RU2242814C1 - Method for recovering reactor graphite waste - Google Patents

Abstract

FIELD: recovery of high-activity wastes.

SUBSTANCE: proposed method includes crushing of reactor graphite waste and its separation into several fractions, that is fine fraction having particle size of 50 μm and below and coarse fraction with particles measuring 50 to 100 μm. Then fine and coarse fractions of crushed reactor graphite waste are mixed up, mass proportion of fine and coarse fractions being 95 : 5 to 10 : 90. Introduced in mixture of crushed reactor graphite waste are rutile concentrate as oxide of carbide-forming element, powdered aluminum as energy carrier, and modifier, proportion of ingredients being as follows, mass percent: graphite waste, 7 - 11; powdered aluminum, 23 - 30; rutile concentrate, 53 - 64; modifier, 2 - 15. Then mixture prepared in this way is charged in container and igniter composition layer is placed on top. Mixture is given heat treatment under self-spreading high-temperature fusion conditions to form carbide oxide material including radionuclides. Prior to charging mixture inner surfaces of container walls and its bottom are covered with asbestos sheets. End product formed within container is allowed to cool down under natural conditions.

EFFECT: prevented escape of radionuclides into environment; enhanced strength of end product obtained.

4 cl, 1 dwg, 4 tbl, 3 ex

Description

The invention relates to the field of nuclear energy, in particular to a technology for the processing of high-level waste of irradiated graphite generated during operation of uranium-graphite reactors. Such wastes include a number of radionuclides, including the long-lived carbon isotope ¹⁴ C, formed under the influence of graphite, and impurities of the neutron flux contained in it.

Currently, in a number of countries (in Russia, Great Britain, the USA, France, Germany, and others), resource-using uranium-graphite reactors are beginning to be decommissioned. Over the years of operation, as a result of various anomalous situations and technological operations, a number of defects (burrs, chips, shells, pores, etc.) were formed in the graphite blocks of these reactors, and the resulting graphite chips crumbled. On the surface of graphite blocks, a loose crumbling layer forms over time. When the fuel elements were depressurized, particles of nuclear fuel were shed. As a result, so-called “spills” were formed in which transuranic - neptunium, plutonium, americium, curium - and other radionuclides were formed as a result of a long stay in the reactor core: carbon - 14, cesium - 137, strontium - 90, calcium - 41 nickel - 59, zirconium - 93, iron - 55, etc.

Graphite wastes from uranium-graphite reactors also include periodically replaced graphite bushings, in which radionuclides were formed as a result of neutron irradiation, including carbon - 14.

The carbon isotope ¹⁴ C is long-lived (with a half-life of 5730 years) and, oxidizing to ¹⁴ CO ₂ , is included in the natural carbon cycle, which therefore poses a serious danger to humans due to the ability to be absorbed by the body.

In this regard, when decommissioning uranium-graphite reactors, the task is to process graphite waste with reliable isolation of the long-lived carbon isotopes ¹⁴ C and other radionuclides contained in them from the environment.

A known method of processing a carbon-containing material [I] in the form of particles in which carbon is present as free or atomic carbon and which is contaminated with toxic elements, including waste graphite from a nuclear reactor, comprising loading the carbon-containing material into an upper immersed blast reactor and melting said material in said reactor in the presence of liquid slag; injecting, during the melting, oxygen-containing gas into the slag through an upper immersion tube to compress burning essentially all of the carbon contained in the carbon-containing material, removing volatile toxic elements with reactor exhaust gases in the form of smoke, introducing substantially completely non-volatile toxic elements into the slag, the process being carried out at a temperature of 1100 to 1400 ° C, and the slag is silicon anhydride slag, including iron oxide and at least one of the other oxides selected from aluminum oxide, calcium oxide and magnesium oxide, and iron oxide performs the function of an oxygen carrier in the slag, sp special burning of carbon contained in the carbon-containing material by reactions:

2FeO (slag) + 1 / 2O ₂ = 2FeO _1.5 (slag),

2FeO _1.5 (slag) + С = 2FeO (slag) + СО,

and these reactions are supported by the turbulent movement of the slag generated by the upper submerged injection of oxygen-containing gas.

The disadvantages of this method are:

- the complexity of the technological process for the processing of carbon-containing material, including graphite waste from nuclear reactors, due to the process in the reactor with slag melt, and oxygen-containing gas is blown into the liquid slag through tubes immersed into it from above, which is necessary to support the combustion of carbon contained in the carbon-containing material;

- transfer of all carbon contained in the carbon-containing material, including graphite waste from nuclear reactors, and hence the long-lived carbon isotope ¹⁴ C, into the gas phase in the form of carbon monoxide CO and ¹⁴ CO, which requires special additional measures to exclude the yield carbon monoxide ¹⁴ СО into the environment and its conversion into a form suitable for long-term safe disposal.

