RADIOACTIVE WASTE SOLIDIFICATION METHOD

Abstract

 In a radioactive waste solidification method, radioactive waste (for example, zeolite to which Cs-137 was adsorbed) and glass raw materials (soda lime glass) are supplied into the solidifying vessel (3). The solidifying vessel (3) filled with the radioactive wastes and glass raw materials is disposed in an adiabatic vessel (7). A lid is attached to the adiabatic vessel (7) to seal it. The radioactive waste and glass raw materials in the adiabatic vessel (7) are heated by thermal energy generated due to radiation emitted from Cs-137. The glass raw materials are melted and osmose in clearances among the radioactive waste, producing a vitrified radioactive waste. Since the radioactive waste and glass raw materials are disposed in the adiabatic vessel (7), they are uniformly heated and a uniform vitrified radioactive waste is obtained. No melting facility is needed. The radioactive waste solidification method can produce a uniform vitrified radioactive waste through uniform heating without having to use a melting facility.

Description

BACKGROUND OF THE INVENTION

[Technical Field]

[0001] The present invention relates to a radioactive waste solidification method, and more particularly to a radioactive waste solidification method suitable to processing of high-dose radioactive waste having a high radioactive level.

[Background Art]

[0002] Radioactive waste generated from a nuclear facility and the like are solidified with cement or glass and are then converted to a form suitable for storage, transportation, and burial processing. Solidification with cement of various types of solidification processing is a method in which radioactive waste is solidified with cement and water, so this method is inexpensive and is also advantageous in that processing is easily performed. When high level radioactive waste is solidified with cement in a solidifying vessel, however, moisture included in the radioactive solidified body generated by the cement solidification is subjected to radiolysis, generating a hydrogen gas. This hydrogen gas may affect the solidified body itself or a facility after burial processing (refer to Japanese Patent Laid-open No. 2007-132787). Therefore, in a radioactive waste solidification method described in Japanese Patent Laid-open No. 2007-132787, radioactive waste, cement, and water are mixed in a drum which is a solidifying vessel to produce a solidified body, and the solidified body is dried to eliminate moisture from the solidified body through heating or pressure reduction at a stage in which uniaxial compression strength is 1.5 MPa or more and is 75% or less of predicted strength.

[0003] In solidification with glass in which water is not used, so even if a radioactive waste is at a high radioactive level, there is no fear that a hydrogen gas is generated. As described in Japanese Patent Laid-open No. 2011-46996, however, solidification with glass involves processing at a high temperature, so that a large melting facility and the like are needed.

[0004] Japanese Patent Laid-Open No. 62(1987)-124499 describes a radioactive waste solidification method. In this radioactive waste solidification method, solid or liquid radioactive waste are mixed with glass with a low melting point (the melting point is 400°C to 800°C), and the resulting mixture of the waste and glass is subjected to molding and baking or is melted by being heated and a solidified body is produced.

[0005] Japanese Patent Laid-Open No. 62(1987)-165198 describes a hydrothermal solidification method for high-level radioactive waste. In this hydrothermal solidification method for high-level radioactive waste, high-level radioactive waste, glass, and quartz powder are mixed, and the resulting mixture is further mixed with water. This mixture is supplied into a canister. The mixture in the canister is heated to 300°C due to decay heat of the high-level radioactive waste, producing a solidified body through a hydrothermal reaction. In this hydrothermal solidification method for high-level radioactive waste, the surfaces of glass and quartz powder are melted due to decay heat and high-level radioactive waste is bonded.

[Citation List]

[Patent Literature]

[0006] 

[Patent Literature 1] Japanese Patent Application No. 2007-132787

[Patent Literature 2] Japanese Patent Application No. 2011-46996

[Patent Literature 3] Japanese Patent Application No. 62(1987)-124499

[Patent Literature 4] Japanese Patent Application No. 62(1987)-165198

SUMMARY OF THE INVENTION

[Technical Problem]

[0007] As for solidification of high-dose radioactive waste, solidification with glass in which a hydrogen gas due to radiation is not generated is preferable. Although conventional methods of solidification with glass are problematic in that a large melting facility is needed, the hydrothermal solidification method for high-level radioactive waste described in Japanese Patent Laid-Open No. 62(1987)-124499 can solve the above problem because decay heat of high-level radioactive waste is used to melt glass and crystal powder. In the method described in Japanese Patent Laid-Open No. 62(1987)-124499, however, high-level radioactive waste is solidified in a hydrothermal solidification method in which added water is used. Although decay heat of high-level radioactive waste is used, part of the decay heat is used to evaporate water because a hydrothermal solidification method is used. Accordingly, the high-level radioactive waste is solidified by melting only the surfaces of glass and crystal powder. The produced solidified body is a non-uniform substance including water and steam and radioactive nuclides are thereby likely to leak from the solidified body.

[0008] An object of the present invention is to provide a radioactive waste solidification method that can produce a uniform vitrified radioactive waste through uniform heating without having to use a melting facility.

[Solution to Problem]

[0009] A feature of the present invention for attaining the above object is a radioactive waste solidification method comprising steps of:

supplying radioactive waste including radioactive nuclides and glass raw materials into a first vessel;

disposing the first vessel in which the radioactive waste and glass raw materials exist, in an adiabatic area in a second vessel;

heating the radioactive waste and glass raw materials in the first vessel existing in the adiabatic area in the second vessel by heat generated by radiation emitted from the radioactive nuclides; and/or

producing a vitrified radioactive waste by melt of the heated glass raw materials.

