Method for solidifying radioactive iodine-containing waste and method for manufacturing radioactive iodine-containing solidified waste

Abstract

To provide a solidification method for a radioactive iodine-containing waste capable of confining the radioactive iodine, and a manufacturing method for a radioactive iodine-containing solidified body capable of producing a solidified body having radioactive iodine confinement performance.SOLUTION: A solidification method for a radioactive iodine-containing waste comprises: a filling process in which a radioactive iodine-containing waste and a metal-containing base material are filled into a metal container; and a pressurizing process in which the filled material obtained in the filling process is pressurized by a hot isostatic pressurizing method, wherein a melting point of the metal contained in the metal-containing base material is 1,500°C or higher, and a shape of the metal-containing base material is granular, powder, plate, bar, lump, or tubular, an average particle diameter of the metal-containing base material is 5 μm or higher when the metal-containing base material filled in the filling process is granular or powder, and a processing temperature in the pressurization process is 1,100°C or higher.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for solidifying radioactive iodine-containing waste and a method for producing a radioactive iodine-containing solidified body.

Background Art

[0002] Radioactive iodine generated in a nuclear fuel reprocessing plant is usually collected by a silver adsorbent. It is necessary to dispose of the silver adsorbent with a predetermined amount of adsorbed iodine as radioactive waste. Among radioactive iodine, iodine-129 has a very long half-life of about 15 million years, so there is a possibility of long-term exposure. Therefore, when disposing of waste containing iodine-129, it is necessary to dispose of it as a solidified body having a lifespan of 100,000 years or more.

[0003] As a method for solidifying solid waste containing radioactive iodine, Patent Document 1 describes a method in which granular radioactive waste containing radioactive iodine and copper powder are filled and solidified under the condition of a temperature of 860 ° C by a hot isostatic pressing method (hereinafter sometimes referred to as "HIP method").

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When a solid waste containing radioactive iodine and a metal powder are mixed and filled and subjected to a treatment by the HIP method (hereinafter sometimes referred to as "HIP treatment"), if the treatment temperature is low, the radioactive iodine contained in the solid waste may not be confined.

[0006] The present invention has been made based on the circumstances described above, and its purpose is to provide a method for solidifying radioactive iodine-containing waste that can contain radioactive iodine, and a method for producing a radioactive iodine-containing solidified body that can produce a solidified body having radioactive iodine containment performance. [Means for solving the problem]

[0007] One aspect of the present invention, made to solve the above problems, is a method for solidifying radioactive iodine-containing waste, comprising a filling step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material, and a pressurizing step of pressurizing the filling obtained in the filling step by a hot isostatic pressurizing method, wherein the melting point of the metal contained in the metal-containing base material is 1500°C or higher, the shape of the metal-containing base material is granular, powdery, plate-shaped, rod-shaped, lump-shaped, or cylindrical, and if the metal-containing base material filled in the filling step is granular or powdery, the average particle size of the metal-containing base material is 5 μm or higher, and the processing temperature in the pressurizing step is 1100°C or higher.

[0008] Another aspect of the present invention, made to solve the above problems, is a method for producing a radioactive iodine-containing solidified body, comprising a filling step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material, and a pressurizing step of pressurizing the filling obtained in the filling step by a hot isostatic pressurizing method, wherein the melting point of the metal contained in the metal-containing base material is 1500°C or higher. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a method for solidifying radioactive iodine-containing waste that can contain radioactive iodine, and a method for producing a radioactive iodine-containing solidified body that has radioactive iodine containment capabilities. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a cross-sectional view illustrating the concept of an apparatus used in the hot isostatic pressurization method. [Modes for carrying out the invention]

[0011] [1] A method for solidifying radioactive iodine-containing waste according to one embodiment of the present invention comprises a filling step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material, and a pressurizing step of pressurizing the filling obtained in the filling step by hot isostatic pressurizing, wherein the melting point of the metal contained in the metal-containing base material is 1500°C or higher, the shape of the metal-containing base material is granular, powdery, plate-shaped, rod-shaped, lump-shaped or cylindrical, and if the metal-containing base material filled in the filling step is granular or powdery, the average particle size of the metal-containing base material is 5 μm or higher, and the processing temperature in the pressurizing step is 1100°C or higher. According to this method for solidifying radioactive iodine-containing waste, radioactive iodine contained in the radioactive iodine-containing waste can be contained. The term "melting point" refers to the melting point peak temperature measured under atmospheric pressure using a differential scanning calorimeter (DSC) in accordance with JIS-K-7121 (2012).