A known method of processing solid highly active graphite-containing wastes [2], including crushing the wastes, additionally introducing titanium and / or aluminum as an energy carrier and oxide of the carbide-forming element into these wastes, the components of the mixture being taken in an amount sufficient to completely bind the graphite, the prepared mixture is compacted and placed in a steel mold or gas thermostat, heat treatment of the mixture is carried out in a sealed reactor in the mode of self-propagating high-temperature synthesis (SHS) put This involves initiating the combustion reaction of the components of the mixture by applying a short-term current pulse through a tungsten spiral in contact with the mixture, followed by compacting the synthesis product during or after burning the mixture to form a high-density product for environmentally safe disposal.

The disadvantages of the method are the complexity in the technological design due to the operation of compaction of the prepared mixture, as well as the dangerous stage of compaction of the final product in a sealed reactor during the SHS process or after its completion at temperatures above 2000 K using powerful press equipment, high energy consumption associated with the use of press equipment, insufficiently reliable ignition when applying a short-term current pulse through a tungsten spiral in contact with the mixture.

Closest to the claimed method is a method of processing reactor graphite waste [3], including grinding reactor graphite waste, introducing powdered aluminum as a carrier energy into the crushed reactor graphite waste, titanium dioxide or dichrome trioxide as a carbide-forming element and zircon as a modifier , aluminum, copper oxide or nickel oxide, while the components of the mixture are taken in the following ratio, wt.%:

graphite waste 5-12

powdered aluminum 25-40

titanium dioxide or dichromium trioxide 50-66

modifier 2-15

placing the prepared mixture in a container, on top of the mixture in the container, place a layer of igniter composition covering at least half of its open surface, and the igniter composition has a combustion temperature of at least 2500 K and does not emit gaseous products when burning, heat treatment of the mixture in the container placed in an inert atmosphere reactor in the SHS mode by initiating the combustion reaction of the mixture components with the formation of carbide oxide material including radionuclides s designed for environmentally safe disposal.

The disadvantages of this method are:

- high heat loss in the contact zone of the reacting mixture with the walls of the container, as a result of which the reaction in this zone does not proceed completely, not all graphite on the surface of the final product is bound in the carbide oxide material, due to which partial shedding of graphite from the surface of the resulting block of carbide oxide material occurs, which can lead to the ingress of graphite with the radionuclides contained in it, including the carbon isotope ¹⁴ C, into the environment;

- insufficient strength of the resulting carbide oxide material;

- significant ablation of ¹³⁷ Cs and / or ⁹⁰ Sr radionuclides during SHS;

- high leaching rate of ¹³⁷ Cs and / or ⁹⁰ Sr radionuclides from the resulting carbide oxide material;

- the high cost of the components used, primarily titanium dioxide and zircon.

The technical result that can be achieved by implementing the proposed method is to prevent the carbon isotope ¹⁴ C and other radionuclides contained in reactor graphite from entering the environment, increasing the strength of the resulting carbide oxide final product; reduced entrainment of ¹³⁷ Cs and / or ⁹⁰ Sr radionuclides during SHS; a decrease in the leaching rate of ¹³ Cs and / or ⁹⁰ Sr radionuclides from the resulting carbide oxide end product; reduction in the cost of the components used.

To achieve a technical result, a method for processing reactor graphite waste is proposed, including crushing reactor graphite waste, separating the crushed reactor graphite waste to be heat treated into several fractions, at least two fractions - with particle sizes of 50 μm or less (hereinafter referred to as fine fraction) and with particle sizes from 50 to 100 μm (hereinafter referred to as the coarse fraction), combining the fine and coarse fractions of crushed reactor graphite waste into a mixture with a ratio of sce between fine and coarse fractions from 95: 5 to 10:90; the introduction of powdered aluminum into the mixture of fine and coarse fractions of crushed waste from reactor graphite as an energy carrier, rutile concentrate as an oxide of a carbide-forming element, and as a modifier - substances that decompose and / or melt at the temperature of heat treatment of the prepared mixture and do not react with other the components of the mixture, in the following ratio, wt.%:

graphite waste 7-11

powdered aluminum 23-30

rutile concentrate 53-64

modifier 2-15

drawing on the inner surface of the walls and the bottom of the container in which the mixture is heat treated before loading the above-mentioned prepared mixture, asbestos sheet, loading the prepared mixture into the container, placing on top of the mixture in the container, a layer of igniter composition covering at least half of it an open surface, and the igniter composition has a combustion temperature of at least 2500 K and does not emit gaseous products during combustion, heat treatment of the mixture in it is an onteiner in the mode of self-propagating high-temperature synthesis by initiating the combustion reaction of the mixture in the container with the formation of a carbide oxide material, including radionuclides, trapping aerosols released during the combustion of the mixture in the container, and cooling of the final product formed naturally in the container. As a modifier, quartz sand, barium metatitanate, calcium metasilicate (wollastonite) are used.

