RU2096844C1 - Method for insulating and chemically immobilizing radioactive wastes - Google Patents

Abstract

FIELD: storage of highly radioactive solid wastes. SUBSTANCE: mixture of highly active wastes and materials enhancing formation of cermet compounds is prepared by mixing wastes with metals of series s-, p-, d-, and f-elements and their oxides so as to ensure thermal characteristics of 40-60 kJ/g-atom ; inorganic binder is introduced in mixture, and briquette is formed. The latter is placed in container in a spaced relation to all surfaces, and heat-emitting materials are placed in these spaces. Then high-temperature fusion of cermet containing radioactive wastes is initiated. EFFECT: provision for single-stage high-temperature binding of radioactive wastes in metal and cermet matrixes; improved reliability of radionuclide isolation and immobilization. 4 cl, 4 dwg , 2 tbl

Description

 The claimed invention relates to the treatment of radioactive waste, in particular, solid radioactive waste, and is intended for the isolation and chemical immobilization of highly active solid radioactive waste (solid HLW) in cermet compositions.

 Immobilization refers to the achievement of the effect, chemical stability and the difficulty of migration of HLW from cermet compositions when exposed to external factors.

A known method of isolation of solid radioactive waste by incorporating waste in the form of oxide calcinate in a lead matrix. In this method, powdered calcine containing solid radioactive waste is granulated by agglomerating it with additives of a binder, granules 1.10 mm in size are sintered at 1100 ^° C and then filled with lead melt [1, p. 6-7]
A known method of isolation of solid radioactive waste by incorporating glass granules containing radioactive waste in a metal matrix (Pamela-process), developed by Gelsenberg (Germany) and Eurochemic (Belgium) [1, p. 10-13] The method consists in obtaining granules of glass containing radioactive waste, and in the subsequent pouring of glass granules with metal melts based on lead or aluminum. To carry out the process of incorporating solid forms of highly active waste into metal matrices, molten metal must have good casting properties. The temperature of the poured metal is limited to not more than 580 ^o C for phosphate glass and not more than 640 ^o C for borosilicate glass, so that upon contact with the metal melt the glass granules remain unchanged and the formation of a reaction zone at the granule-metal interface is excluded. A restriction is introduced on the size of the granules, which should not be less than 0.1 mm in order to ensure the penetration of metal into the voids between the granules.

There is also a method of isolating solid radioactive waste in the form of cermets obtained by sintering finely divided powders of iron or aluminum with calcinate containing solid radioactive waste. The method includes mixing the mixture, pressing under pressure of 500 MPa and heating in an inert atmosphere to a temperature slightly below the melting point of the corresponding metal. The resulting composition with a content of up to 80 wt. calcines possessed high mechanical strength, and thermal conductivity increased by almost two orders of magnitude compared to pure calcinate. However, the cermet obtained by this method turned out to be insufficiently resistant to leaching in water. So a composition of 50 wt. calcinate and 50 wt. aluminum lost almost 12 wt. sodium after 150-170 hours of exposure in water, and after 500-650 hours 14.4 wt. sodium [1, p. 21]
Closest to the claimed method of isolation of high-level waste (HLW) in sintered metal-ceramic compositions developed by Oakridge National Laboratory of the USA [1, 2] In this way, the isolation of HLW is achieved by distributing ceramic microparticles containing HLW components (for example, oxides, aluminosilicates or titanates) in a metal matrix. The method for incorporating HLW into cermet compositions is, in essence, a technology of powder metallurgy for producing cermet with preliminary reduction by hydrogen from oxides of the components of the metal matrix, in which HLA non-reducible oxides of HLW are distributed in the form of pure oxides or oxide compounds. The metal matrix is a sintered metal, such as iron, copper, nickel, cobalt, etc. obtained by restoring them with hydrogen from compounds present in the waste or added specifically. Liquid wastes in the form of nitrate aqueous solutions are concentrated by evaporation, urea, aluminosilicates and titanium salts are introduced into the concentrate in order to chemically bond cesium and strontium into durable compounds. Nitrate ions are destroyed in the process of chemical reduction, subsequent heating and calcination. Calcinate obtained by calcination is reduced at a temperature not exceeding 800 ^o C. Easily reducible cations are transformed into finely divided metal particles. Easily reducible HLW oxides (for example, ruthenium oxide) are also converted into metals in the future by the components of the metal matrix. Oxides that cannot be reduced under the conditions of the process, including refractory HLW oxides, remain in the form of oxide particles. After the recovery process, finely dispersed metal and oxide particles are mixed with an organic binder, pressed, sintered at 1200 ^o C. Thus, as a result of the implementation of the method, cermet containing HLW wastes is formed both in the metal matrix and in the ceramic oxide inclusions of the composite material. The volatility of cesium, strontium, and especially ruthenium at all stages of the process is small. The maximum loss was 0.27 wt. cesium, 0.23 wt. ruthenium and 0.0001 wt. strontium. The method allows you to isolate sludge and metal waste radiochemical production, but is especially promising for the processing of liquid waste containing a large number of recoverable cations, and in particular iron.

