Micro heat-exhausting element for nuclear reactor

FIELD: nuclear engineering, in particular, engineering of micro heat-exhausting elements for nuclear reactors.

SUBSTANCE: first layer of micro heat-exhausting element with four-layer protective cover is made of SiC-PyC composition with content of 1,0-10,0 % of mass of silicon carbide with thickness of layer equal to 0,02-0,2 of diameter of fuel micro-sphere, second layer is made of SiC-PyC composition with content of 20,0-45,0 % of mass of silicon carbide with thickness of layer equal to 0,03-0,40 diameter of fuel micro-sphere, third layer is made of silicon carbide, while fourth layer is made of titanium nitride with thickness equal to 0,01-0,08 of diameter of fuel micro-sphere.

EFFECT: increased exploitation resource of nuclear reactor due to increased corrosion resistance and radiation stability.

3 dwg, 1 tbl

 

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

MICROTEL (MT) nuclear reactor is the fuel microsphere (TM) of nuclear material with layers of protective coating of pyrocarbon (PyC) and silicon carbide (SiC) (Beginig D. high-temperature Gas-cooled reactors. TRANS. with it. M: Atomizdat, 1975, 224).

Protective coating microtalon nuclear reactor perform multipurpose functions:

- The retention of the gaseous and solid fission products within MICROTEL, reducing, thus, the cost of protection and operation of structures outside the reactor core.

- Compensation of mismatches in the coefficients of linear expansion of material of the fuel microspheres and high density of the coating layers.

- Protection of the fuel material from embrittlement, corrosion from exposure to fluid and impurities of the material of the fuel element.

The creation of the "free" volume for containment of gaseous fission products within the particles during exposure - this function performs the buffer pyrocarbon (first from the fuel microspheres highly porous protective layer).

The thickness of the coating microtalon optimized for specific operating conditions of a nuclear reactor.

In the irradiation process, each of the protective layers of MICROTEL nuclear react the RA vzaimosoglasovannyie prevents gaseous (GPA) and solid fission products (TPD) outside particles: a highly porous PyC protects high-density PyC layer from direct bombardment by the recoil nucleus and localizes gaseous fission products; internal high-density PyC is the first diffusion barrier against gaseous and solid fission products, while protecting the SiC from the corrosive effects of the TPD; because of their excellent physical, mechanical and thermophysical characteristics of SiC is the main power layer MT and diffusion barrier in relation, first of all SOPS.

The integrity of the multilayer coatings MT irradiation process depends primarily on the degree of structural changes of pyrocarbon. The behavior of the PyC coatings by irradiation with fast neutrons is largely similar to the behavior of other carbon-graphite materials: anisotropy of properties leads to various dimensional changes depending on the direction of the axes of the crystallographic orientation.

In the plane of the PyC deposition is significant shrinkage, temperature-dependent irradiation source and the density of the material. In the direction perpendicular to the plane of deposition, is the initial shrinkage that with increase of neutron fluence goes into the swelling.

The degree of dimensional stability PyC associated with the isotropy of the material, the Anisotropy of the radiation-dimensional changes under irradiation leads to an increase in stresses in the PyC. As a result, the point located on the inner side of the PyC, where p is sbivautsa maximum voltage, there are cracks (figure 1). In addition to the violation of the mutually agreed coexistence system layer coatings MT formed lanceolated shrinkage cracks (Fig 1, a), provides direct access to the main power layer design MT-SiC solid fission products, limiting its integrity due to corrosion (figure 1, b).

Silicon carbide at temperatures 1000°and more is an excellent diffusion barrier against most of the TPD, has high strength and conductivity, to a substantially lesser degree than PyC exposed to radiation dimensional changes.

Known MICROTEL nuclear reactor fuel microsphere mixture of ThO2-UO2with a diameter of 450 microns, including as protective coatings: low-density buffer PyC thickness of 104 μm, inner dense PyC thickness of 5 μm and the outer layer of silicon carbide in the mixture with PyC (SiC-composition) containing 33 wt.% silicon (Caae J.L., Sterling S.A., Yang L. Improvements in the performance of Nuclear fuel particles offered by silicon - alloyed carbon coatings. - Nucl. Technol, vol. 35, No. 2, 1977, R).