[0010] According to the present invention, since the first vessel in which the radioactive waste including radioactive nuclides and glass raw materials exist is disposed in the adiabatic area in the second vessel and the glass raw materials in the first vessel are melted in the adiabatic area by heat generated from radiation emitted from the radioactive nuclides, a melting facility is not needed and the radioactive waste and glass raw materials in the first vessel are evenly heated, so a uniform vitrified radioactive waste can be obtained.

[Advantageous Effect of the Invention]

[0011] According to the present invention, radioactive waste and glass raw materials are uniformly heated without having to use a melting facility and a uniform vitrified radioactive waste can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] 

FIG. 1 is a flowchart showing a processing procedure in a radioactive waste solidification method according to embodiment 1, which is a preferred embodiment of the present invention.

FIG. 2 is an explanatory drawing showing a concrete example of a radioactive waste solidification method according to embodiment 1 shown in FIG. 1.

FIG. 3 is an explanatory drawing showing a radioactive waste solidification method according to embodiment 3, which is another preferred embodiment of the present invention.

FIG. 4 is an explanatory drawing showing a radioactive waste solidification method according to embodiment 4, which is another preferred embodiment of the present invention.

FIG. 5 is an explanatory drawing showing a radioactive waste solidification method according to embodiment 5, which is another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The inventors made various studies and found that when a solidifying vessel filled with a mixture of a radioactive waste and glass raw materials, which are solidifying materials, is disposed in an adiabatic region formed by, for example, surrounding the solidifying vessel with an adiabatic member or vacuating the interior of the solidifying vessel and the glass raw materials in the solidifying vessel are melted by using decay heat of radioactive nuclides included in the radioactive waste, the glass raw materials in the solidifying vessel are uniformly heated and a uniform solidified body of radioactive waste can thereby be produced due to the melted glass raw materials. That is, much radiation energy is emitted from high-dose radioactive waste, so when the radioactive waste itself and glass raw materials, which are solidifying materials, absorb the emitted radiation energy, thermal energy into which the radiation energy has been converted is stored in the radioactive waste and glass raw materials. Accordingly, the temperature in the glass raw materials in the solidifying vessel can be substantially uniformly raised to a temperature needed to melt the glass raw materials, regardless of the positions of the glass raw materials in the solidifying vessel.

[0014] Therefore, the solidifying vessel filled with the radioactive waste and glass raw materials is uniformly heated, and a more uniform solidified body of radioactive wastes can thereby be produced.

[0015] Embodiments of the present invention in which the above study result is considered will be described below.

[Embodiment 1]

[0016] A radioactive waste solidification method according to embodiment 1, which is a preferred embodiment of the present invention, will be described with reference to FIGs. 1 and 2. In the radioactive waste solidification method in the present embodiment, 100 kg of high-dose radioactive waste that includes 10¹⁶ Bq of Cs-137 (for example, zeolite to which Cs-137 has been adsorbed) as high-dose radioactive waste and 100 kg of soda lime glass, which is a glass raw material, with a glass softening point of about 700°C were supplied into a solidifying vessel and a vitrified radioactive waste is produced. The radioactive waste solidification method of the present embodiment will be described below with reference to the procedure illustrated in FIG. 1.

[0017] Radioactive waste and glass raw materials are supplied into a solidifying vessel (step S1). Specifically, a metallic (or ceramic) vacant solidifying vessel (a first vessel) 3 is disposed below a waste tank 1 in which radioactive waste 4 has been stored. High-dose radioactive waste 4 that include 10¹⁶ Bq of Cs-137 in the waste tank 1 are supplied by 100 kg into the solidifying vessel 3 through a waste supply pipe 2. In the present embodiment, the radioactive waste 4 supplied into the solidifying vessel 3 is, for example, zeolite to which the above Cs-137 was adsorbed. After that, the solidifying vessel 3 filled with the radioactive waste 4 is moved to a position immediately below a glass raw material tank 1A. 100 kg of soda lime glass, which is a glass raw material 6 in the glass raw material tank 1A, is supplied into the solidifying vessel 3 through a glass raw material supply pipe 5. Glass raw materials 6 may be supplied into the solidifying vessel 3 first, and after the supply of the glass raw materials 6, radioactive waste 4 may be supplied into the solidifying vessel 3. Alternatively, the glass raw materials 6 and radioactive waste 4 may be supplied into the solidifying vessel 3 at the same time. The radioactive waste 4 supplied into the solidifying vessel 3 may be a liquid radioactive waste, a solid radioactive waste, or a mixture of a liquid radioactive waste and a solid radioactive waste.

[0018] Adiabatic processing is performed for the solidifying vessel 3 filled with the radioactive waste 4 and the glass raw materials 6 is melted (step S2). Specifically, the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is disposed in an adiabatic vessel (a second vessel) 7 with its upper end open. The adiabatic vessel 7 has a lid 7A, which is removable, at the upper end. When the solidifying vessel 3 is to be disposed in the adiabatic vessel 7, the lid 7A is removed, making the solidifying vessel 3 upwardly open. In this state, the solidifying vessel 3 is disposed in the adiabatic vessel 7 from above. After that, the lid 7A is attached to the upper end of the adiabatic vessel 7 to seal the adiabatic vessel 7 in which the solidifying vessel 3 is disposed. The adiabatic vessel 7 and lid 7A are made of an adiabatic material. For example, they are made of glass wool. In the sealed adiabatic vessel 7, an adiabatic area, which is thermally insulated by the adiabatic vessel 7, is formed. The solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 are disposed in this adiabatic area.