The disadvantages of the method include the following. The method is divided in time into several fundamentally different technological operations occurring in the equipment corresponding to this operation: calcification of HLW; hydrogen reduction of a mixture of oxides; sintering press manufacturing; sintering compacts in a non-oxidizing atmosphere at 1200 ^o C.

 Special sophisticated technological equipment is required at each stage of the process.

The sintering temperature of 1200 ^o C does not allow immobilization of HLW in the most chemically stable forms. At present, it is believed that radionuclides are most strongly retained in a synroc polymorphic ceramic, consisting of gallandite BaAl ₂ Ti ₆ O ₁₆ , zirconolite CaZrTi ₂ O ₇ , perovskite CaTiO ₃ and obtained by sintering of oxide materials [3, 4] However, by melting the starting components were obtained mineral-like compounds (pyroxenes, garnets, titanosilicates, etc.), which have higher chemical resistance compared to synroc [5]
The sintering method is inefficient, worked out for parts of small mass.

 The objective of the invention is the creation of a method with increased reliability of isolation and chemical immobilization of solid HLW.

 The technical result that can be obtained by carrying out the invention is that it provides a one-step high-temperature process for the binding of HLW in metal and ceramic matrices with the formation of mineral-like oxide compounds containing HLW.

The essence of the invention lies in the fact that in the method of isolation and chemical immobilization of solid HLW, including the preparation of a mixture of waste in the form of oxides and (or) metals with materials that contribute to the formation of cermet compositions, with which they isolate HLW, add binders, compact the mixture before imparting forms of a briquette, according to the invention, as materials contributing to the formation of cermet compositions, a cermet mixture is introduced into the mixture, the components of which are selected from poisons of s-, p-, d-, f-elements and their oxides, for example, Cr ₂ O ₃ , F ₂ O ₃ , NiO, SiO ₂ , D ₂ O ₃ , TiO ₂ , ZrO ₂ , CaO, BaO, Al ₂ O ₃ , CrFe, Ni, Mg, Al, Si, Ti, ensuring the tightness of the mixture, including HLW, within 40.60 kJ / gram atom, the binder materials are selected inorganic, after compaction, the briquette is placed in a metal container to ensure gaps between the surfaces of the briquettes and the internal surfaces of the container, then initiate in the briquette high-temperature synthesis of cermet compositions containing HLW, for which heat-generating materials are placed in the gaps.

 In addition, HLW is made in the form of a dispersed system with particle sizes of not more than 8.0 mm.

As the heat-generating material, a melt of oxides or metals with a temperature of 1000.1600 ^o C. is chosen.

 As a heat-generating material, a metallothermal mixture can be selected, which, after filling the gaps, is ignited.

 The claimed invention meets the criterion of "novelty", because contains features that distinguish it from the prototype.

 When searching, no solutions were found that have features that match the distinguishing features of the claimed invention, therefore, we can conclude that its criterion is "inventive step".

 Compliance with the criterion of "industrial applicability" is proved by the indication at the beginning of the description of the purpose of the claimed subject matter of the invention, the following specific example and the accompanying graphic materials.

In FIG. 1 shows a container assembly (initial state);
in FIG. Stage 2 of the formation of cermets and cast insulating layer after filing in the gaps of the melt;
in FIG. 3 two-layer container with a cermet composition (end of the process);
in FIG. 4 photograph of a longitudinal fracture of a block.

 The method is implemented as follows.