[0012] In general, the hot isostatic pressing (HIP) method for solidifying radioactive iodine-containing waste involves adding an alkaline solution or a metal such as copper to the radioactive iodine-containing waste, filling a metal container with these, and then pressurizing the entire filled metal container (the object to be treated) using the HIP method to obtain a solidified body. However, when an alkaline solution is used, the object to be treated contains moisture, resulting in a porous solidified body that is not sufficiently stable or corrosion-resistant in terms of long-term stability. Furthermore, when using metal materials, the temperature at which HIP treatment is possible is generally limited to about 80% to 90% of the melting point of the added metal. For example, with copper, which has a low melting point of 1085°C, the limit of the treatment temperature during HIP treatment is about 870°C. Using such metals results in a low treatment temperature during HIP treatment, which can lead to a solidified body with low density due to insufficient sintering. On the other hand, if the treatment is performed at a temperature exceeding the melting point of the added metal, insufficient sintering or voids may occur due to the melting of the added metal. A drawback of using these solidification methods is that they cannot contain the radioactive iodine contained in the waste.

[0013] On the other hand, the inventors have found that by performing HIP treatment using a metal-containing base material containing a metal with a melting point of 1500°C or higher (hereinafter, the metal contained in the metal-containing base material may be referred to as the "base metal") and radioactive iodine-containing waste, radioactive iodine contained in the waste can be contained. The reason for this is not certain, but it is thought to be as follows: By performing HIP treatment at high temperatures using a metal that does not melt even in a high-temperature environment, strong bonds are formed between the particles of the base metal. That is, depending on the treatment temperature, the bond distance between the base metal particles becomes shorter, and a denser structure can be formed. It is thought that this dense structure formed by the base metal particles restricts the free movement of radioactive iodine particles, making it possible to contain radioactive iodine in this method of solidifying radioactive iodine-containing waste.

[0014] The shape of the metal-containing base material described above may be granular, powdery, plate-like, rod-like, lump-like, or cylindrical. Furthermore, if the metal-containing base material filled in the filling process is granular or powdery (hereinafter also referred to as granular, etc.), the average particle size of the metal-containing base material must be 5 μm or larger. In addition, the processing temperature in the pressurizing process must be 1100°C or higher. Note that if the metal-containing base material is granular, etc., and the specific surface area of ​​the metal-containing base material is large, insufficient sintering or voids may occur due to oxygen generated by the decomposition of components adsorbed on the surface of the metal-containing base material or surface oxides of the metal-containing base material before and during HIP treatment. This may prevent the containment of radioactive iodine contained in the waste. On the other hand, even if the metal-containing base material is granular, by making the average particle size 5 μm or more, the specific surface area is reduced, which prevents insufficient sintering and the generation of voids caused by components adsorbed on the surface of the metal-containing base material or oxygen generated by the decomposition of surface oxides of the metal-containing base material, and allows for the formation of a dense structure. In this case, if the processing temperature during the pressurization process (HIP treatment) is 1100°C or higher, the base metal sintersects stably, making it easy to contain radioactive iodine. When the melting point of the base metal is 1500°C, 80% of that temperature is 1200°C, but by using the metal-containing base material with an average particle size of 5 μm or more, the processing temperature can be lowered to below 1200°C. In this invention, "average particle size" means the particle size at which the cumulative volume accounts for 50% in the particle size distribution measured by laser diffraction scattering.

[0015] [2] The method for solidifying radioactive iodine-containing waste described in [1] above is preferable if the corrosion rate of the metal contained in the metal-containing base material in a low-oxygen environment is 0.1 μm / year or less. With such a method for solidifying radioactive iodine-containing waste, the resulting solidified material can maintain its radioactive iodine containment performance for a long period of time in a geological disposal environment where there is almost no dissolved oxygen. The "corrosion rate in a low-oxygen environment" refers to the amount of metal corrosion per year caused by water reduction that occurs after the remaining oxygen has been consumed following the closure of the geological disposal tunnel. At the assumed disposal depth of approximately 300m to 1000m, the ground temperature is approximately 24°C to 45°C.

