RU2517436C2 - Method of producing ceramic material for nuclear reactor core melt localising apparatus - Google Patents

Abstract

FIELD: physics, atomic power.

SUBSTANCE: invention relates to a method of producing ceramic sacrificial material for an apparatus for localising nuclear reactor core melt, which involves preparing a mixture containing iron oxide, aluminium oxide, a neutron absorber additive and a sintering activator, grinding and calcining the powder. Combined grinding of aluminium oxide, neutron absorber additive and sintering activator is performed first, followed by further combined grinding of all components of the mixture until achieving grain size of less than 10 mcm.

EFFECT: method enables to obtain material of good quality with lower labour and energy costs.

5 cl, 1 tbl

Description

FIELD OF THE INVENTION

The invention relates to the technology of ceramic materials for nuclear energy, designed to reduce enthalpy, reduction potential and corium density in the event of an out-of-accident accident of VVER-type nuclear reactors and placed for this purpose in a melt localization device. In a severe accident of a nuclear reactor with loss of coolant as a result of uncontrolled heating of the fuel elements, a melt (corium) is formed due to the melting and destruction of the fuel assemblies and internals of the nuclear reactor. Corium as the main constituents contains uranium and zirconium oxides in the oxide phase, as well as uranium, zirconium and metal structures in the metal phase of the melt pool [Asmolov V.G. The concept of severe accident management at WWER nuclear power plants. “Safety issues for WWER nuclear power plants. Investigation of the process during beyond design basis accidents with core destruction. ” Trud. Seminar, St. Petersburg, September 12-14, 2000 St. Petersburg: Ed. AEP, 2000, p.1-22]. After the destruction of the reactor vessel, the molten corium enters the melt localization device, which should ensure its cooling and retention for an unlimited time without serious damage to the containment structures and environmental consequences for the environment.

State of the art

Corium, according to calculations, has a high temperature - up to 3000 K and high chemical activity. There are two fundamentally different concepts for solving this problem. According to the first concept [Fischer M. Main Features of the EPR Melt Retention Concept, OECD Wockshop on Ex-Vessel Debris Coolability. Karlsruhe, Germany, November 15-18, 1999, 10 p.] The melt from the body flows into a storage ring, where it loses some of the heat due to the melting of sacrificial materials, which are used as lithium, sodium, potassium borates; oxides of magnesium, calcium, strontium and barium; phosphates or carbonates of the same elements [US Patent 4,121,970].

According to the second concept [Asmolov VG, Bezlepkin VV, Kukhtevich IV, and others. The concept of localization of the corium melt during the off-stage stage of the beyond design basis accident of nuclear power plants with VVER-1000. “Safety issues for WWER nuclear power plants. Investigation of the process during beyond design basis accidents with core destruction. ” Tr. scientific researcher seminar, St. Petersburg, September 12-14, 2000 St. Petersburg: Publishing House. AEP, 2000, pp. 23-36] during an accident, the melt and fragments of the reactor structure pass through the guiding funnel into the melt localization device, where due to the interaction with the sacrificial material, the corium metal components are oxidized, the melt density decreases, and the melt enthalpy decreases to the level at which by the time the melt exits to the water-cooled walls of the melt localization device, there is no heat exchange crisis. The design of such a melt localization device (ULR) is patented [RF Patent 2165106].

The present invention relates to the field of technology of non-metallic (oxide) materials, currently designated by the term - sacrificial materials, designed to ensure localization of the core melt in the water-cooled reactor vessel during an beyond design basis accident. One of the types of sacrificial materials are ceramic sacrificial materials placed inside the cylindrical container of the melt localization device. In the event of an beyond design basis accident, such ceramic materials should work at high temperatures in contact with the corium, interact with the latter, changing its density, enthalpy and recovery potential.

A number of requirements are imposed on sacrificial materials of a ceramic type for a melt localization device:

1. Sacrificial material located directly in the OHRM should, in any likely accident scenario:

- reduce the enthalpy of corium;

- form unlimited solutions with a melt of the oxide phase of the corium;

- oxidize the most aggressive components of corium - metallic uranium and zirconium without the formation of gaseous reaction products;

- the solidus temperature of the multicomponent oxide melt formed after the interaction of the corium with the sacrificial material should be minimal.

2. The density of sacrificial non-metallic materials should ensure a decrease in the density of the formed oxide melt to values lower than the density of molten steel.

3. Subject to the above requirements, the sacrificial material must also have sufficient compressive strength (at least 20 MPa) to ensure the mechanical reliability of the design of the ULR.

