halogen-containing reagents, isolates the fuel microsphere from the penetration of hydrogen chloride formed during the application of dense layers of silicon carbide, and is a diffusion barrier for fission products;

- the third thin layer of SiC, being porous, separates the dense layers of silicon carbide in order to prevent the development of a through crack, which is likely to arise in the second layer of SiC experiencing tensile stresses as a result of the development of pressure of gaseous fission products at the stages of deep burning of fuel;

- the fourth layer of high-density silicon carbide carries the main power load and is the main barrier to fission products.

The essence of the proposed solution is as follows.

In the first porous layer of silicon carbide adjacent directly to the fuel microsphere, oxygen is formed during the operation of the oxide fuel SiC + 2O → SiO ₂ + С, as a result of which a solid compound SiO _{2 is formed} , which reduces the internal pressure in MT and, accordingly, the probability of microfuel destruction by the mechanism of the pressure vessel. The contact of porous silicon carbide with the fuel microsphere also makes it possible to bind palladium, which corrodes the main carrier layer of silicon carbide, into refractory (1950 ° С) and diffusion-resistant compounds U-Si-Pd-C. The coatings have close physical properties, as a result of which the destruction of the coating layers at their boundaries is prevented. The ultimate resistance of pyrocarbon layers is limited by the fast neutron fluence (2-4) · 10 ²¹ n / cm ² , while in fast neutron reactors a fluence greater than 2.0 · 10 ²³ n / cm ² can be expected, which makes the application unacceptable microfuel containing pyrocarbon coatings in fast neutron reactors. To solve this problem, the inventive microfuel contains only silicon carbide protective coatings, which are characterized by high resistance to hard neutron fluxes (Tecdoc 1645 “High Temperature Gas Cooled Reactor Fuels and Materials)), IAEA, Vienna, Austria, 2010).

To study the properties of porous silicon carbide, which was not previously used as a compensation volume for gaseous fission products and swelling fuel microspheres, experiments were carried out to test the possibility of such a layer to withstand significant deformations without fracture. The boundary of the porous SiC layer in the samples before and after thermal shock, carried out 10 times by abrupt heating (at a speed of 100-1000 deg / s) to 1000 ° C and subsequent abrupt cooling to 20 ° C, is shown in FIG. 1 and FIG. 2, respectively. This experiment confirms the good deformation characteristics of the porous layer of silicon carbide, which is necessary when using it as a compensation volume for a swelling fuel core and gaseous fission products, and also confirms the possibility of using a layer to bind palladium, because layer detachment from the fuel microsphere does not occur and there are all conditions for the formation of a stable U-Si-Pd-C compound.

The experimental design of the MT was simulated. The calculations were performed using the GOLT-v3 code developed by VNIINM OJSC (Golubev I.E., Kurbakov SM., Chernikov A.S. "Calculation and experimental studies of the pyrocarbon and silicon carbide barriers of microfuel VTGR", Atomic Energy, Volume 105, Vol. 1, July 2008, p. 14-25). The code was tested on problems with an analytical solution, and also verified during benchmark calculations carried out by the IAEA in the framework of the CRP-6 project (Tecdoc 1674 Advances in High-Temperature Gas-Cooled Reactor Fuel Technology), IAEA, Vienna, Austria, 2012 )

For comparison, calculations were performed for two MT designs:

1) the proposed design: a spherical microsphere UO ₂ (diameter 400 μm, 20% enrichment according to U-235) with a four-layer coating: porous SiC (thickness 100 μm, density 1.0 g / cm ³ ), dense SiC (thickness 35 μm , density 2.8 g / cm ³ ), porous SiC (thickness 10 μm, density 2.0 g / cm ³ ), high-density β-SiC (thickness 40 μm, density 3.21 g / cm ³ );

2) prototype construction (TRISO type coated particle): spherical microsphere UO ₂ (400 μm diameter, 20% enrichment according to U-235) with a four-layer coating applied on it: porous pyrocarbon (BPyC buffer layer, thickness 100 μm, density 1.0 g / cm ³ ), dense pyrocarbon (IPyC, thickness 35 μm, density 1.85 g / cm ³ ), a layer of β-SiC (thickness 40 μm, density 3.21 g / cm ³ ), dense pyrocarbon (OPyC, thickness 40 μm, density 1.85 g / cm ³ ).