The disadvantage of this MICROTEL nuclear reactor is that as exposure in the structure of protective coatings proceed noticeable transformation: shrinkage low-density buffer PyC, the formation of cracks in it, the output of cracks on the inner surface of the high-density PyC then RA is the destruction and formation of channels direct access fission products to the composition of the SiC-C, which then collapses under corrosive solid fission products and the internal pressure of the GPA. The situation on the radiation resistance PyC and health microtalon compounded in the case of integral high doses of irradiation with fast neutrons. For PyC high density at fluences greater than (2-4)·1021n/cm2total power lead to the destruction of the material, and at fluences above (1-2)·1022n/cm2they cause the formation of new cracks and fracture coatings MT containing PyC layers.

The disadvantages of silicon carbide should be attributed to its low corrosion resistance in alkaline environments and when in contact with metals type Fe, Ni, Cr, Ti, Al, etc. to interact with occurs with appreciable rate at temperatures 700°and more.

The closest analogue is the prototype of the proposed technical solution is MICROTEL of a nuclear reactor containing the TM of the UO2and four-layer protective coating (figure 2), the first layer which is made of highly porous PyC density ≤1.0 g/cm3thickness of 90 μm, a second layer of high-density isotropic PyC density ≥1,80 g/cm3and a thickness of 60 μm, the third layer of the SiC density 3,20 g/cm3and a thickness of 50 μm and the fourth (outer) layer of high-density isotropic PyC density ≥1,80 g/cm3and what thickness of 50 μm (Ascherio, Lntermodal, III, AVI, Sedcoorbo. The fuel rods on the basis of spherical fuel particles with a protective coating for reactors with advanced security. - Atomic energy, t, issue 6, December 1999, s-462).

The disadvantage of this MICROTEL nuclear reactor is a low resource use associated with low corrosion resistance to the metallic structural elements of the active zone, limited temperature 700°and With low radiation stability, limited to fluences of fast neitronov (2,0-4,0)·1021n/cm2.

These shortcomings stem from the fact that the first PyC layer, as in the initial state, highly porous, in the irradiation process undergoes substantial shrinkage and originated on its inner surface crack at high speed extends throughout its thickness. Second PyC layer being a high-density, also sits. While for this reason, in the stressed state, it is not a significant barrier for cracks propagating from low-density PyC layer. Leaving the boundary of the second PyC-SiC layer, the crack opens up direct access to the carbide coating solid fission product (Cs, Ag, Pd, Ba, Sr, and others). The last cause corrosion of the third SiC layer, reducing its strength and increase permeability through him TPD and GPA. After a bit the solutions of the three inner layers of MICROTEL due to a sharp increase in pressure GPA significantly increases the likelihood of damage to the latter double protective PyC coating.

Depressurization of all four layers of the coating leads to a complete loss GPA and from MICROTEL and accelerate the interaction UO2fuel microspheres with PyC. These factors limit resource use MICROTEL. Four-layer system PyC-SiC MICROTEL has low corrosion resistance when in contact with metals and limited temperature compatibility ˜700°C. this is Due to the fact that the fourth PyC layer when heated intensely dissolves the metal impurity in the fuel element, the structural element of the cassette in contact with microtalon). With the penetration of the metal to the third SiC layer with the latter are formed of low-melting eutectic, destroying the floor and open access metal to the second PyC layer. The combination of these processes reduces corrosion resistance, and resource use MICROTEL in General.

The authors proposed technical solutions faced the challenge of increasing resource use MICROTEL nuclear reactor by increasing the corrosion resistance and radiation stability.

The problem is solved in that in microtube of a nuclear reactor containing the fuel microsphere and four-layer protective coating, in which the first, second and fourth layer contains the pyrocarbon, the third layer contains silicon carbide, the first layer further comprises 1.0 to 1.0 wt.% silicon carbide with a layer thickness of 0.02-0.20 diameter of the fuel microspheres, the second layer further comprises 20,0-of 45.0 wt.% silicon carbide with a layer thickness of 0.02-0.40 diameter of the fuel microspheres, and a fourth layer made of titanium nitride with a thickness of 0.01-0.08 diameter of the fuel microspheres (figure 3).

From the point of view of radiation dimensional changes in the composition of pyrocarbon - SIC under irradiation is more stable than pure pyrocarbon. This result was confirmed in studies of radiation-induced alteration of the properties of this material.

Experimental results indicate that the composition of pyrocarbon - silicon carbide has a higher radiation stability than pure pyrocarbon and its stability increases with increasing silicon content. The strength of the composition pyrocarbon - silicon carbide is higher than the net pyrocarbon and also increases with increasing silicon content.

Resource irradiation show that the radiation dimensional changes of SiC isotropic, small in absolute value, the strength characteristics of the stable.

The titanium nitride for a long time resistant to the molten elements such as Cr, Fe, Ni, Co, Al, and alloys (Fe-Ni (3%), (Ni-C), (Ni (80%) - Cr (20%), steel GN (up to 1500°).