Distinctive features of the proposed method are:

- separation of the ground waste of irradiated reactor graphite to be heat treated into several fractions, at least two fractions - with particle sizes of 50 μm or less (hereinafter referred to as the fine fraction) and with particle sizes from 50 to 100 μm (hereinafter referred to as coarse fraction), combining small and large fractions of crushed reactor graphite waste into a mixture with a weight ratio between small and large fractions from 95: 5 to 10:90; the introduction of a mixture of small and large fractions of crushed waste reactor graphite of rutile concentrate as oxide of a carbide-forming element in the following ratio of components, wt.%:

graphite waste 7-11

powdered aluminum 23-30

rutile concentrate 53-64

modifier 2-15

- drawing on the inner surface of the walls and the bottom of the container in which the mixture is heat treated, asbestos sheet before loading the aforementioned prepared mixture into it.

Additional distinctive features of the proposed method are the use of quartz sand, barium metatitanate, calcium metasilicate (wollastonite) as a modifier in the mixture.

As studies have shown, by separating the crushed waste of reactor graphite into several at least two fractions, fine, with particle sizes of 50 microns or less, and large, with particle sizes of 50 to 100 microns, and changes in the ratio between fine and coarse fractions in the crushed graphite waste introduced into the mixture result in a carbide-corundum end product with different strengths (table 1).

According to the data given in table 1, the strength of the final product can be controlled over a fairly wide range by changing the ratio between the fine and coarse fractions in the crushed reactor graphite waste to be processed, and the introduction of rutile concentrate graphite as an oxide of a carbide-forming element into the waste recycling mixture gives a significant increase in the strength of the final product.

The rutile concentrate introduced into the mixture for thermal processing of reactor graphite waste in the SHS mode as the oxide of the carbide-forming element is enriched natural ore of the following chemical composition, wt.%: TiO ₂ - not less than 94, Al ₂ O ₃ - not more than 0.6, Fe ₂ O ₃ - not more than 3, SiO ₂ - not more than 1.5, ZrO ₂ - not more than 1.0, P ₂ O ₅ - not more than 0.07, moisture - not more than 0.5 [4]. The titanium dioxide TiO ₂ included in the composition of the rutile concentrate acts as the oxide of the carbide-forming element. The moisture contained in the rutile concentrate evaporates, only partially taking part in the combustion reaction. Zirconium dioxide ZrO ₂ has the ability to contain radionuclides isomorphically, thereby improving the containing properties of the final product. The oxides Al ₂ O ₃ and Fe ₂ O ₃ at the SHS reaction temperature form spinels, which also have host properties for a number of radionuclides. Phosphorus pentoxide P ₂ O ₅ forms stable phosphates of elements - radionuclides. Silicon dioxide SiO ₂ melts due to the heat of the combustion reaction, interacts with elements of impurities contained in graphite waste, forms a silicate melt, which includes these impurities in the glass phase and fills the porous space in the carbide-corundum matrix, giving the final product glass-ceramic properties with increased strength.

Thus, the oxides of aluminum, iron, silicon, zirconium, and phosphorus contained in the rutile concentrate improve the properties of the carbide-alumina matrix formed during the SHS process: they increase the retention capacity with respect to radionuclides (the entrainment of ¹³⁷ Cs and ⁹⁰ Sr decreases by about 2 times while maintaining the leaching rate by the level of indicators of a mixture with titanium dioxide TiO ₂ ), reduce porosity, increase the strength of the final product in comparison with the carbide-corundum material formed when carbide form is used as oxide element of pure titanium dioxide TiO ₂ (see tables 1 and 2).

Reducing the leaching rate of ¹³⁷ Cs and ⁹⁰ Sr in the case of mixtures based on rutile concentrate is achieved by introducing modifiers into the reactor waste treatment mixture.

Rutile concentrate not only improves the target characteristics of the final product, but it is also approximately 5 times cheaper than titanium dioxide: the price of titanium dioxide is 97,000 rubles / ton, the price of rutile concentrate is 18,000 rubles / ton.

When the content of crushed reactor graphite waste in the initial mixture is less than 7%, the necessary energy of the SHS process is not provided due to insufficient heat generation during the formation of titanium carbide TiC, when more than 11% of graphite waste is introduced into the initial mixture, not all carbon, including the carbon isotope ¹⁴ C , will bind to stable titanium carbide, while carbon ¹⁴ C is not excluded in the environment.