A ceramic-metal charge is prepared, the components of which are selected from a number of s-, p-, d-, f-elements and their oxides, for example Cr ₂ O ₃ , Fe ₂ O ₃ , NiO, SiO ₂ , B ₂ O ₃ , TiO ₂ , ZrO ₂ , CaO, BaO, Al ₂ O ₃ , Cr, Fe, Ni Mg, Al, Si, Ti. The choice of specific components and their amount is determined by the final composition of the products of the cermet process and the possibility of implementing the process of synthesis of cermets with a given temperature and burning rate. Highly active waste in the form of oxides, metals or mixtures thereof of a fraction of not more than 8 mm and inorganic binders are introduced into the prepared cermet charge. The fractional composition of the injected mixture covers the range of particle sizes that make up the sludge and solid waste of radiochemical production or are formed at the stage of solidification of liquid waste. The maximum particle size is determined by the width of the reaction zone of the high-temperature synthesis of cermet, which is the basis of the method. High-level waste is mixed with a ceramic-metal charge in mixers or when mixing two streams of bulk materials, for example, in a gravity pipe. The amount of HLW introduced is determined by the final composition of the cermets and by maintaining the thermal content of the mixture within 40.60 kJ / g-atom. The mixture is sealed in special equipment or directly in a thin-walled aluminum jacket. The finished briquette 1 is dried and installed in a single non-returnable metal container 2 with a gap (see Fig. 1) so that a cavity 3 is formed between the briquette 1, the bottom, the walls and the roof of the container 2, into which the molten metal or oxides 4 are poured with a temperature 1000.1600 ^o C. The temperature of the molten melt 4 is due to the nature of the melt and its fluidity.

As a result of the heat pulse from the molten melt 4 in the briquette 1, a high-temperature process is initiated, which is an exothermic reaction of metallothermal reduction of oxides, which are realized in the form of condensed (gas-free) combustion of a metallothermal mixture that spontaneously propagates from the surface of briquette 1 to its center. The peculiarity of the process implementation is that at each moment of time only part of the mixture is involved in the combustion, and the so-called combustion wave moves along the briquette 1, a characteristic feature of which is the presence of the combustion zone 5 with a maximum heat release (Fig. 2). Combustion zone 5 has the following parameters: zone width 1.10 mm; combustion temperature 1700.2100 ^o C; linear velocity of propagation of the combustion zone 0.02.1.2 mm / s. This combination of parameters of the combustion zone 5 ensures a smooth flow of the process (without spraying and emissions of combustion products) and the formation of the required macrostructure of the ceramic-metal composition. In the combustion zone 5, which has a high temperature, the metal and oxide combustion products are in a liquid or in a two-phase solid-liquid state, and during their stay in the combustion zone 5 do not have time to completely separate.

 After passing through the combustion zone 5, cooling and solidification of the combustion products, the metal material of the block macrostructure 6 is formed, consisting of two interpenetrating and inextricable matrixes of metal and oxide ceramic. As a result of chemical interaction in the reaction zone, i.e., the zone in which combustion occurs (combustion zone 5), afterburning, and structure formation, the components of the metallothermal mixture containing HLW are redistributed between the metal and ceramic matrices of the condensed combustion composite product. The metal matrix consists of the metal products of metallothermal reduction reactions, from metals specially introduced into the mixture to form the required composition of the metal matrix, and from metal HLW of the metallothermal mixture. The ceramic matrix consists of oxide metallothermal reduction products, from oxides specially introduced into the mixture to form the required composition of the ceramic matrix, and from HLW oxides that are not reduced to metals under the conditions of this process.

 In the cavity between the metal container 2 and the briquette, the melt solidifies, which forms a protective layer 7, which together with the metal container 2 isolates the synthesis process and the ceramic-metal composition from the environment. The solidification of the melt is ahead of the synthesis process in time and ends before the end of the synthesis of cermet.

 A variant of the method of isolation and chemical immobilization of solid HLW in cermet compositions is a method that is essentially different from that described in that the cavity between the briquette and the container is filled with a metal-thermal mixture that does not contain HLW. In this layer of the metallothermal mixture, a metallothermal process is initiated, as a result of which a protective layer is formed consisting of cermet material that does not contain HLW, and the heat generated initiates in the briquette containing HLW the process of high-temperature synthesis of the cermet composition.