The calculations were carried out for the irradiation conditions characteristic of high-temperature gas reactors (HTGR):

- heat in a single core: 0.1 W;

- maximum burnup: 20% FIMA;

- maximum fast neutron fluence (E> 0.18 MeV): 4 × 10 ²⁵ m ^-2 ;

- maximum temperature of the fuel microsphere: 1250 ° C.

In FIG. Figure 3 shows plots of the tangential thermal stresses at the beginning of the fuel campaign after reaching the rated power for two MT variants. From the above graphs it follows that during the initial heating in the power layer of silicon carbide in the coating of the prototype MT, tensile stresses develop, and in the proposed construction, the stresses in the external SiC power layer are of the order of -64 MPa. Such a stress distribution in the proposed multilayer coating design increases its resistance to the action of the internal pressure of the gaseous fission products and, consequently, the fuel burnup depth increases.

In FIG. Figure 4 shows the dependences on the fluence of fast neutrons of internal pressure that develops under MT coatings during fuel burnout. From a comparison of the obtained curves for the 2 options, it follows that in the proposed design MT the final value of the gas pressure is 4 times lower than in the design of the prototype, and accordingly, the burnup depth of the fuel increases.

In FIG. Figure 5 shows the dependences of the maximum stresses in the power layers of the coating on the fluence of fast neutrons. From the graphs shown in this drawing, it follows that the stresses in the MT of the prototype at the beginning of irradiation from tensile thermal ones quickly move to the compression region, and then again to the tension region and tend to increase significantly as the fuel burns out and the gaseous fission products leave the fuel microsphere in porous SiC buffer layer. On the contrary, in the strength layer of silicon carbide coatings MT of the proposed design, the stresses are in the compression region during the entire calculation period and have a slight upward trend. From the obtained results we can conclude: the proposed MT design is promising for use in fast neutron reactors.

An example embodiment of the invention

The apparatus for coating in a fluidized bed is heated to a temperature of 1100 ° C, with an argon flow rate of 800 l / h, 100 g of fuel microspheres with a diameter of 400 μm (3 · 10 ⁵ pieces) are filled in, after which a pyrolysis gas mixture is fed instead of argon.

The first low-density layer is applied by pyrolysis of a mixture of methylsilane and argon at a methylsilane concentration of 5-15 vol.% And a total gas flow of 900 l / h, at a temperature of 1100 ° C, the deposition time of the layer is 20 minutes. The second dense layer is obtained by feeding a mixture of methylsilane 2-10 vol.% With argon at a total gas flow of 900 l / h, at a temperature of 800 ° C, the deposition time of the layer is 50 minutes. The third low-density layer of silicon carbide is applied by pyrolysis of a mixture of methylsilane and argon at a methylsilane concentration of 2-5 vol.% And a total gas flow rate of 900 l / h, at a temperature of 1100 ° C, the deposition time of the layer is 5 minutes. The fourth high-density layer is deposited from methyltrichlorosilane vapor in a hydrogen atmosphere at a concentration of methyltrichlorosilane 1.2-1.5 vol.%, Hydrogen consumption for fluidization of 900 l / h at a temperature of 1500 ° C, the deposition time of the layer is 120 minutes.

Thus, a microfuel of a nuclear reactor was created, which has a fuel microsphere of oxide fuel, and a protective coating, in which all layers are made of silicon carbide of various densities and structures. The resulting microfuel has an increased resource of fuel operation (burnup depth) in the reactor.

Claims (1)

A microtel of a nuclear reactor containing an oxide-based fuel microsphere and a protective coating comprising a first low-density layer from a fuel microsphere 84-110 μm thick, a second dense layer 30-36 μm thick, a third silicon carbide layer and a fourth high-density layer 36-42 μm thick characterized in that all the layers are made of silicon carbide, wherein the first layer has a density of 0.9-1.2 g / cm ³ , the second layer has a density of 2.5-2.9 g / cm ³ , the third layer has a density 1.5-2.2 g / cm ³ and a thickness of 7-13 microns, and the fourth layer has a density of 3.2-3.3 g / cm ³ .

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