RU2575863C1 - Method to manufacture ceramic tube for fuel element shell - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method to manufacture a ceramic tube for a fuel element shell, made of silicon carbide layers, includes stages: tubular frame is formed from fibre with structure β-SiC, its impregnation is carried out by a ceramic-forming precursor in inert gas atmosphere, subsequent stepped thermal treatment to produce a ceramic matrix and formation of ceramic composite in process of final heat-temperature thermal processing.

EFFECT: elimination of one of main faults of ceramic products - brittleness, provision of environmental purity of the method.

4 cl, 5 dwg

Description

The invention relates to nuclear energy, and in particular to processes for creating high-temperature silicon carbide composite materials that can be used in the manufacture of ceramic tubes for the shells of fuel elements (fuel elements) and other components of the fuel assembly.

A known method of manufacturing a composite material based on silicon carbide, including: processing a suspension of carbon filaments (2-8 wt.% Carbon nanotubes, 5-20 wt.% Polymer binder, organic solvent), processing a suspension of beams (10-30 wt.% Polymer a binder, 3-15% thermally expanded graphite), the synthesis of a matrix and a silicon carbide layer by carbonization and siliconizing (RF Patent 2457192, IPC C04B 35/573, C04B 35/80, B82B 3/00, publ. 03.20.12).

The disadvantages of the method are the complication or exclusion of obtaining silicon carbide of stoichiometric composition (Si / C = 1.05) due to the need for the siliconization process and, as a result, high porosity, low corrosion resistance in water vapor at high temperatures, low mechanical strength.

A known method for producing a composite material by deposition of silicon carbide from a gas phase containing methylsilane CH3SiH3 onto a porous framework followed by pyrolysis of methylsilane at a temperature of 650-800ºC (RF Patent No. 2130509, IPC C23C 16/32, publ. 05.20.1999).

The disadvantages of the analogue are that this method does not provide uniform in thickness and structure of the coating layer and does not provide uniform saturation of the multilayer porous frame.

A known method of producing products from a carbon-containing composite material, including forming a frame corresponding to the shape of the manufactured product, impregnating the frame with a binder, drying and subsequent heat treatment in a gas medium, characterized in that organic fibers — mycoton — are used as the framework, and aminborane solution is used as the binder in hydrofuran, or as a skeleton, a neutron moderator, and as a binder, a material - a polymer reagent from the series including polycarbosilane, or polysil zan, or polysiloxane, while the heat treatment is carried out under conditions providing a high density of the gas medium (Patent No. 2370436, IPC C01B 31/02 (2006.01), C04B 35/532, publ. 2009).

The disadvantages of this analogue are:

- the use as a frame of a fiber from mycotone, which has increased hygroscopicity and low thermal conductivity, therefore, cannot be used for the intended purpose of the proposed method;

- the use of materials that increase the capture of thermal neutrons from 0.09 to 3.2 barns, which is unacceptable for the shells of the fuel cell, as a structural material, as it reduces the efficiency of the fuel cell.

A known method of producing a cellular material based on silicon carbide by forming a cellular structure from a polymer material and processing it at elevated temperatures, characterized in that to form a cellular structure, polycarbosilane is poured at 200-280ºC in an inert gas medium into a mold for the formation of elastic cells or a film consisting of one row of elastic cells whose walls can rotate relative to each other, the mold is cooled to room temperature, the cells or film are removed from spring molds and in an atmosphere of air several cells or films are placed in a container one on top of the other and compacted by applying an air pressure of 0.02-0.03 MPa, then gradually raise the temperature to 300-310ºC, hold it for 30-60 minutes, then raise the temperature to 1200-1300ºC and hold for 30-60 minutes until the formation of an elastic foam block (RF patent 2074151, IPC C04B 35/565, C04B 35/571, C04B 38/00, publ. 1997).

The disadvantages of the analogue are:

- the method of finishing heat treatment involves increasing the temperature from 300 to 1200ºC, using an air atmosphere, which will lead to the oxidation of silicon carbide;

- the material contains an increased amount of oxygen in the form of SiO ₂ and free carbon, which under the operating conditions of the fuel cell will lead to oxidation of the shell, its swelling and destruction;

- the cellular material obtained by the proposed method is not a composite;

- mechanical properties characteristic of rubbers, which is not applicable to the use of a fuel cell as a tube under the conditions of operation of thermal reactors.

Known fuel element and a method of manufacturing a tube that serves as the main part of its shell, which are described in application for US patent 2009/0032178 from 2008, publ. 2009, US Class 156/143, 427/237, "Multi-layered Ceramic Tube for Fuel Containment Barrier and Other Applications in Nuclear and Fossil Power plants". The fuel rod proposed in this application consists of three layers - internal, central and external. The inner layer is made of monolithic beta-phase stoichiometric silicon carbide.