 As a result of the implementation of any of the considered options for high-temperature synthesis in a briquette containing HLW, a ceramic-metal composition of a HLW preservative is formed, consisting of a corrosion-resistant metal matrix and a mineral-like oxide matrix.

 The advantages of the method are: in a one-stage high-temperature process, a result is achieved, which in the prototype is a consequence of the implementation of several independent technological operations; in a one-stage high-temperature process, oxide reduction is carried out; the formation of a metal matrix occurs simultaneously with the formation of a ceramic matrix, which is not formed at all in the prototype method; the stage of molding and sintering of the reduction products is eliminated. An additional positive effect is the receipt of the part, ready for storage without additional processing operations.

 The implementation of the chemical redistribution of HLW through the high-temperature stage of the existence of combustion products in liquid form accelerates (compared with solid-phase sintering) chemical reactions of binding of HLW in a metal and ceramic matrix in the form of solutions or chemical compounds and makes it possible to obtain mineral-like oxide compounds containing HLW by melting , which is fundamentally impossible with low-temperature sintering. The high-temperature liquid-phase stage allows one to reduce (as compared to solid-phase sintering) the duration of the formation of metal and ceramic matrices, composition averaging and strong adhesion of individual matrix fragments have time to occur during the residence of the combustion products in the liquid and solid-liquid states.

 No special heating equipment is needed to carry out the recovery and sintering processes. The required amount of heat is generated directly in the briquette due to chemical reactions during metallothermal condensed combustion.

 No special unit is required for the implementation of the process; its role is played by a metal disposable container and a protective layer between the briquette and the container. The molten metal or oxides, as well as the metallothermal mixture for forming the protective layer, are prepared in radiation-friendly units, which facilitates the maintenance and repair of these units.

 The method is more productive in comparison with the prototype by reducing the duration of the recovery stage and eliminating the sintering stage in the proposed method, the duration of the formation of cermet is determined by the duration of the burning of the briquette, which is completed within 5.20 minutes compared to several hours of exposure in traditional powder recovery and sintering technologies. The method allows to obtain parts with a cermet composition, the total mass of which varies from 40 to 6000 kg.

 An example of a specific implementation.

 A specific implementation of the method is considered on the example of obtaining a cermet composition in a cast steel shell and the use of fused corundum as a simulator of solid HLW.

The initial thermal charge was prepared by dry mixing in a mixer the following components: chromium concentrate, aluminum powder, pulverized silica, titanium dioxide and ferrotitanium. A simulated solid HLW fused corundum was introduced into the mixed metallothermal charge in an amount of 20. 25% of the mass of the metallothermal charge was mixed until the mixture was homogeneous (a mixture weighing 300 kg was mixed in the mixer for 10-15 minutes). An inorganic binder of water glass with a density of 1.20.1.25 g / cm ³ in the amount of 6.7% by weight of the dry mixture was introduced into the dry mixture and mixed until ready for 5-10 minutes. 40 kg briquettes with an internal wire frame were molded from the crude mixture in a reusable snap. Together with the equipment, the briquette was blown with carbon dioxide in order to carbon-cure the mixture. After this operation, the briquette was removed from the snap and produced a thermal drying of the briquette at 200.250 ^o C for 2.3. 3.0 hours. After heat drying, the briquette was fed to the assembly of the mold. The mold consisted of two half-molds made of sand-clay molding sand. After drying the half-molds, the mold was assembled and the briquette was installed in the mold so that a cavity was formed between the briquette and the inner surfaces of the mold. The distance from the surface of the briquette to the inner surfaces of the mold (cavity size) was 15.20 mm. The size of the cavity can be changed by changing the size of the briquette and (or) the dimensions of the inner surfaces of the form. Liquid steel was poured into the mold by a siphon, liquid metal enters the cavity between the briquette and the inner surface of the mold by the gate system. At the same time, 8 castings (blocks) were made in one form. Upon completion of pouring the metal into the mold, a quick (within 0.5.1.0 minutes) hardening of the molten steel located in the cavity between the briquette and the internal surfaces of the mold took place, as a result of which a steel shell was formed around the briquette, acting as a cast container, inside of which a high-temperature synthesis of cermets.