A content of aluminum powder in the initial mixture of less than 23% will not provide the necessary energy release, complete reduction of titanium from titanium dioxide contained in rutile concentrate, complete carbon binding in titanium carbide, and, thus, SHS process. As a result of the incomplete binding of carbon in titanium carbide, the carbon isotope ¹⁴ C is not excluded from the environment. The content of aluminum powder in the initial mixture of more than 30% is excessive and will negatively affect the energy of the SHS process.

If the content of rutile concentrate in the initial mixture is less than 53%, the reduced titanium will not be enough for the complete binding of carbon, including the carbon isotope ¹⁴ C, to titanium carbide TiC. A content of more than 64% in the initial mixture for processing waste of reactor graphite of rutile concentrate is more than 64%, which leads to the formation of lower titanium oxides in the final product, a decrease in the reaction temperature, and, as a result, a decrease in the strength of the final product. Moreover, as a result of incomplete binding of carbon in titanium carbide, reliable isolation of the carbon isotope ¹⁴ C from the environment is not ensured.

When the content of the modifier reactor graphite in the initial mixture for waste treatment is less than 2%, the necessary increase in strength, decrease in porosity, and improvement in the enclosing properties of the final product are not provided. When the modifier in the initial mixture contains more than 15%, its melting requires a large heat consumption, as a result of which the temperature decreases, the completeness of the SHS reaction is not achieved, the carbide formation process does not end, not all graphite (carbon) binds to titanium carbide and, therefore, cannot be excluded carbon - 14 into the environment.

The silica sand introduced into the mixture for processing crushed waste of reactor graphite as a modifier has the following composition (wt.%): Silicon dioxide SiO ₂ from 99.8 to 98.5, aluminum oxide Al ₂ O ₃ from 0.1 to 0, 6, iron oxide Fe ₂ O ₃ from 0.01 to 0.05 [5]. Due to the heat of the chemical reaction, silicon dioxide and other oxides contained in quartz sand melt, forming spinel and a glass phase, which fills the pores in the carbide-corundum matrix of the final product, thereby increasing its strength by almost 2 times (see Table 3), and improves the enclosing properties with respect to radionuclides contained in graphite waste and smelted during SHS. So, while maintaining the entrainment value of ¹³⁷ Cs and ⁹⁰ Sr at about the level of indicators for a mixture without the addition of a modifier and for a mixture according to the prototype, the leaching rates of the same radionuclides are reduced by 2.7-3 times (Table 4), which provides more reliable isolation from environmental radionuclides ¹³⁷ Cs and ⁹⁰ Sr.

Barium metatitanate introduced into the mixture for processing crushed waste of reactor graphite as a modifier has a melting point of 1888 K and does not decompose upon melting [6]. Barium metatitanate BaTiO ₃ at the reaction temperature during SHS melts and fills the pore space in the carbide oxide matrix of the final product. The strength of the resulting carbide oxide end product of the processing of graphite waste increases slightly (see Table 3). The ablation of ¹³⁷ Cs and ⁹⁰ Sr radionuclides in the SHS process remains approximately at the level of indicators for the prototype mixture. However, the leaching rate of ¹³⁷ Cs and ⁹⁰ Sr is three times lower than the final product synthesized using the prototype mixture (Table 4), which significantly increases the reliability of isolation of these radionuclides from the environment.

The calcium metasilicate CaSiO ₃ (wollastonite) introduced into the mixture for processing crushed waste of reactor graphite as a modifier has a melting point of 1817 K and does not decompose upon melting [6]. Wollastonite at the temperature of the SHS process due to the heat of the exothermic reaction melts and fills the pore space in the carbide oxide matrix of the final product. At the same time, the strength of the resulting carbide oxide end product of graphite waste processing increases by more than 2.5 times (Table 3). For ¹³⁷ Cs and ⁹⁰ Sr radionuclides, in comparison with the prototype mixture, the ablation is reduced by approximately 1.3 times and the leaching rate by 2-3 times (Table 4), resulting in a more reliable isolation of the radionuclides from the environment .

In studies of the proposed method for processing crushed reactor graphite wastes mixed with powdered aluminum, rutile concentrate and a modifier loaded into a metal container, it was found that on the surface of the resulting carbide-corundum block of the final product directly in contact with the container walls, a layer of unreacted mixture remains, which manifests susceptibility to shedding even with minor mechanical influences, which can lead to environmental exposure th medium graphite particles with carbon contained therein ^14C and other radionuclides. The thickness of this layer can reach approximately 2-3 mm.