[0019] The adiabatic vessel 7 has a double structure of a metallic outer vessel (not shown) and an inner vessel (not shown) which is disposed in the outer vessel. Glass wool is disposed in an annular area between the outer vessel and the inner vessel and in a space between a bottom of the outer vessel and a bottom of the inner vessel. An upper end of the annular area between the outer vessel and the inner vessel is sealed with a ring-shaped plate attached to both upper ends of the outer vessel and inner vessel. The lid 7A is formed by a metal hollow case (not shown) filled with glass wool.

[0020] After that, adiabatic processing is performed on the solidifying vessel 3 disposed in the sealed adiabatic vessel 7. The adiabatic processing means a processing to suppress heat of the solidifying vessel 3 from being emitted to the outside. In the solidifying vessel 3 that has been subjected to the adiabatic processing, heat (decay heat) is generated based on radiation emitted from Cs-137 included in the radioactive waste 4 in the solidifying vessel 3. Emission of this decay heat to the outside is suppressed by the adiabatic vessel 7 sealed with the lid 7A, and the decay heat is stored in the interior of the adiabatic vessel 7 sealed with the lid 7A, that is, in the adiabatic vessel 7 sealed with the lid 7A. The glass raw materials 6 in the solidifying vessel 3 are heated due to this decay heat and melted.

[0021] Since the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is surrounded by the adiabatic vessel 7 and lid 7A, the solidifying vessel 3 is heated by decay heat of the radioactive waste 4. The temperatures of the radioactive waste 4 and glass raw materials 6 stored in the solidifying vessel 3 in the adiabatic vessel 7 become substantially uniform; these temperature do not become non-uniform depending on the positions of the radioactive wastes 4 and glass raw materials 6 in the solidifying vessel 3.

[0022] For example, Cs-137, which is a radioactive nuclide included in the radioactive waste 4, emits radiation with about 1.15 MeV of energy per disintegration. This radiation is absorbed in the radioactive waste 4 and glass raw materials (soda lime glass) 6 and is then converted to thermal energy (decay heat). Since the radioactive waste 4 filled in the solidifying vessel 3 includes 10¹⁶ Bq of Cs-137, if radiation emitted from each Cs-137 is all absorbed in the radioactive wastes 4 and glass raw materials 6 in the solidifying vessel 3, thermal energy of 1.15 MeV x 10¹⁶Bq (= 1.15E22 eV/s), that is, at a heat generation rate of 1840 J/s, is obtained. If the specific heat of the mixture of the radioactive waste (zeolite to which Cs-137 has been adsorbed) 4 and glass raw material (soda lime glass) 6 is 0.5 J/(g.K), the temperatures of the radioactive waste 4 and glass raw materials 6 are each raised by about 66°C per hour. Soda lime glass, which is the glass raw material 6, is melted due to this temperature rise and flows into clearances among the radioactive waste 4. Preferably, as described in embodiment 4 below, after the radioactive waste 4 and glass raw materials 6 have been supplied into the solidifying vessel 3, they are mixed with an agitator. If a liquid is included in the radioactive waste 4, this liquid is heated by the heat described above and is turned into a vapor.

[0023] A vitrified radioactive waste is produced (step S3). Specifically, since some heat is emitted to the outside through the adiabatic vessel 7 and lid 7A, an actual temperature rise rate in the solidifying vessel 3 is lower than 66°C/h. However, a temperature rise is continued with time, so all glass raw materials 6 in the solidifying vessel 3 are melted. As a result, clearances between radioactive waste 4 are filled with the melted substances of the glass raw materials 6, and a vitrified radioactive waste 9 in which a glass-solidified substance 8 exists in the solidifying vessel 3 is produced. In the vitrified radioactive waste 9, the radioactive waste 4 have been integrated by the melted substances of the glass raw materials 6. After the vitrified radioactive waste 9 has been produced, the lid 7A is removed. Then, the vitrified radioactive waste 9 is taken out of the adiabatic vessel 7 and a lid (not shown) is attached to the solidifying vessel 3 of the vitrified radioactive waste 9 to seal it. Thereafter, the vitrified radioactive waste 9 is stored as a waste body at a prescribed storage place (not shown).

[0024] In the present embodiment, since the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is surrounded by the adiabatic vessel 7 sealed with the lid 7A, radiation emitted as a result of the decay of radioactive nuclides included in the radioactive waste 4 is absorbed in the radioactive waste 4 and glass raw materials 6 disposed in the adiabatic area in the adiabatic vessel 7 and the resulting thermal energy (decay heat) heats the radioactive waste 4 and glass raw materials 6. Therefore, the radioactive waste 4 and glass raw materials 6 existing in the adiabatic area are uniformly heated, so the temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 become more uniform. This suppresses corrosion of the solidifying vessel 3 at high temperatures and volatilization of the radioactive wastes 4. In addition, a uniform vitrified radioactive waste of the radioactive waste 4 is obtained. This vitrified radioactive waste is stable.

[0025] As described above, the radioactive waste 4 and glass raw materials 6 are heated by thermal energy generated by the absorption of radiation emitted due to the decay of the radioactive waste 4 in the solidifying vessel 3. In the present embodiment, therefore, a melting facility to melt the glass raw materials 6 is not needed, unlike solidification of radioactive waste with glass, which is described in Japanese Patent Laid-open No. 2011-46996. That is, a simple system can be used to solidify the radioactive waste 4 with the glass raw materials 6.