An analysis of the principles of selecting sacrificial material for a melt trap according to the second concept showed that the optimal composition is a mixture of iron and aluminum oxides, which is close in composition to an equimolar mixture [Asmolov VG, Zagryazkin VN, Udalov Yu.P. et al. The choice of the sacrificial material of the trap for holding the melt of the VVER-1000 core. - Atomic energy, 2002, vol. 92, issue 1, pp. 7-18]. Experiments have shown that such a sacrificial material effectively interacts with both the oxide component of the corium and the metal component of the corium (zirconium). To prevent the occurrence of secondary criticality, it is recommended to introduce neutron absorbers into the sacrificial materials - for example, gadolinium oxide GcbCb, which has an anomalously high thermal neutron capture cross section. Varieties of oxide materials are known for use in the considered purposes: for example, ceramic materials based on SiO2, Al2O3, ZrO2, MgO, VO2, ThO2, as well as iron and aluminum oxides [US Patent 5,343,506]. However, the authors of this patent do not specify the quantitative composition of the ceramic material and its characteristics (density, porosity, etc.); there is no justification for the optimal composition in this patent. The material proposed in US Pat. No. 5,343,506 is incapable of solving one of the important functions of the sacrificial material, the oxidation of metallic zirconium and chromium in corium, since it does not contain enough oxides that can easily and in large quantities give oxygen.

Known compositions of ceramic materials based on iron and aluminum oxides, in which various additives are introduced to achieve the necessary density and strength as a result of high-temperature sintering. So in US patent 4,810,290 to provide sintering of iron ore is introduced up to 2 wt.% Calcium oxide CaO. For similar purposes, it is proposed to introduce up to 0.8 wt.% Alkali element silicates [US Pat. No. 6,293,994], or colloidal silica [US Pat. No. 6,384,126], or high alumina cement in an amount of 3 to 30 in iron ore (consisting mainly of magnetite Fe304) wt.% [US patent 6,409,964]. The disadvantages of such materials from the point of view of their use as sacrificial materials for a nuclear reactor melt trap include the fact that their synthesis at high temperatures leads to a partial loss of hematite oxygen with its transition to magnetite (it should be noted that the authors of the above patents sought this goal) and the fact that the density and strength of the ceramic material was not controlled.

A known method of manufacturing a synthetic refractory material based on a mixture of iron oxide and aluminum with a ratio in the initial mixture of Pe ₂ O ₃ : Al2O3 from 30:70 to 60:40 and the addition of magnesium oxide MgO from 20 to 60 wt.% [Canadian patent 2379885]. In this ceramic material, along with a set of properties that partially satisfy the requirements for non-metallic sacrificial materials for HRM (low melting point, solubility in an unlimited range of concentrations in molten corium, the ability to lower the density of corium), there are the following disadvantages: the use of MgO as a sintering activating additive results in a loss of intensity oxide Pe ₂ O ₃ in the burning step transition portion of the oxygen in magnetite Fe ₂ O _4, so as to achieve sufficient strength (ve Jicin it is not specified in the patent) the ceramic material is fired at a temperature from 1350 to 1600 ° C. As a result, the material loses some of its valuable properties (iron oxide Fe ₂ O ₃ decomposes under these conditions to Pe ₃ O ₄ with the release of 3.1% oxygen by weight of iron oxide). In addition, the release of oxygen from Fe ₂ O ₃ at the stage of firing makes sintering difficult, which does not allow a single firing to obtain the required bulk density of the ceramic material. The reason for this behavior of hematite when using a magnesium oxide additive as a sintering activator is as follows. During the diffusion of divalent magnesium ions into the crystal lattice of ferric oxide Fe ₂ O ₃ , magnesium ions are inserted into the sites of iron ions with the formation of oxygen vacancies to compensate for the lack of charge in the cationic sublattice. As a result, a Schottky defect is formed in which the oxygen ion migrates to the surface, where it loses charge and leaves the crystal lattice with the formation of gaseous oxygen. This type of interaction is observed for all monovalent and divalent cations (which could potentially be fusible sintering activators). Cations of trivalent metals cannot be sinter activators, since, as a rule, they have a very high melting point (higher than that of iron oxide). The cations of ferrous and pentavalent metals can, in principle, be sintering activators for ferric oxide, since when they diffuse into the Fe ₂ O ₃ oxide crystal lattice, metal cations replace ferric iron cations with the formation of vacancies in the cationic sublattice to compensate for the excess charge (i.e., Schottky defects due to the departure of iron cations to the surface of the crystal), and the sublattice of oxygen ions remains unchanged and oxygen loss does not occur.

The known compositions of the charge for the manufacture of sacrificial ceramic products from ceramics based on iron and aluminum oxides, in which to activate the sintering of the basic oxides, silicon oxide is introduced in its pure form to 4 wt.% [Patent RU 2178924], or silicon oxide in the composition of kaolin, the content of which in the charge 2.1-2.3 times higher than the specified content of silicon dioxide in the material [patent RU 2206930].