[0016] [3] The method for solidifying radioactive iodine-containing waste described in [1] or [2] above preferably contains at least one metal selected from zirconium and zirconium alloys as the metal-containing base material. Zirconium and zirconium alloys are inexpensive and have high melting points, making them highly practical metals. In the method for solidifying radioactive iodine-containing waste, using zirconium or a zirconium alloy as the base material metal makes it possible to perform the HIP treatment at high temperatures of, for example, 1000°C or higher, which makes the structure formed between the metal particles denser and allows for strong containment of the radioactive iodine.

[0017] [4] In the method for solidifying radioactive iodine-containing waste described in [3] above, it is possible to use spent fuel cladding waste as the metal contained in the metal-containing base material. For example, it is possible to use spent fuel cladding waste made of Zircaloy (hereinafter referred to as "Zircaloy Hull") as the zirconium alloy. Generally, Zircaloy Hull needs to be disposed of as long-half-life, low-heat radioactive waste. This method for solidifying radioactive iodine-containing waste, by using Zircaloy Hull which is radioactive waste, makes it possible to firmly contain the radioactive iodine while making the structure formed between the metal particles more dense, similar to the zirconium alloy described above, and further reduces the costs associated with the disposal of radioactive iodine-containing waste.

[0018] [5] In the method for solidifying radioactive iodine-containing waste described in [3] above, it is preferable that the main metal of the metal container is a metal that does not form a low-melting-point alloy with zirconium. A "low-melting-point alloy" refers to an alloy whose eutectic temperature is lower than the lower of the melting point of the base metal or the melting point of the main metal of the metal container. The "main metal" refers to a metallic material that makes up 50% by weight or more of the entire metal container. By performing HIP treatment at high temperatures, a low-melting-point alloy may be formed between the main metal of the metal container (hereinafter sometimes referred to as "container metal") and zirconium. If HIP treatment is continued in the presence of a low-melting-point alloy, the metal container may melt even at a temperature below the melting point of the container metal. As a result, the metal container may not be able to maintain its shape. In this method for solidifying radioactive iodine-containing waste, using a metal that does not form a low-melting-point alloy with zirconium as the container metal allows for HIP treatment at higher temperatures without deforming the metal container, and makes it possible to create a denser structure between the base metal particles.

[0019] [6] In the method for solidifying radioactive iodine-containing waste described in [3] above, it is more preferable that the main metal of the metal container is at least one metal selected from niobium and titanium. Niobium and titanium do not form the low-melting-point alloy described above with zirconium. By using niobium or titanium as the container metal, for example, HIP treatment at high temperatures of 1200°C or higher becomes possible, and the structure formed between the base metal particles becomes denser. It also improves thermal diffusion within the solidified body and makes the internal composition more uniform.

[0020] [7] In the method for solidifying radioactive iodine-containing waste described in any of [1] to [6] above, it is preferable that the processing temperature in the pressurization step is lower than the eutectic temperature between the main metal of the metal container and the metal contained in the metal-containing base material. If the HIP treatment is performed at a temperature higher than the eutectic temperature, the metal container may melt. By setting the processing temperature of the HIP treatment lower than the eutectic temperature between the container metal and the base material metal, the method for solidifying radioactive iodine-containing waste can solidify the radioactive iodine-containing waste without deforming the metal container.

[0021] 〔8〕The method for manufacturing a radioactive iodine-containing solidified body according to an embodiment of the present invention includes a step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material, and a step of pressurizing the filled material obtained in the filling step by a hot isostatic pressing method. The melting point of the metal contained in the metal-containing base material is 1500°C or higher.

[0022] According to the method for manufacturing the radioactive iodine-containing solidified body, a radioactive iodine-containing solidified body having a radioactive iodine confinement performance can be manufactured. By performing HIP treatment using a metal that does not melt even in a high-temperature environment, the particles of the base metal are firmly bonded to each other and further shrink. Depending on the high treatment temperature, the bonding distance between the base metal particles becomes closer, and a denser structure can be formed. In the radioactive iodine-containing solidified body manufactured in this way, it becomes difficult for radioactive iodine to freely move outside the solidified body. Therefore, it is considered that the radioactive iodine-containing solidified body obtained by the method for manufacturing the radioactive iodine-containing solidified body has a radioactive iodine confinement performance. In addition, by performing HIP treatment, a homogeneous radioactive iodine-containing solidified body can be manufactured.