As a result of heating from the steel shell in the surface layers of the briquette, a high-temperature synthesis process was initiated, which started at 780.800 ^o C and spread inside the briquette in the form of a condensed combustion zone, which had the following parameters: combustion zone width 6.8 mm, linear burning rate 0.2.0.4 mm / s, the temperature in the combustion zone 2000. 2100 ^o C. In the combustion zone molten metal and oxides formed, consisting of products of the metallothermic reaction and specially introduced metal and oxide materials SiO ₂ , TiO ₂ , Al ₂ O ₃ (simulator VAO), non-reducible oxides of chromium concentrate — CaO, MgO, SiO ₂ , Al ₂ O ₃ and residues of unreduced oxides Cr ₂ O ₃ and FeO. After passing through the combustion zone, the metal and oxide melts solidified, forming a cermet material, consisting of two interpenetrating and inextricable matrices of highly alumina ceramic and corrosion-resistant metal. The cast block had dimensions of 400 x 180 x 600 mm, weight 100 kg. Figure 4 presents a longitudinal fracture of the block. The strength of the composite material in the core of the block is high after an autogenous incision of the steel shell, the block was able to break after 2-3 strokes of the copra (load weighing 3000 kg, falling from a height of 15 m).

The ceramic-metal composition in the core of the block had the following chemical composition:
metal matrix (wt.): 14.65 Cr; 0.75 Ni; 0.96 Si; 0.45 Mn; 0.52 Ti; 0.25 Mo; 0.1 V; Fe is the rest;
ceramic matrix (wt.): 66.14 Al ₂ O ₃ ; 6.14 SiO ₂ ; 13.5 MgO; 2.6 Cr ₂ O ₃ ; 1.0 FeO; 5.4 TiO ₂ ; 0.42 CaO; 15.5 Na ₂ O; 3.3 MnO.

 The composition of the charge materials (see table 1 and 2).

LITERATURE
1. Scarlet A. S. Shashukov E. A. Solidification of radioactive waste in the form of glass-metal and cermet compositions: Review M. TsNIIatominform, 1984. Issue. 2 (9) .s. 7-9 prototype, p. 6-7, 10-13, 21.

 2. Aaron W.S. Kobisk E.N. Quinby T.C. // Trans. Amer. Nucl. Soc. 1978. v. 30. N 2-P. 284.

 3. Ringwood A.E. Kelly P.M.//Phil. Trans R. Soc. Lond. 1986. v. A 319. P. 63-82.

 4. Coles D.G. Bozan F. Continuous-Flow Leaching Stadied of Crushed and Cores Sunroc // Nucl. Techn. 1982 v. 56. p. 226-237.

 5. Nikiforov A.S. and others. Some ways to improve the reliability of localization of HLW during disposal // Radioactive waste problems, solutions. In 2 parts / A. S. Nikiforov, V. I. Vlasov, N. V. Krylova, R. N. Salomatina, T. V. Smelova. M. IAE them. I.V. Kurchatova, 1992. Part 2. S.401-405.

Claims (4)

 1. The method of isolation and chemical immobilization of solid high-level radioactive waste (HLW), according to which a mixture of HLW is prepared in the form of oxides and / or metals with materials that contribute to the formation of cermet compositions by which the HLW is isolated, binders are added, the mixture is compacted until it is imparted briquette shape, characterized in that as materials contributing to the formation of cermet compositions, a metallothermic mixture is introduced into the mixture, the components of which are selected from the series s-, p-, d-, f- elements, and their oxides, providing a thermal mixture, including HLW, in the range of 40-60 kJ / mol, the binder materials are selected inorganic, after compacting, the briquette is placed in a metal container to provide gaps between the surfaces of the briquettes and the inner surfaces of the container, then high-temperature synthesis of cermet is initiated in the briquette compositions containing HLW, for which heat-generating materials are placed in gaps.  2. The method according to claim 1, characterized in that the HLW is made in the form of a dispersed system with a particle size of not more than 8 mm 3. The method according to claim 1, characterized in that the gaps are filled with a melt of oxides or metal, the melt temperature is selected in the range of 1000 1600 ^o C.  4. The method according to claim 1, characterized in that the gaps are filled with a metallothermal mixture and set fire to it.

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