[0026] In the present embodiment, zeolite to which Cs-137 was adsorbed was supplied into the solidifying vessel 3 as the radioactive waste 4 and was solidified with the glass raw materials 6. In the present embodiment, however, clinoptilolite, mordenitem, chabazite, insoluble ferrocyanide, or a titanate compound may be supplied into the solidifying vessel 3 and may be solidified by melting the glass raw materials 6 by the decay heat, as the radioactive waste 4.

[0027] As substitute for soda lime glass, any one of silicate glass and borosilicate glass may be used as the glass raw material 6. Furthermore, as the glass raw material 6, any one of lead glass, phosphate glass, and vanadium-based glass, which have a softening point lower than soda lime glass, silicate glass, and borosilicate glass may be used. When one of lead glass, phosphate glass, and vanadium-based glass is used, solidification with glass is possible in an area at a lower temperature. Therefore, even under conditions in which the amount of heat generated by the decay of the radioactive waste 4 in the solidifying vessel 3 is low and in which a highly adiabatic state cannot be assured, the radioactive waste 4 can be solidified with glass in the solidifying vessel 3 and volatilization of the radioactive waste 4 can be suppressed to a lower level.

[0028] Glass wool used in the adiabatic vessel 7 and lid 7A, which are used adiabatic processing in the present embodiment, may be replaced with any one of cellulose fiber, which is a fiber-based adiabatic material, carbonized cork, urethane foam, phenol foam, polystyrene foam, and a potassium silicate board, which is a porous adiabatic material. These adiabatic materials may be used in the adiabatic vessel 7 and lid 7A in embodiments 2 and 4 described below.

[Embodiment 2]

[0029] A radioactive waste solidification method according to embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to FIGs. 1 and 2. In the radioactive waste solidification method according to the present embodiment, 100 kg of high-dose radioactive waste that includes 10¹⁶ Bq of Sr-90 (for example, spent adsorbents, for radioactive nuclides, the main component of which is a titanate compound to which Sr-90 was adsorbed (the adsorbent will be referred to as the titanate compound adsorbent)) as high-dose radioactive waste and 100 kg of borosilicate glass, which is a glass raw material, with a glass softening point of about 800°C were supplied into a solidifying vessel and a vitrified radioactive waste was produced. The radioactive waste solidification method of the present embodiment will be described below.

[0030] In the present embodiment as well, a vitrified radioactive waste 9 is produced in steps S1, S2, and S3, as in embodiment 1.

[0031] In step S1, 100 kg of high-dose radioactive waste 4, including 10¹⁶ Bq of Sr-90, stored in the waste tank 1 and 100 kg of borosilicate glass, which is glass raw materials 6, stored in the glass raw material tank 1A are supplied into the solidifying vessel 3. In the present embodiment, the radioactive waste 4 supplied into the solidifying vessel 3 is a titanate compound adsorbent to which Sr-90 was adsorbed.

[0032] After the radioactive wastes 4 and glass raw materials 6 were supplied into the solidifying vessel 3, step S2 is executed. That is, as in the embodiment 1, the solidifying vessel 3 which was filled with the radioactive waste 4 and glass raw materials 6 is disposed in the adiabatic vessel 7, and the lid 7A is attached to the adiabatic vessel 7 to seal it. Radiation emitted by the decay of Sr-90 included in the radioactive waste 4 existing in the solidifying vessel 3 enclosed with the lid 7A and adiabatic vessel 7 is absorbed by the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 and is then converted to thermal energy. The radioactive waste 4 and glass raw materials 6 enclosed with the lid 7A and adiabatic vessel 7 are heated due to this thermal energy and their temperatures are raised. The temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 enclosed with the lid 7A and adiabatic vessel 7 become substantially uniform; these temperatures do not become non-uniform depending on the positions of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3. An adiabatic area is formed in the sealed adiabatic vessel 7 as described above. In the present embodiment as well, the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is disposed in this adiabatic area.

[0033] For example, Sr-90 emits about 2.8 MeV of energy per disintegration. If this radiation is absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, the radiation is converted to thermal energy. Since the radioactive waste 4 includes 10¹⁶ Bq of Sr-90, if radiation emitted from each Sr-90 is all absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, thermal energy of 2.8 MeV x 10¹⁶ Bq (= 2.8E22 eV/s), that is, at a heat generation rate of 4520 J/s, is obtained. If the specific heat of the mixture of the radioactive waste (titanate compound adsorbent to which Sr-90 has been adsorbed) 4 and glass raw material (borosilicate glass) 6 is 0.5 J/(g.K), the temperatures of the radioactive waste 4 and glass raw materials 6 are each raised by about 160°C per hour. Borosilicate glass, which is the glass raw material 6, is melted due to this temperature rise and flows into clearances among the radioactive waste 4.

[0034] In step S3, a vitrified radioactive waste 9 is produced. Specifically, since some heat is emitted to the outside through the adiabatic vessel 7 and lid 7A, an actual temperature rise rate in the solidifying vessel 3 is lower than 160°C/h. However, a temperature rise is continued with time, so all glass raw materials 6 in the solidifying vessel 3 are melted. As a result, clearances between radioactive waste 4 are filled with the molted substances of the glass raw materials 6, and a vitrified radioactive waste 9 in which a glass-solidified substance 8 exists in the solidifying vessel 3 is produced; in the vitrified radioactive waste 9, the radioactive waste 4 have been integrated by the melted substances of the glass raw materials 6. The vitrified radioactive waste 9 is taken out of the adiabatic vessel 7 and a lid (not shown) is attached to the solidifying vessel 3 of the vitrified radioactive waste 9 to seal it. Thereafter, the vitrified radioactive waste 9 is stored as a waste body at a prescribed storage place (not shown).