[0023] 〔9〕The method for manufacturing a radioactive iodine-containing solidified body according to the above 〔8〕 further includes a recovery step of recovering at least one metal selected from zirconium and zirconium alloys, and the metal-containing base material preferably contains the metal recovered in the recovery step. In the method for manufacturing the radioactive iodine-containing solidified body, for example, by using a zirconium alloy recovered from a used fuel cladding tube, which is waste, it is possible to manufacture a radioactive iodine-containing solidified body having a radioactive iodine confinement performance while suppressing raw material costs and improving the efficiency of the manufacturing process. In addition, the recovery step and the filling step can be made continuous, and the manufacturing efficiency can be improved.

[0024] Hereinafter, a method for solidifying radioactive iodine-containing waste according to an embodiment of the present invention and a method for manufacturing a radioactive iodine-containing solidified body according to another embodiment will be described in detail. The names of the respective components used in each embodiment may be different from the names of the respective components used in the background art.

[0025] <Method for Solidifying Radioactive Iodine-Containing Waste> In the method for solidifying radioactive iodine-containing waste according to an embodiment of the present invention, a filling step and a pressurizing step are provided, and the melting point of the metal contained in the metal-containing base material is 1500 °C or higher.

[0026] (Filling Step) In the above filling step, radioactive iodine-containing waste and a metal-containing base material are filled into a metal container. Before the above filling step, the radioactive iodine-containing waste and the metal-containing base material may be uniformly mixed by a stirrer or the like. Also, the metal-containing base material may be filled by introducing it into a metal container containing radioactive iodine-containing waste generated in a reprocessing plant. By the above filling step, a filler in which radioactive iodine-containing waste and a metal-containing base material are filled into a metal container can be obtained.

[0027] (Metal-Containing Base Material) The above metal-containing base material refers to a base material containing a metal. The base material refers to a material that fills voids, and in the present invention, it particularly refers to a material that makes the solidified body dense and confines radioactive iodine. Also, the above base material may contain alumina, silica gel, or the like. By containing such components, radioactive iodine can be sufficiently confined.

[0028] (Base Metal) The metal contained in the above-mentioned metal-containing base material (base metal) is a metal having a melting point of 1500°C or higher. The inclusion of a high-melting-point metal in the metal-containing base material allows for a higher HIP treatment temperature, resulting in strong bonds between the base metal particles and a reduction in the bond distance. This enables the formation of a dense structure within the solidified material, allowing for the containment of radioactive iodine. Examples of such metals include zirconium, titanium, hafnium, vanadium, niobium, tantalum, palladium, rhodium, iridium, ruthenium, osmium, chromium, molybdenum, tungsten, iron, and alloys thereof. The base metal may be one of these metals alone or a mixture of two or more. It is also preferable to reuse metals from the decommissioning of nuclear facilities, etc., where the radioactive material content is below regulatory limits.

[0029] The base metal described above should preferably have a corrosion rate of 0.1 μm / year or less in a low-oxygen environment. By using a metal with a low corrosion rate in a low-oxygen environment as the base metal, the resulting solidified material can maintain a high level of containment performance for a long period of time without corrosion progressing even in deep geological (geological disposal) environments where there is almost no dissolved oxygen. Examples of such metals include zirconium, titanium, hafnium, vanadium, niobium, tantalum, palladium, rhodium, iridium, ruthenium, osmium, and alloys thereof.

[0030] The base metal can preferably be zirconium or a zirconium alloy, from the viewpoint of price, stability, melting point, and practicality. Examples of the zirconium alloy include Zircaloy-2, Zircaloy-4, zirconium-niobium alloy, and zirconium-cobalt alloy. Multiple types of zirconium alloys may be mixed and used. When such zirconium or a zirconium alloy is used as the base metal, the HIP treatment can be performed at high temperatures of, for example, 1000°C or higher, making the structure formed between the particles of the base metal denser and allowing radioactive iodine to be firmly contained.