The central layer is made in the form of a winding thread on the inner layer. This thread is made of carbon fiber coated with a thin layer of silicon carbide (see section "Detailed Description of Preferred Embodiments)) applications, p. 3, paragraph [0038]) or of silicon carbide (see paragraph 21 of the claims) and in this case the same stoichiometric silicon carbide is used in the central layer as in the inner layer. The outer layer is made of monolithic silicon carbide. It is proposed to obtain stoichiometric silicon carbide with a crystal structure of a cubic modification of β-SiC (hereinafter β-SiC) used in these layers by chemical vapor deposition or, in other words, by gas-phase deposition.

The disadvantage is that due to the use of winding a 15-micron carbon fiber filament, coated on top with a thin layer of 1-micron silicon carbide in the central layer of the fuel rod, the effect of neutrons leads to a significant swelling of the carbon fiber along the filament, i.e. lengthens it; this leads to a weakening of the winding and makes it pointless from the point of view of increasing the strength of the tube with the winding in relation to the internal pressure. The disadvantage is the use of winding in the Central layer, as a result of which radial thermal conductivity is significantly reduced, which is unacceptable for fuel rods.

As a prototype, the patent of the Russian Federation No. 2481654, IPC G21C 3/00, publ. May 10, 2013). In this patent, the fuel cladding tube is made of alternating layers of nanocrystalline silicon carbide and separation layers of high-temperature material that does not coincide in structure with silicon carbide, and the tubes at both ends of the tube are made of silicon carbide, which prevents gases from passing through the tubes. The essence of the manufacture of a ceramic tube for the shell of a fuel element consists in applying by gas-phase deposition onto an outer surface of a solid or hollow graphite rod heated to 1300-1600ºC alternating layers of nanocrystalline cubic silicon carbide and separation layers of high-temperature material that does not coincide in structure with silicon carbide. For the formation of nanocrystalline cubic silicon carbide using a mixture of gases containing hydrides and chlorides of silicon and carbon.

The separation of the graphite rod from the coating according to the prototype method is carried out by its oxidation in air at a temperature of from 700 to 1000ºC.

The disadvantages of the prototype are:

1) the prototype tube has one of the main disadvantages inherent in ceramic products, fragility, as it consists of monolithic silicon carbide;

2) the use of a mixture of gases containing chlorine and hydrogen, which leads to the formation of environmentally hazardous gaseous hydrogen chloride;

3) the need to use the laborious operation of obtaining a tube by oxidizing (burning) graphite at a temperature of 800-1000ºC in air, since the presence of an operation (process) of applying silicon carbide to the surface of a heated graphite rod creates a sufficiently strong adhesion of silicon carbide and graphite, which is confirmed by the use of this methods for applying protective heat-resistant coatings to carbon coatings (Tkachenko L.A., Shaulov A.Yu., Berlin A.A. “Protective heat-resistant coatings of carbon materials”, Inorganic materials Rials 2012, №3);

4) it is not clear from the description how the indicated temperature range for heating a graphite rod from 1300 to 1600ºC was observed with increasing coating thickness and how the composition of the gas phase is replaced when applying an intermediate layer.

The objective of the invention is the manufacture of a ceramic tube for the shell of a fuel element by a technologically advanced environmentally friendly method.

The technical result is the advantages of the proposed method over the prototype:

- the formation of a ceramic composite during the final high-temperature heat treatment eliminates one of the main disadvantages of ceramic products - fragility. Fragility is eliminated both by creating a tubular skeleton of a fiber with a β-SiC structure impregnated with a ceramic-forming precursor, and by ensuring compatibility by thermal expansion coefficient (CTE) over a wide temperature range from room temperature to 1800ºC, as can be seen from the presented example of the manufacture of a ceramic tube for a shell fuel rod;

- in the proposed method using components (SiC fiber and ceramic-forming precursor, not containing chlorine), which reduces the environmental load.

The technical result is achieved in a method of manufacturing a ceramic tube for a shell of a fuel element, including layers of silicon carbide, whereby a tubular frame of fiber with a β-SiC structure is formed, it is impregnated with a ceramic-forming precursor in an inert gas atmosphere, followed by heat treatment with a stepwise increase in temperature to obtain a ceramic matrix and the final high-temperature heat treatment, providing the formation of ceramic composite.

As a ceramic-forming precursor, polycarbosilane is used.

The tubular cage is impregnated cyclically at a residual pressure of 0.001 MPa in an inert gas atmosphere.

Heat treatment is carried out by a gradual increase in temperature with a heating rate of 5-20 deg / h to 350ºC and holding for at least an hour at 350ºC. From 350ºC with a heating rate of 5-20 deg / h to 700ºC and holding for at least an hour at a pressure of 0.1 MPa in an inert gas atmosphere.

The final high-temperature heat treatment is carried out by heating from 700ºC to 1800ºC with an inert gas pressure of 0.1 MPa and with a holding time of at least 2 hours.