The causal link between the essential features and the technical solution is as follows.

Each of the protective layers Ave is Loginova of MICROTEL nuclear reactor performs the following functions:

layer of PyC + SiC to SiC content is 1.0-10.0 wt.% and a thickness of 0.02-0.2 diameter TM is the amount for localization GPA, compensates for the mismatch of the coefficients of linear thermal expansion between TM and subsequent layers, protects the second layer from damage by debris fission fuel material (recoil). The content of SiC is less than 1.0 wt.% leads to decrease of its radiation stability, increasing the SiC content to values above 10.0% vol. impractical due to the decrease in free volume for containment of the GPA. Thickness less than 0.02 diameter of TM leads to the decrease of the free volume, more than 0.2 of the diameter of the TC is to increase the resistance to heat transfer from TM to the second high-density PyC + SiC layer:

the second high-density layer of PyC + SiC to SiC content of 20.0-of 45.0 wt.% and a thickness of 0.02-0.4 diameter TM is a diffusion barrier for the GPA and TPD, protects the SiC layer from the corrosive effects of the TPD. When the SiC content of 20.0-of 45.0 wt.% layer is an effective diffusion barrier for GPA and TPD, has a high radiation stability compared to pure PyC and protects the third SiC layer from the corrosive effects of the TPD. When the SiC content less than 20.0 wt.% reduced the effectiveness of this layer as a diffusion barrier decreases the strength and radiation stability. In that case, when containing the s SiC more than 45.0 wt.% implemented strained layered PyC-SiC structure, having low radiation stability.

When the thickness of this layer is less than 0.02 diameter of the fuel microspheres lost function of the diffusion barrier and the protection of the third SiC layer from the corrosive effects of the TPD. The increasing thickness of more than 0.4 of the diameter of the fuel microspheres leads to increased stresses in the coating and increase the likelihood of fracture during irradiation;

the third SiC is the main power coating and diffusion barrier for TPD;

fourth TiN is the main floor of the MT, which protects the inner layers from corrosion of the contacting metal fuel elements or metal structures of the active zone of the nuclear reactor.

An example implementation of technical solutions.

Fuel microsphere uranium dioxide with a diameter of 0.5-3.0 mm precipitated in a fluidized bed at a temperature of pyrolysis 1450±20°With the first layer of the composition pyrocarbon - silicon carbide by pyrolysis of CH3SiH3-C2H2-Ar mixture. The number entered in the coating composition of the silicon phase is governed by the ratio of CH3SiH3and C2H2served at the entrance to the pyrolysis zone. After deposition of the desired thickness of the composition of the porous pyrocarbon - silicon carbide stop the supply of the reaction gases (CH3SiH3and C2H2), acustica fuel microspheres supported in a state of fluidization by supplying an inert carrier gas argon. By adjusting the input to the heater of the furnace electrical power reduce the temperature of the fluidized bed up to 1350±20°C. the Deposition of the second layer of the composition pyrocarbon-silicon carbide at this temperature carried out due to decomposition of CH3SiH3-C3H6-N2-Ar mixture.

The third silicon layer is precipitated at a temperature of 1550-1600°due to pyrolysis of CH3SiCl3-H2of the mixture.

Outer (fourth) layer of titanium nitride is precipitated either by the process of chemical deposition from the gas phase by pyrolysis TiCl4with NH3or by using physical methods of sputtering of TiN and deposition of the layer of titanium nitride on the surface of the particles located on the vibrating pan.

The table below shows the comparison of the operational characteristics of known MICROTEL nuclear reactor microtalon on the proposed technical solution.

As follows from the table, the proposed MICROTEL nuclear reactor (examples 2, 3, 4) in comparison with the known microtalon (example 1) provides increased resource exploitation at the expense of greater corrosion and radiation resistance. With exorbitant settings microtalon (examples 5 and 6) corrosion and radiation resistance decreases sharply.

MICROTEL of a nuclear reactor containing the fuel microsphere and four-layer protective coating, in which the first, second and fourth layer contain pyrocarbon, the third layer contains silicon carbide, wherein the first layer further comprises 1.0 to 10.0 wt.% silicon carbide with a layer thickness of 0.02-0.20 diameter of the fuel microspheres, the second layer further comprises 20,0-of 45.0 wt.% silicon carbide with a thickness of 0.02-0.40 diameter of the fuel sphere, and the fourth layer is made of titanium nitride with a thickness of 0.01-0.08 diameter of the fuel microspheres.



 

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