[0031] Zirconium alloys are used in nuclear fuel rods, particularly in the fuel cladding that constitutes the fuel rods. In the nuclear fuel reprocessing process, they are recovered from the fuel cladding and disposed of as long-half-life, low-radioactive waste. In the solidification method for this radioactive iodine-containing waste, Zircaloyhull can be suitably used as the base metal from the viewpoint of reducing raw material costs. When using Zircaloyhull as the base metal, Zircaloyhull may be used alone, or it may be mixed with zirconium or a zirconium alloy.

[0032] The shape of the metal-containing base material, including the base metal mentioned above, can be granular, powdery, plate-shaped, rod-shaped, lump-shaped, or cylindrical. The shape of the metal-containing base material may be a combination of granular, powdery, plate-shaped, rod-shaped, lump-shaped, or cylindrical. Furthermore, if there are multiple metal-containing base materials, all of them may have the same shape, or some of the metal-containing base materials may have different shapes from others. In other words, metal-containing base materials of different shapes may be mixed together. As for the granular or powdery metal-containing base material, it may be obtained by crushing the rod-shaped or plate-shaped metal-containing base material using a crusher or the like. Also, when using Zircaloyhull as the base metal, fuel cladding containing zirconium alloy can be used as the metal-containing base material, and cut pieces or residues after acid treatment can be used. When using cut pieces of fuel cladding, the length of these pieces is preferably between 1 mm and 50 mm.

[0033] The metal-containing base material, which includes the above-mentioned base metal, may be in granular or powder form. The granular or powder form of the metal-containing base material facilitates uniform mixing of the radioactive iodine-containing waste and the metal-containing base material during the filling process. Furthermore, when the metal-containing base material is granular or powder, the lower limit of the average particle size of the metal-containing base material is 5 μm, preferably 5.2 μm, and more preferably 5.5 μm. By setting the average particle size of the metal-containing base material to be above this lower limit, it is possible to prevent insufficient sintering and void formation caused by components adsorbed on the surface of the metal-containing base material or oxygen generated by the decomposition of surface oxides of the metal-containing base material, and to form a dense structure. On the other hand, the upper limit of the average particle size of the metal-containing base material can be, for example, 10 μm. The average particle size of the metal-containing base material can be adjusted using a pulverizer or the like.

[0034] (Waste containing radioactive iodine) The above-mentioned radioactive iodine-containing waste is not particularly limited, but is typically an iodine adsorbent that has adsorbed radioactive iodine. Examples of iodine adsorbents include silver alumina, silver zeolite, silver silica gel, cerium oxide, and activated carbon. The above-mentioned radioactive iodine-containing waste is preferably in granular or powder form. Powdered radioactive iodine-containing waste can usually be obtained by crushing granular radioactive iodine-containing waste. By using powdered radioactive iodine-containing waste, a filler in which the granules are more uniformly dispersed can be obtained in the above-mentioned filling process, and uniform solidification can be achieved by subsequent HIP treatment. To obtain the above-mentioned powdered radioactive iodine-containing waste, crushers and classifiers are used. Examples of crushing methods include using a mortar and pestle, ball mill, sand mill, vibrating ball mill, planetary ball mill, jet mill, counter jet mill, swirling airflow type jet mill, etc.

[0035] (Radioactive iodine) The radioactive iodine contained in the above-mentioned radioactive iodine-containing waste is not particularly limited, but is typically radioactive iodine-129 adsorbed on an iodine adsorbent. The above-mentioned radioactive iodine reacts with silver (Ag) to form silver iodide (AgI), which is then adsorbed and collected on the above-mentioned iodine adsorbent. In the method for solidifying the radioactive iodine-containing waste, the radioactive iodine contained within the solidified material includes the above-mentioned silver iodide, other iodine compounds, unreacted radioactive iodine, etc.

[0036] (metal container) The above-mentioned metal container is used to fill the above-mentioned metal-containing base material and the above-mentioned radioactive iodine waste. It also serves as the outer shell of the solidified body obtained after the pressurization process. As the main metal (container metal) of the above-mentioned metal container, a metal appropriate from the viewpoint of corrosion resistance in the geological disposal environment and cost can be selected. Examples of such metals include zirconium, titanium, niobium, nickel, copper, alloys thereof, and stainless steel. The capacity and shape of the metal container can be selected arbitrarily.