As shown by measurements made using tungsten-rhenium thermocouples, the temperature directly at the boundary between the reacting mixture and the inner surface of the container wall during the SHS process is ~ 1400 K at a temperature in the volume of the reacting mixture of 2327 K (see Fig. 1). Thus, in a layer of a reacting mixture with a thickness of 2-3 mm, the average temperature is ~ 1850 K, which is due to intense heat removal through the wall of the container to the environment, as a result of which there are no conditions in this layer for the SHS reaction to completely bind carbon, including its isotope ¹⁴ C, in titanium carbide. Obviously, to ensure the completeness of the SHS reaction, it is necessary to sharply reduce the heat removal from the mixture layer directly adjacent to the inner surface of the container wall. In the proposed method for the processing of reactor graphite waste for this purpose, sheet asbestos is applied to the inner side surface and the bottom of the container.

Asbestos is a good heat insulator and can dramatically reduce the heat flux from the volume of the reacting mixture through the wall of the container into the environment, i.e. reduce heat loss. As a result, as measurements using tungsten-rhenium thermocouples show, in the zone of contact of the reaction mixture with asbestos, the temperature is ~ 2250 K (see Fig. 1), and the average temperature in the layer of the reaction mixture directly adjacent to the asbestos coating of the container wall, is ~ 2290 K. Thus, in the reactive mixture of graphite wastes with powdered aluminum, rutile concentrate and a modifier in the zone of its contact with the asbestos coating of the container wall, the temperature conditions guarantee the completeness of the process sa SHS and, therefore, the completeness of incorporation of carbon titanium (graphite) into the carbide, including the carbon isotope ¹⁴ C. This ensures reliable fixation of carbon ¹⁴ C and other radionuclides contained in the waste of reactor graphite in the volume resulting from the SHS process carbide-corundum matrix block, which eliminates their ingress into the environment.

In the proposed method for the treatment of reactor graphite waste, asbestos not only serves as a heat insulator, providing a reduction in heat loss from the volume of the reacting mixture, but also improves the surface properties of the synthesized carbide-corundum final product, which is due to the chemical composition of asbestos. Chemically, all types of asbestos are silicates of magnesium, calcium, iron, and sodium of a rather complex composition [7]. The content of silicon dioxide in them is 50-60%. During the SHS process in a mixture of reactor graphite wastes with powdered aluminum, rutile concentrate and a modifier in the container, the surface of the asbestos layer in contact with the reacting mixture, as noted above, is heated to ~ 2200 K. At this temperature, asbestos decomposes into oxides with the formation of a glassy melt into silica based. The formed melt, being in contact with the surface of the porous carbide-alumina matrix block of the final product, penetrates into its pores and covers its surface, forming a glass-ceramic protective layer on the surface, additionally isolating the carbon isotope ¹⁴ C and other radionuclides from the environment.

The drawing shows schematically in vertical section a part of a container loaded with a mixture of 1 reactor graphite waste with powdered aluminum, rutile concentrate and a modifier, moreover, the metal wall 2 of the container does not have a layer of heat insulator (left side of the section), and the part of the container with the aforementioned mixture 1, on the inner surface of the side wall 2 of which, according to the proposed method, applied asbestos sheet 3 (the right side of the cut). The radius is plotted on the abscissa, and the temperature is plotted on the ordinate (the axis of symmetry). For illustrative purposes, the proportions and scales in the diagram are not respected.

The following notation is used in the drawing: R - radial coordinate; R _m is the radius of the cylindrical cavity of the container filled with the mixture; N _m - the wall thickness of the container; N _cm - the thickness of the layer of the mixture directly adjacent to the wall of the container, with intense heat sink; R _a is the radius of the filled cylindrical cavity of the container with the sheet asbestos applied to its inner surface; N _a - the thickness of the layer of sheet asbestos deposited on the inner surface of the container; N _sa - the thickness of the layer of the mixture directly adjacent to asbestos deposited on the inner surface of the container wall; T is the temperature; T _p - temperature in the reacting mixture during the SHS process; T _VA - temperature on the surface of asbestos in contact with the reacting mixture; T _int - temperature on the inner surface of the container, not protected by asbestos; T _cm - temperature on the outer surface of the container; T _on - temperature at the interface between the container wall and sheet asbestos applied to it; T _mn - temperature on the outer surface of the metal container.

In the case of using reactor graphite container for waste processing without a sheet of asbestos applied to its inner surface (the left part from the vertical axis), the values indicated on the drawing have the following values: temperature T _p = 2320-2330 K; temperature on the inner surface of the wall of the container T _VM = 1300-1500 K; temperature on the outer surface of the container wall T _nm = 500-550 K; container wall thickness N _m = 3 mm; the thickness of the layer of the mixture directly adjacent to the wall of the container N _cm = 2-3 mm Thus, the temperature in the mixture layer adjacent to the container wall decreases from the reaction temperature of 2325 K to the temperature of the inner surface of the container wall ~ 1400 K, and the average temperature in this layer is ~ 1850 K, which is completely insufficient for the completeness of the SHS reaction.