The closest way (prototype) to obtain a sacrificial ceramic material from iron and aluminum oxides is the technical solution according to patent RU 2206930. According to this patent, the manufacture of a sacrificial ceramic material includes, in particular, the following operations: preparation of a mixture of iron and aluminum oxides, silicon dioxide, grinding it with obtaining a press powder, pressing samples of a certain configuration and size, subsequent firing with exposure for 2-14 hours. The calcined product is then subjected to crushing, grinding, fractionation and re-pressing. The final operation is repeated high-temperature firing with exposure for 4-14 hours.

Obviously, the method under consideration, including double high-temperature processing, crushing of the fired briquettes after the first firing, and repeated pressing of the product, is very labor-consuming and energy-consuming, which necessitates the replacement of it more perfect.

SUMMARY OF THE INVENTION

A ceramic material based on aluminum and iron oxides should provide the maximum possible amount of bound oxygen in the form of compounds based on Fe ₂ O ₃ and have a given bulk density (from 3.6 to 4.0 g / cm ³ ) after a single firing. In principle, the following options are possible for solving the problem of obtaining a ceramic material with properties comparable to the material described in patent RU 2206930, using a single high-temperature treatment:

1) the intensification of firing due to the introduction into the mixture of special additives called mineralizers;

2) intensification of firing due to the use of mechanical activation of the mixture.

It is possible that both the first and second options are used at the same time, since they are completely compatible. The effectiveness of the first option was previously shown, as described in Eurasian patent No. 003961. The present invention uses a combined sintering intensification method.

To this end, in a method of manufacturing a ceramic sacrificial material for a device for localizing a melt of a nuclear reactor, including preparing a mixture containing components of iron oxide, aluminum oxide, an additive of a neutron absorber and an activator of sintering, grinding and firing of the powder, the aluminum oxide, additives of the neutron absorber are co-milled and sintering activator, and then additional joint grinding of all components of the mixture to achieve a powder grain size of less than 10 microns.

Thus, given the large difference in grinding ability of iron and aluminum oxides, solid alumina is initially ground together with a sintering activator. Iron oxide is added when performing additional grinding of the mixture. This streamlines the technology. Also, the selected composition and preparation procedure of the charge increases the residual amount of reactive oxygen in iron oxide and activates sintering of oxides at relatively low sintering temperatures. The effectiveness of the proposed method is determined by the fact that finely ground alumina and silica from the sintering activator, when heated already at 1200 ° C, begins to form fine-grained mullite, which, on the one hand, prevents the possibility of the formation of hercinitis (compounds FeAl ₂ O ₄ with a low oxygen content compared with hematite Fe ₂ O ₃ ), and on the other hand, increases the strength of ceramics.

In a specific embodiment of the method, the mixture contains iron oxide 62-75 wt.%, Alumina 25-38 wt.%, The addition of gadolinium oxide in an amount of up to 0.15 wt.% As a neutron absorber and up to 5 wt.% Over the content of components sintering activator in the form of silicon oxide in pure form or as part of bentonite clay. Bentonite clay plays the role of both a binder and a plasticizer in the briquetting of samples, and a mineralizer - an activator of sintering.

In another particular case, the grinding of aluminum oxide, the addition of a neutron absorber and a sintering activator is carried out until a powder grain size of less than 10 microns is achieved.

In another particular case, the firing is carried out at a temperature of 1320-1350 ° C for 3-6 hours.

In another particular case, the ceramic material after a single firing has a density of 3.6-4.0 g / cm ³ and compressive strength of at least 20 MPa.

The proposed method was implemented on known technological equipment for vibration grinding using a vibration mill and firing in a furnace in an air atmosphere. The results in the form of examples are summarized in table. As can be seen from the table, the material obtained by the proposed method has characteristics quite comparable with materials of double firing both in density and in compressive strength.

Claims (5)

1. A method of manufacturing a ceramic sacrificial material for a device for localizing a melt of a nuclear reactor, including the preparation of a mixture containing components of iron oxide, alumina, an additive of a neutron absorber and an activator of sintering, grinding and firing of powder, characterized in that the aluminum oxide and additives are co-milled initially a neutron absorber and a sintering activator, and then an additional joint grinding of all components of the mixture to achieve a powder grain size of less than 10 microns. 2. The method according to claim 1, characterized in that the mixture contains iron oxide 62-75 wt.%, Alumina 25-38 wt.%, The addition of gadolinium oxide in an amount of up to 0.15 wt.% As a neutron absorber and up to 5 wt.% In excess of the content of sinter activator components in the form of silicon oxide or bentonite clay. 3. The method according to claim 1, characterized in that the grinding of aluminum oxide, the addition of a neutron absorber and a sintering activator is carried out until a powder grain size of less than 10 microns is achieved. 4. The method according to claim 1, characterized in that the firing is carried out at a temperature of 1320-1350 ° C for 3-6 hours. 5. The method according to claim 1, characterized in that the ceramic material after a single firing has a density of 3.6-4.0 g / cm ³ and compressive strength of at least 20 MPa.