[0035] The present embodiment can be obtained the effects generated in the embodiment 1.

[0036] As substitute for borosilicate glass, any one of glass raw materials described in embodiment 1 may be used as the glass raw material 6. In the present embodiment, the radioactive waste 4 solidified with the glass raw material 6 may be zeolite, clinoptilolite, mordenitem, chabazite, or insoluble ferrocyanide, besides a titanate compound adsorbent.

[Embodiment 3]

[0037] A radioactive waste solidification method according to embodiment 3, which is another preferred embodiment of the present invention, will be described with reference to FIGs. 1 and 3. In the radioactive waste solidification method of the present embodiment, 100 kg of high-dose radioactive waste that include 10¹⁵ Bq of Cs-137 (for example, a spent adsorbent, for radioactive nuclides, the main component of which is insoluble ferrocyanide and to which Cs-137 was adsorbed (the adsorbent will be referred to as the insoluble ferrocyanide compound adsorbent)) as high-dose radioactive waste and 100 kg of vanadium-based glass, which is a glass raw material, with a glass softening point of about 300°C were supplied into a solidifying vessel and a vitrified radioactive waste was produced. The radioactive waste solidification method of the present embodiment will be described below.

[0038] In the present embodiment as well, a vitrified radioactive waste 9 is produced in steps S1, S2 and S3, as in embodiment 1. In adiabatic processing in step S2 in the present embodiment, however, a pressure reducing vessel (second vessel) 10 is used instead of the adiabatic vessel 7 used in step S2 in embodiments 1 and 2.

[0039] In the present embodiment, 100 kg of insoluble ferrocyanide compound adsorbent, which is the radioactive waste 4 in the waste tank 1, and 100 kg of vanadium-based glass, which is the glass raw material 6 in the glass raw material tank 1A, are supplied into the solidifying vessel (first vessel) 3 in step S1. The solidifying vessel 3 into which the radioactive waste 4 and vanadium-based glass were supplied is then disposed in the pressure reducing vessel 10 in step S2. A lid 10A is attached to the pressure reducing vessel 10 to seal it.

[0040] An exhaust pipe 12 connected to the pressure reducing vessel 10 is connected to a pressure reducing pump 11. In addition, an exhaust pipe 13 is connected to the pressure reducing pump 11. In step S2, the pressure in the sealed pressure reducing vessel 10 is reduced as described below.

[0041] After the lid 10A is attached to the pressure reducing vessel 10 to seal it, the pressure reducing pump 11 is driven and an opening/closing valve (not shown) attached to the exhaust pipe 12 is opened. When the opening/closing valve is opened, the gas in the sealed pressure reducing vessel 10 in which the solidifying vessel 3 is disposed is released to the outside through the exhaust pipe 12. This exhaust of the gas by the exhaust pipe 12 is performed until the pressure in the pressure reducing vessel 10 drops to one-tenth the atmospheric pressure. When the pressure in the pressure reducing vessel 10 becomes one-tenth the atmospheric pressure, the pressure reducing pump 11 is stopped and the opening/closing valve attached to the exhaust pipe 12 is closed. When the pressure in the sealed pressure reducing vessel 10, which surrounds the solidifying vessel 3 filled with the radioactive waste (for example, an insoluble ferrocyanide compound adsorbent) 4 and glass raw materials (vanadium-based glass) 6, is reduced to one-tenth the atmospheric pressure, adiabatic performance is improved by a factor of about 10 as compared with the use of the adiabatic vessel 7.

[0042] When the pressure in the sealed pressure reducing vessel 10 is reduced, an adiabatic area in which the pressure was reduced is formed in the pressure reducing vessel 10. The solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is disposed in the adiabatic area in the sealed pressure reducing vessel 10.

[0043] When the solidifying vessel 3 is thermally insulated by pressure reduction as described above, radiation generated due to the decay of Cs-137 included in the radioactive waste 4 in the solidifying vessel 3 is absorbed by the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 and is then converted to thermal energy. The radioactive waste 4 and glass raw materials 6 enclosed with the lid 10A and pressure reducing vessel 10 and existing in the area at a reduced pressure (adiabatic area) are heated due to this thermal energy. Accordingly, the temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 are raised. These temperatures become substantially uniform; these temperatures do not become non-uniform depending on the positions of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3.

[0044] For example, Cs-137 emits radiation with about 1.15 MeV of energy per disintegration, as described in embodiment 1. When this radiation is absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, the radiation is converted to thermal energy. Since the radioactive waste 4 includes 10¹⁵ Bq of Cs-137, if radiation emitted from each Cs-137 is all absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, thermal energy of 1.15 MeV x 10¹⁵ Bq (= 1.15E21 eV/s), that is, at a heat generation rate of 184 J/s, is obtained. If the specific heat of the mixture of the radioactive waste (an insoluble ferrocyanide compound adsorbent to which Cs-137 was adsorbed) 4 and glass raw material (vanadium-based glass) 6 is 0.5 J/(g.K), the temperatures of the radioactive waste 4 and glass raw materials 6 are each raised by about 6.6°C per hour.