In the case of using reactor graphite waste container for processing waste, a sheet of asbestos (the right part from the vertical axis) is applied to the walls and bottom of the container, the values indicated on the drawing have the following values: temperature T _p = 2320-2330 K; the temperature on the inner surface of the sheet asbestos layer T _wa = 2220-2270 K; the surface temperature between the section of the container wall and coated with a sheet of asbestos _on T = 550-600 C. Thus, the temperature in the layer of mixture adjacent the Asbestos coating applied to the internal surface of the container wall, the temperature decreases from 2325 K the reaction to an internal temperature the surface of the asbestos coating is ~ 2250 K, and the average temperature in this layer is ~ 2290 K, which ensures the completeness of the SHS reaction.

The recycling process of reactor graphite waste according to the proposed method is as follows.

The crushed waste of reactor graphite is divided into several at least two fractions: fine, with particle sizes of 50 microns or less, and large, with particle sizes of 50 to 100 microns. Combine small and large fractions of crushed graphite waste in a certain ratio by weight between fractions. Powdered aluminum, rutile concentrate and a modifier are introduced into the prepared mixture of ground graphite wastes in the required amounts. Sheet asbestos is applied to the inner surface of the walls and the bottom of the container for processing reactor graphite waste. The prepared mixture of reactor graphite waste, powdered aluminum, rutile concentrate and a modifier is loaded into a container. On top of the mixture loaded into the container, have an igniter (termite) composition, which serves to initiate the SHS process in the mixture.

The igniter (termite) composition is ignited using a hot nichrome or tungsten wire introduced into its layer. Upon combustion of the thermite composition, condensed products are formed with a temperature above 2700 K, under the action of which a melt of aluminum and rutile concentrate is formed directly from the mixture of reactor graphite, powdered aluminum, rutile concentrate and modifier, and a chemical interaction begins in this melt, initiating the SHS process in the entire volume of the mixture loaded into the container.

As a result of the exothermic reaction during SHS, a sintered carbide oxide porous material is formed in the container in the form of a block of the final product, which consists mainly of titanium carbide TiC with ¹⁴ C carbon isotope and Al ₂ O ₃ alumina (corundum) included in its composition. The impurities of oxides included in the composition of the rutile concentrate fill the pore space in the matrix of the final product, form compounds such as spinels, phosphates and silicates, incorporating radionuclides and their compounds in their structure, which improve the containing properties of the final product and give additional strength to the final product. A similar role is played by silica sand, barium metatitanate BaTiO ₃ and calcium metasilicate CaSiO ₃ (wollastonite) introduced into the mixture for processing reactor graphite waste as modifiers.

Due to the fact that asbestos deposited on the inner surface of the walls and the bottom of the container is a good heat insulator, there is practically no heat loss in the layer of the reaction mixture directly adjacent to the surface of asbestos, as a result of which the reaction proceeds with the complete inclusion of carbon (graphite), including carbon isotope ¹⁴ C, in the composition of the resulting titanium carbide. In addition, due to the heat of the exothermic reaction during SHS, the asbestos mineral substance decomposes with the formation of a silicate melt, which is embedded in the surface layer of the carbide-corundum block of the final product formed in the SHS container, giving this layer glass-ceramic properties, which leads to an increase in the strength of the block in as a whole, and also contributes to the retention of radionuclides in the carbide-corundum matrix and prevents their entry into the environment.

Aerosol particles formed during the SHS process in the reaction zone and carried away from it are captured from the exhaust gases using filters.

The proposed method for processing reactor graphite waste is implemented as follows.

Example 1. A portion of a mixture weighing 10 kg is prepared from crushed reactor graphite waste 9.5 wt.%, Divided into two fractions: fine, with particle sizes less than 50 microns, and large, with particle sizes from 50 to 100 microns, with a ratio between fine and large fractions by weight of 50:50, powdered aluminum 27.5 wt.%, powder of rutile concentrate 57 wt.%, quartz sand 6 wt.%. A 6 mm thick layer of sheet asbestos is applied to the inner surface of the walls and the bottom of the container intended for processing the prepared mixture. The prepared mixture is loaded into the container with asbestos deposited on its inner surface; a layer of termite composition covering at least half of the open surface of the mixture is placed on top of the mixture loaded into the container. The container with the mixture loaded into it and the termite composition placed on its open surface is installed under an exhaust hood with filters for trapping aerosol particles, after which the thermite composition is ignited, by which the SHS process is initiated in the mixture.