In the proposed method, polycarbosilane of molecular weight 800-1500 is used.

In FIG. 1 shows a loading curve of a prototype sample at three-point bending.

In FIG. 2 shows a loading curve of a sample made by the proposed method with three-point bending.

The nature of the curve in FIG. 1 indicates brittle fracture of the material. From the loading curve shown in FIG. 2, it is clearly seen that loading is accompanied by deformation, the possible formation of cracks does not lead to destruction of the sample, since their development is quenched at the boundaries of the fibers that make up the composite.

In FIG. 3 shows the surface microstructure and preliminary analysis of the content of elements of the obtained ceramic tube for the shell of the fuel element.

Analysis of the microstructure of the sample indicates a uniform filling of the porous structure with the SiC matrix in the entire volume of the sample.

In FIG. 4 by scanning electron microscopy (SEM) with X-ray microanalysis (EMF), the surface was evaluated and a preliminary analysis of the content of elements in the tubes was obtained. The results of the analysis are shown in table 1.

In FIG. 5 by the method of x-ray phase analysis (XPA) it was found that the ceramic matrix corresponds to the β-SiC modification.

From the analysis it is seen that the tube consists mainly of silicon carbide cubic modification of β-SiC, the carbon content of 45.84% at. and silicon - 54.16% at., while there is no oxygen.

The method is as follows.

Example

The experimental development of the processes of liquid-phase saturation of the experimental samples with silicon carbide was carried out on a special installation.

A sample is loaded into the reaction container — a tubular carcass of fiber with a β-SiC structure, which is formed by winding or weaving the fiber onto a cylindrical carcass of the required size and is installed in an electrical resistance furnace. Before starting the process, a three-time argon purge of the reaction container with the samples loaded into it is carried out. The purge includes 3 successive stages of "pumping (up to 0.001 MPa) - filling with argon (0.1 MPa)." Purge the reaction container at a temperature of 60 ° C. A temperature of 60 ° C is selected to remove moisture from the pores of the tubular frame. Pouring a ceramic-forming precursor, for example, polycarbosilane in a toluene solution (from 30 to 60%) includes: pouring into a container and three successive cycles: "pumping (up to -0.001 MPa) - filling with an inert gas (0.1 MPa)." Cycles are conducted to increase the uniformity of the filling of the porous volume of the tubular casing with a liquid ceramic-forming precursor. The process is carried out by a gradual increase in temperature with a heating rate of 8 deg / h to 350ºC and with exposure at 350ºC for at least one hour for polymerization of polycarbosilane. Further, to obtain a ceramic matrix, heating from 350ºC to 700ºC is continued at a speed of 8 deg / h with holding at 700ºC for 1 h at an inert gas pressure of 0.1 MPa.

The selected heat treatment regimes (temperature, holding time, heating rate, inert gas pressure) of the tubular frame made of fiber with a β-SiC structure are optimal and make it possible to obtain a ceramic matrix of silicon carbide.

To form a cubic modification of β-SiC and obtain a ceramic tube of three or more layers of silicon carbide of a cubic structure after the pyrolysis process, the final high-temperature heat treatment is carried out up to 1800 ° C with an inert gas pressure of 0.1 MPa and with a holding time of at least 2 hours, providing the formation of a ceramic composite.

Thus, the formation of a ceramic composite during the final high-temperature heat treatment eliminates one of the main disadvantages of ceramic products - fragility, by reinforcing SiC fibers to ensure compatibility by thermal expansion coefficient (CTE) with a matrix of SiC.

In addition, the proposed method uses components (fiber with a β-SiC structure and a ceramic-forming precursor that does not contain chlorine), which ensures the ecological purity of the method.

Claims (4)

1. A method of manufacturing a ceramic tube for a shell of a fuel element, comprising layers of silicon carbide, characterized in that they form a tubular frame of fiber with a β-SiC structure, impregnate the tubular frame cyclically at a residual pressure of 0.001 MPa in an inert gas atmosphere, heat treatment is carried out with a step temperature rise to obtain a ceramic matrix and the final high-temperature heat treatment, providing the formation of ceramic composite. 2. The method according to p. 1, characterized in that polycarbosilane is used as a ceramic-forming precursor. 3. The method according to p. 1, characterized in that the heat treatment is carried out by gradually increasing the temperature with a heating rate of 5-20 deg / h to 350 ° C and holding for at least an hour at 350 ° C, from 350 ° C with a heating speed of 5-20 deg / h to 700 ° C and holding for at least an hour at a pressure of 0.1 MPa in an inert gas atmosphere. 4. The method according to p. 1, characterized in that the final high-temperature heat treatment is carried out by heating from 700 ° C to 1800 ° C with an inert gas pressure of 0.1 MPa and with a holding time of at least 2 hours.

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