[0045] In step S3, a vitrified radioactive waste 9 is produced. Specifically, since some heat is emitted to the outside through the adiabatic vessel 7 and lid 7A, an actual temperature rise rate in the solidifying vessel 3 is lower than 6.6°C/h. However, a temperature rise is continued with time, so all glass raw materials 6 in the solidifying vessel 3 are melted. As a result, clearances between radioactive waste 4 are filled with the molted substances of the glass raw materials 6, and a vitrified radioactive waste 9 in which a glass-solidified substance 8 exists in the solidifying vessel 3 is produced; in the vitrified radioactive waste 9, the radioactive waste 4 have been integrated by the melted substances of the glass raw materials 6. The vitrified radioactive waste 9 is taken out of the pressure reducing vessel 10 and a lid (not shown) is attached to the solidifying vessel 3 of the vitrified radioactive waste 9 to seal it. Thereafter, the vitrified radioactive waste 9 is stored as a waste body at a prescribed storage place (not shown).

[0046] The present embodiment can be obtained the effects generated in the embodiment 1. Particularly, in the present embodiment, the radioactive waste 4 and glass raw materials 6 existing in the adiabatic area in the sealed pressure reducing vessel 10 are uniformly heated, so the temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 become more uniform. Furthermore, in the present embodiment, the solidifying vessel 3 including the radioactive waste 4 and glass raw materials 6 is disposed in the sealed pressure reducing vessel 10 and the pressure in the pressure reducing vessel 10 is reduced, so a desired adiabatic effect can be obtained by adjusting the degree of pressure reduction.

[0047] As substitute for vanadium-based glass, any one of the glasses described in embodiment 1 may be used as the glass raw material 6. In the present embodiment, the radioactive waste 4 solidified with the glass raw material 6 may be zeolite, clinoptilolite, mordenitem, chabazite, or a titanate compound, besides an insoluble ferrocyanide compound adsorbent.

[Embodiment 4]

[0048] A radioactive waste solidification method according to embodiment 4, which is another preferred embodiment of the present invention, will be described with reference to FIGs. 1 and 4. In the radioactive waste solidification method of the present embodiment, 100 kg of high-dose radioactive waste that includes 10¹⁵ Bq of Co-60 (for example, a solid-state radioactive waste the main component of which is an iron oxide including Co-60 (the radioactive waste will be referred to as the iron oxide)) as high-dose radioactive waste and 100 kg of soda lime glass, which is a glass raw material, with a glass softening point of about 700°C were supplied into a solidifying vessel to produce a vitrified radioactive waste. The radioactive waste solidification method of the present embodiment will be described below.

[0049] In the present embodiment as well, a vitrified radioactive waste 9 is produced in steps S1, S2 and S3, as in embodiment 1. In the present embodiment, however, the radioactive waste (for example, iron oxides) 4 and glass raw materials (soda-lime glass) 6 supplied into the solidifying vessel 3 are mixed with an agitator in step S1 and in adiabatic processing in step S2, the adiabatic vessel 7 to which an air supply pipe that has an air supply pump is connected and further, an exhaust pipe is also connected is used instead of the adiabatic vessel 7 used in step S2 in embodiments 1 and 2.

[0050] In the present embodiment, 100 kg of iron oxide, which is the radioactive waste 4 supplied from the waste tank 1, and 100 kg of soda lime glass, which is the glass raw material 6 supplied from the glass raw material tank 1A, are supplied into the solidifying vessel 3 in step S1. After that, an agitator 14 is inserted into the solidifying vessel 3 in step S1. The radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 are mixed with this agitator 14.

[0051] Upon completion of the mixing of the radioactive waste 4 and glass raw materials 6, the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is disposed in the adiabatic vessel 7 in step S2. The lid 7A is attached to the adiabatic vessel 7 to seal it. An air supply pipe 17 to which an air supply pump 16 and an opening/closing valve (not shown) are attached and an exhaust pipe 18 to which an opening/closing valve (not shown) is attached are connected to the adiabatic vessel 7. A thermometer 19 is attached to the adiabatic vessel 7.

[0052] In a state in which the adiabatic vessel 7 is sealed, adiabatic processing in step S2 is performed for the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6. Radiation emitted by the decay of Co-60 included in the radioactive waste 4 existing in the solidifying vessel 3 enclosed with the sealed adiabatic vessel 7 is absorbed by the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 and is then converted to thermal energy. The radioactive waste 4 and glass raw materials 6 enclosed with the lid 7A and adiabatic vessel 7 are heated due to this thermal energy and their temperatures are raised. The temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 enclosed with the lid 7A and adiabatic vessel 7 become substantially uniform; these temperatures do not become non-uniform depending on the positions of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3.

[0053] For example, Co-60 emits radiation with about 2.5 MeV of energy per disintegration. When this radiation is absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, the radiation is converted to thermal energy. Since the radioactive waste 4 includes 10¹⁵ Bq of Co-60, if radiation emitted from each Co-60 is all absorbed in the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, thermal energy of 2.5 MeV x 10¹⁵ Bq (= 2.5 E22 eV/s), that is, at a heat generation rate of 4000 J/s, is obtained. If the specific heat of the mixture of the radioactive waste (iron oxide including Co-60) 4 and glass raw material (soda lime glass) 6 is 0.5 J/(g.K), the temperatures of the radioactive waste 4 and glass raw materials 6 are each raised by about 144°C per hour.