At the end of the SHS process, the final product is formed in the container, which is a durable sintered carbide-corundum material with carbon chemically bonded in the form of titanium carbide, including the ¹⁴ C long-lived carbon isotope, with a porous space filled with spinels, phosphates and silicates, which have host properties for a number of of radionuclides, as well as silicon dioxide SiO ₂ , including impurities of radionuclides contained in the waste of reactor graphite in the glass phase and imparting properties of the stack to the final product Lokeramics. The outer layer of the final product formed in the container is impregnated with silicate melt formed from asbestos during thermal exposure to exothermic reactions during the SHS process, due to which this layer acquires the properties of glass ceramics and gives the entire block of the final product additional strength.

The compressive strength of the obtained product is ~ 22 MPa, the entrainment of ¹³⁷ Cs and ⁹⁰ Sr is 4.2 and 3.2%, respectively, the leaching rates for ¹³⁷ Cs and ⁹⁰ Sr are ~ 10 ^-4 and ~ 5 · 10 ^-5 g / (cm ³ · day), respectively.

The container with the final product formed in it as a result of the SHS reaction is cooled due to a natural decrease in temperature, sealed with a lid and sent for burial.

Example 2. Prepare a portion of the reaction mixture weighing 5 kg from the crushed waste of reactor graphite 9 wt.%, Divided into two fractions: fine, with particle sizes less than 50 microns, and large, with particle sizes from 50 to 100 microns, with a ratio between fine and coarse fractions by weight 60:40, powdered aluminum 28 wt.%, curb concentrate 59 wt.%, barium metatitanate 4 wt.%. A layer of sheet asbestos with a thickness of 4 mm is applied to the inner surface of the walls and the bottom of the container intended for processing the prepared mixture. The prepared mixture is loaded into the container with asbestos deposited on its inner surface; a layer of termite composition covering at least half of the open surface of the mixture is placed on top of the mixture loaded into the container. The container with the mixture loaded into it and termite composition placed on its open surface is placed in an inert atmosphere (argon) reactor equipped with filters for trapping aerosol particles, after which the thermite composition is ignited, by which the SHS process is initiated in the mixture.

At the end of the SHS process, a final product is formed in the container, which is a durable sintered carbide-corundum material with carbon chemically bonded in the form of titanium carbide, including a ¹⁴ C long-lived carbon isotope, with a porous space filled with spinels, phosphates, and silicates, which have host properties for a number of radionuclides. In the porous space of the carbide-alumina material of the final product, barium metatitanate is also included, which, due to its perovskite structure, contains radionuclide impurities in the waste of reactor graphite. The outer layer of the final product formed in the container is impregnated with silicate melt formed from asbestos during thermal exposure to exothermic reactions during the SHS process, due to which this layer acquires the properties of glass ceramics and gives the entire block of the final product additional strength.

The compressive strength of the obtained product is ~ 19 MPa, the entrainment of ¹³⁷ Cs and ⁹⁰ Sr is 3.8 and 2.7%, respectively, the leaching rate for ¹³⁷ Cs and ⁹⁰ Sr is ~ 5.6 · 10 ^-5 and ~ 2.7 · 10 ^-5 g / (cm ³ · day), respectively.

The container with the final product formed in it as a result of the SHS reaction is cooled due to a natural decrease in temperature, sealed with a lid and sent for burial.

Example 3. Prepare a portion of the reaction mixture weighing 8 kg from the crushed waste of reactor graphite 8 wt.%, Divided into two fractions: fine, with particle sizes less than 50 microns, and large, with particle sizes from 50 to 100 microns, with a ratio between fine and coarse fractions by weight 40:60, powdered aluminum 26 wt.%, rutile concentrate 58 wt.%, calcium metasilicate (wollastonite) 8 wt.%. A layer of sheet asbestos with a thickness of 5 mm is applied to the inner surface of the walls and the bottom of the container intended for processing the prepared mixture. Further, everything is as in example 2.

At the end of the SHS process, the final product is formed in the container, which is a durable sintered carbide-corundum material with carbon chemically bonded in the form of titanium carbide, including the ¹⁴ C long-lived carbon isotope, with a porous space filled with spinels, phosphates and silicates, which have host properties for a number of radionuclides. In the porous space of the carbide-corundum material of the final product, calcium metasilicate is also included, including radionuclide impurities contained in the waste of reactor graphite, as well as giving the whole material high strength. The outer layer of the final product formed in the container is impregnated with silicate melt formed from asbestos during thermal exposure to exothermic reactions during the SHS process, due to which this layer acquires the properties of glass ceramics and gives the entire block of the final product additional strength.