[0054] In step S3, a vitrified radioactive waste 9 is produced. Specifically, since some heat is emitted to the outside through the adiabatic vessel 7 and lid 7A, an actual temperature rise rate in the solidifying vessel 3 is lower than 144°C/h. However, a temperature rise is continued with time, so all glass raw materials 6 in the solidifying vessel 3 are melted. At that time, the temperature of the solidifying vessel 3 in the adiabatic vessel 7 is measured with the thermometer 19. When the temperature of the solidifying vessel 3 reaches 800°C, which is suitable for the melting of the glass raw materials (soda lime glass) 6, in order to prevent the temperature of the solidifying vessel 3 from being further raised, the opening/closing valve attached to the air supply pipe 17 and the opening/closing valve attached to the exhaust pipe 18 are opened and the air supply pump 16 is driven. As the result, an external gas (air) is supplied into the interior of the adiabatic vessel 7 through the air supply pipe 17 and the temperature of the solidifying vessel 3 is maintained at an appropriate temperature. The air supplied into the adiabatic vessel 7 is exhausted to the outside of the adiabatic vessel 7 through the exhaust pipe 18. The amount of air to be supplied into the adiabatic vessel 7 according to the temperature of the solidifying vessel 3 can be adjusted by controlling the rotational speed of the air supply pump 16.

[0055] As a result, clearances between the radioactive waste 4 are filled with the molted substances of the glass raw materials 6, and a vitrified radioactive waste 9 in which a glass-solidified substance 8 exists in the solidifying vessel 3 is produced; in the vitrified radioactive waste 9, the radioactive waste 4 have been integrated by the melted substances of the glass raw materials 6. The vitrified radioactive waste 9 is taken out of the adiabatic vessel 7 and a lid (not shown) is attached to the solidifying vessel 3 of the vitrified radioactive waste 9 to seal it. Thereafter, the vitrified radioactive waste 9 is stored as a waste body at a prescribed storage place (not shown).

[0056] The present embodiment can be obtained the effects generated in the embodiment 1. In the present embodiment, since the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 are mixed with the agitator 14, a uniform vitrified radioactive waste 9 can be produced in a shorter time. In addition, in the present embodiment, the amount of gas (for example, air) to be supplied into the adiabatic vessel 7 is adjusted based on the measured temperature of the solidifying vessel 3 in the adiabatic vessel 7, so that it is possible to prevent the temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, which are raised by heat generated due to the decay of radioactive nuclides (for example, Co-60), from exceeding a temperature necessary for glass solidification. Therefore, the evaporation of radioactive nuclides included in the radioactive waste 4 can be suppressed.

[0057] During solidification with the molted glass raw materials 6 as well, the amount of gas to be supplied into the adiabatic vessel 7 can also be adjusted based on the measured temperature of the solidifying vessel 3. Since the temperature during solidification with glass is measured and the amount of gas to be supplied is controlled, the rate at which the glass raw materials 6 are cooled can be adjusted. This can suppress the vitrified radioactive waste 9 from being cracked due to thermal distortion.

[0058] As substitute for soda lime glass, any one of the glasses described in embodiment 1 may be used as the glass raw materials 6. In the present embodiment, the radioactive waste 4 solidified with the glass raw material 6 may be zeolite, clinoptilolite, mordenitem, chabazite, an insoluble ferrocyanide compound, or a titanate compound, besides iron oxides.

[Embodiment 5]

[0059] A radioactive waste solidification method according to embodiment 5, which is another preferred embodiment of the present invention, will be described with reference to FIG. 5. In the radioactive waste solidification method according to the present embodiment, 100 kg of high-dose radioactive waste that includes 10¹⁶ Bq of Cs-137 (for example, zeolite to which Cs-137 was adsorbed) as high-dose radioactive waste and 300 kg of soda lime glass, which is a glass raw material, with a glass softening point of about 700°C were supplied into a solidifying vessel to produce a vitrified radioactive waste. The radioactive waste solidification method of the present embodiment will be described below.

[0060] In the present embodiment as well, a vitrified radioactive waste 9 is produced in steps S1, S2 and S3, as in embodiment 1. In the present embodiment, however, the radioactive waste 4 and glass raw materials 6 are supplied into the solidifying vessel 3 at the same time in step S1 and, in adiabatic processing in step S2, the pressure reducing vessel 10 is used as in embodiment 1.

[0061] In the present embodiment, 100 kg of high-dose radioactive waste 4, including 10¹⁶ Bq of Cs-137, supplied from the waste tank 1 and 300 kg of glass raw material 6 supplied from the glass raw material tank 1A are supplied into the solidifying vessel 3 at the same time in step S1. The radioactive waste 4 is spent adsorbent the main component of which is insoluble ferrocyanide and to which Cs-137 was adsorbed. The glass raw material 6 is soda lime glass. The radioactive waste 4 and glass raw materials 6 are mixed in the solidifying vessel 3. For convenience, the mixed radioactive waste 4 and glass raw materials 6 will be referred to as the mixed filler 15.

[0062] The solidifying vessel 3 filled with the mixed filler 15 is disposed in the pressure reducing vessel 10 in step S2, as in embodiment 3. The thermometer 19 is attached to the pressure reducing vessel 10. The lid 10A is attached to the pressure reducing vessel 10 to seal it. After that, the pressure reducing pump 11 is driven and the pressure in the sealed pressure reducing vessel 10 is reduced to one-tenth the atmospheric pressure, as in embodiment 3.

[0063] In a state in which the solidifying vessel 3 filled with the radioactive waste 4 and glass raw materials 6 is enclosed with the lid 10A and pressure reducing vessel 10 and disposed in the decompressed atmosphere in the pressure reducing vessel 10 the pressure in which was reduced, the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3 are heated by thermal energy generated due to the decay of Cs-137 included in the radioactive waste 4. The temperatures of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3, which is thermally insulated from the outside, are thereby raised. These temperatures become substantially uniform; these temperatures do not become non-uniform depending on the positions of the radioactive waste 4 and glass raw materials 6 in the solidifying vessel 3.

[0064] When this state is maintained, the glass raw materials 6 in the solidifying vessel 3 are melted and this molted glass raw materials 6 osmoses clearances among the radioactive waste 4. When the temperature of the solidifying vessel 3, which is measured with the thermometer 19, is raised to a temperature suitable for the melting of the glass raw materials 6, in order to prevent the temperature of the solidifying vessel 3, that is, the temperatures of the glass raw materials 6 from being further raised beyond the temperature suitable for the melting, the pressure in the pressure reducing vessel 10 is controlled by supplying air to and exhausting air from the pressure reducing vessel 10 with the pressure reducing pump 11. As a result, the temperatures of the glass raw materials 6 are maintained at an appropriate temperature.

[0065] For example, Cs-137 emits radiation with about 1.15 MeV of energy per disintegration, as described above. Therefore, when the radioactive waste 4 includes 10¹⁶Bq of Cs-137, thermal energy at a heat generation rate of 1840 J/s is obtained. If the specific heat of the mixture of the radioactive waste (zeolite to which Cs-137 was adsorbed) 4 and glass raw material (soda lime glass) 6 is 0.5 J/(g.K), the temperatures of the radioactive waste 4 and glass raw materials 6 are each raised by about 33°C per hour.

[0066] In step S3, a vitrified radioactive waste 9 is produced. Specifically, a temperature rise is continued with time, so all glass raw materials 6 are melted. As a result, clearances between radioactive waste 4 are filled with the molted substances of the glass raw materials 6, and a vitrified radioactive waste 9 in which a glass-solidified substance 8 exists in the solidifying vessel 3 is produced; in the vitrified radioactive waste 9, the radioactive waste 4 have been integrated by the melted substances of the glass raw materials 6. The vitrified radioactive waste 9 is taken out of the pressure reducing vessel 10 and a lid (not shown) is attached to the solidifying vessel 3 of the vitrified radioactive waste 9 to seal it. Thereafter, the vitrified radioactive waste 9 is stored as a waste body at a prescribed storage place (not shown).

[0067] The present embodiment can be obtained the effects generated in the embodiment 3. Furthermore, in the present embodiment, since the state of reduction in pressure in the pressure reducing vessel 10 is controlled based on the temperature of the solidifying vessel 3 that is measured during solidification with glass, temperature in solidification with glass can be controlled to a desired value. Furthermore, it becomes also possible to suppress the vitrified radioactive waste 9 from being cracked due to thermal distortion because cooling temperature after glass has been molted is controlled.

[0068] As substitute for soda lime glass, any one of the glasses described in the first embodiment may be used as the glass raw materials 6. In the present embodiment, the radioactive waste 4 solidified with the glass raw material 6 may be clinoptilolite, mordenitem, chabazite, an insoluble ferrocyanide compound, or a titanate compound, besides zeolite.

[0069] Features, components and specific details of the structures of the above-described embodiments may be exchanged or combined to form further embodiments optimized for the respective application. As far as those modifications are apparent for an expert skilled in the art they shall be disclosed implicitly by the above description without specifying explicitly every possible combination.

[REFERENCE SIGNS LIST]

[0070] 1 : waste tank, 1A : glass raw material tank, 3 : solidifying vessel (first vessel), 4 : radioactive waste, 6 : glass raw materials, 7 : adiabatic vessel (second vessel), 8 : glass-solidified substance, 10 : pressure reducing vessel (second vessel), 11 : pressure reducing pump, 14 : agitator, 16 : air supply pump, 19 : thermometer.

Claims

1. A radioactive waste solidification method comprising steps of:

supplying radioactive waste including radioactive nuclides and glass raw materials into a first vessel (3) ;

disposing the first vessel (3) in which the radioactive waste and glass raw materials (1A) exist, in an adiabatic area in a second vessel (7);

heating the radioactive waste and glass raw materials (1A) in the first vessel (3) existing in the adiabatic area in the second vessel (7) by heat generated by radiation emitted from the radioactive nuclides; and

producing a vitrified radioactive waste by melt of the heated glass raw materials.

2. The radioactive waste solidification method according to claim 1, wherein in the disposition of the first vessel (3), in which the radioactive waste and the glass raw material exist, into the adiabatic area, the first vessel (3) is disposed in an adiabatic area formed in an adiabatic vessel being the second vessel (7).

3. The radioactive waste solidification method according to claim 1, wherein in the disposition of the first vessel (3), in which the radioactive waste and the glass raw material exist, into the adiabatic area, the first vessel (3) is disposed in a pressure reducing vessel (10) being the second vessel, and a pressure in a space in which the first vessel (3) is disposed is reduced to form the adiabatic area, the space being in the pressure reducing vessel (10).

4. The radioactive waste solidification method according to any one of claims 1 to 3, wherein a temperature of the first vessel (3) disposed in the adiabatic area in the second vessel is measured, and a flow rate of a gas supplied to the adiabatic area in the second vessel is adjusted based on the measured temperature.

5. The radioactive waste solidification method according to claim 1 or 3, wherein a temperature of the first vessel (3) disposed in the adiabatic area in the second vessel is measured, and a pressure in the adiabatic area in the second vessel is controlled.

Drawing