The compressive strength of the obtained product is ~ 32 MPa, the ablation of ¹³⁷ Cs and ⁹⁰ Sr is 2.8 and 2.1%, respectively, the leaching rates for ¹³⁷ Cs and ⁹⁰ Sr are ~ 1.2 · 10 ^-4 and ~ 0.5 · 10 ^-5 g / (cm ³ · day), respectively.

The container with the final product formed in it as a result of the SHS reaction is cooled due to a natural decrease in temperature, sealed with a lid and sent for burial.

Thus, the proposed method allows to prevent the isotope of carbon ¹⁴ C and other radionuclides contained in reactor graphite in the environment, to increase the strength of the resulting carbide oxide final product; to reduce the entrainment of ¹³⁷ Cs and / or ⁹⁰ Sr radionuclides during SHS; reduce the leaching rate of ¹³⁷ Cs and / or ⁹⁰ Sr radionuclides from the resulting carbide oxide end product and reduce the cost of the components used.

The inventive method can be used for processing waste irradiated reactor graphite containing a long-lived carbon isotope ¹⁴ C and other radionuclides.

To implement the method, materials and components that are widely used in the domestic industry can be used: sheet asbestos - in the power industry, metallurgy and chemical industry, aluminum - in metallurgy, electrical engineering, aircraft, automobile- and machine-building and pyrotechnics, rutile concentrate - in metallurgy and electric welding, quartz sand - in the production of glass and ceramics, in metallurgy, barium metatitanate - in the production of special ceramic materials, calcium metasilicate (wollastonite) - in metallurgists In the manufacture of ceramics, refractory materials and building materials in the paint industry.

In the State Unitary Enterprise of Moscow - MosNPO Radon, research is currently underway to refine the technology for processing reactor graphite waste according to the proposed method.

Sources of information

1. Patent of the Russian Federation No. 2141076 C1, 6 F 23 G 5/00, 09 V 3/30, publ. 11/10/99, BI No. 31.

2. Patent of the Russian Federation No. 2065220 C1, 6 G 21 F 9/32, publ. 08/10/96, BI No. 22.

3. Patent of the Russian Federation No. 2192057 C1, 7 G 21 F 9/32, publ. 10.27.2002, BI No. 30.

4. GOST 22938-78. Rutile Concentrate. Technical conditions

5. GOST 22551-77. Quartz sand, ground sandstone, quartzite and vein quartz for the glass industry. Technical conditions

6. Toropov N.A., Barzakovsky V.P., Lapin V.V., Kurtseva N.N. State diagrams of silicate systems. Directory. Issue 1. Binary systems. - L .: Nauka, 1969.

7. Fedoseev A.D., Grigoryeva L.F., Makarova G.A. Fibrous silicates. Natural and synthetic asbestos. - M .: Nauka, 1966.

Claims (4)

1. A method of processing reactor graphite waste, including grinding reactor graphite waste, introducing powdered aluminum into the reactor waste graphite waste as an energy carrier, oxide of a carbide-forming element and a modifier, which are selected as substances that decompose and / or melt at the temperature of heat treatment of the prepared mixture and do not react with other components of the mixture, the preparation of the mixture from the crushed waste of reactor graphite, powdered aluminum , carbide-forming element oxide and modifier, loading the prepared mixture into the container, placing on top of the mixture in the container, a layer of igniter composition covering at least half of its open surface, the igniter composition having a combustion temperature of at least 2500 K and does not emit gaseous products during combustion conducting heat treatment of the mixture in the container in the mode of self-propagating high-temperature synthesis by initiating the combustion reaction of the mixture in the container with the images the use of carbide oxide material, including radionuclides, capture of aerosols released during the combustion of the mixture in the container, natural cooling of the final product formed in the container, characterized in that the crushed waste of reactor graphite is divided into several at least two fractions: fine, with particle sizes of 50 μm or less, and large, with particle sizes of 50 to 100 μm, combine the small and large fractions of the crushed reactor graphite waste into a mixture with a mass ratio between the fine major fractions from 95: 5 to 10:90, is introduced into a mixture of fine and coarse fractions of the crushed waste reactor graphite oxide as a carbide-forming element rutile concentrate in the following ratio, wt.%: Graphite Waste 7-11 Powdered Aluminum 23-30 Rutile concentrate 53-64 Modifier 2-15 Apply asbestos sheet to the inner surface of the walls and the bottom of the container in which the mixture is heat treated before loading the aforementioned prepared mixture into it. 2. The method according to claim 1, characterized in that quartz sand is used as a modifier. 3. The method according to claim 1, characterized in that barium metatitanate is used as a modifier. 4. The method according to claim 1, characterized in that as a modifier use calcium metasilicate (wollastonite).