[0037] (container metal) Preferably, the main metal (container metal) of the above-mentioned metal container is a metal that does not form a low-melting-point alloy with the base metal. The main metal refers to a metallic material that makes up 50% by weight or more of the total metal container. The above-mentioned metal container may have the main metal as a single component, or it may contain components other than the main metal. Furthermore, the main metal may be laminated on a base material, and the base material is not particularly limited as long as it does not deteriorate due to the HIP treatment. By using such a container metal, a dense structure can be formed in the solidified body by HIP treatment under high-temperature conditions while maintaining the shape of the metal container. The main metal of the above-mentioned metal container can be selected by referring to the phase equilibrium phase diagram of the alloy in the base metal. In the method for solidifying radioactive iodine-containing waste in which the metal-containing base material contains at least one metal selected from zirconium and zirconium alloys, it is preferable that the metal does not form a low-melting-point alloy with zirconium.

[0038] If the metal-containing base material contains at least one metal selected from zirconium and zirconium alloys, it is preferable that the main metal (container metal) of the metal container be at least one metal selected from niobium and titanium. Niobium and titanium do not form the low-melting-point alloys described above with zirconium. The eutectic temperature in a two-phase system of niobium and zirconium is approximately 1750°C, and the eutectic temperature in a two-phase system of titanium and zirconium is approximately 1550°C. In other words, in the method for solidifying radioactive iodine-containing waste, if the base material metal is zirconium, zirconium alloy, or zircaloyhull, using niobium or titanium as the container metal makes it possible to perform HIP treatment at high temperatures of, for example, 1500°C or higher while maintaining the shape of the metal container, and to form a dense structure within the solidified body.

[0039] (Mixing amount) The amount of the above-mentioned metal-containing base material mixed in is not particularly limited as long as it is within a range that can contain the radioactive iodine. From the viewpoint of adequately containing the radioactive iodine, the amount of the above-mentioned metal-containing base material mixed in is preferably in the range of 50% to 99% by weight, and more preferably in the range of 97% to 99% by weight, relative to the total weight of the above-mentioned metal-containing base material and the above-mentioned radioactive iodine waste.

[0040] The amount of base metal mixed in the above-mentioned metal-containing base material is not particularly limited as long as it can contain radioactive iodine. From the viewpoint of sufficiently containing radioactive iodine, the amount of base metal mixed is preferably in the range of 50% to 100% by weight, and more preferably in the range of 60% to 100% by weight, relative to the total weight of the metal-containing base material.

[0041] (Pressurization process) In the above pressurization step, the filling material obtained in the above filling step is pressurized by hot isostatic pressurization. This pressurization step can solidify the radioactive iodine-containing waste and the metal-containing matrix material.

[0042] (Hot isostatic pressurization method) The hot isotropic pressing (HIP) method used in the above pressurization process is a method of processing the filler material by simultaneously applying a temperature of 100°C or higher and an isotropic pressure of 10 MPa or higher to the material to be processed. An inert gas such as argon gas or nitrogen gas is used as the pressure medium. Argon gas can be preferably used because it can greatly increase the heat transfer efficiency through convection. By using an inert gas as the pressure medium, isotropic pressure can be applied to the material to be processed using the gas pressure, and deformation of the filler material can be prevented.

[0043] Figure 1 shows an example of the apparatus used in the HIP method. A metal container 13 filled with a metal-containing base material 11 and radioactive iodine-containing waste 12 is placed inside a pressure vessel 15. An inert gas is introduced into the pressure vessel 15 through a gas injection pipe 16, and at the same time, the metal container 13 is heated by an internal heater 14, thereby compressing it at a high temperature and uniform pressure.

[0044] It is preferable that the processing temperature in the pressurization step described above is lower than the eutectic temperature of the main metal (container metal) of the metal container and the metal contained in the metal-containing base material (base metal). The processing temperature can be set by referring to the phase equilibrium phase diagram in the two-phase system of the metal container and the base metal. By setting the processing temperature as described above, the radioactive iodine-containing waste can be solidified without deforming the metal container during the processing. The processing temperature in the pressurization step described above is 1100°C or higher, preferably 1150°C or higher, and more preferably 1200°C or higher. By setting the processing temperature within the above range, it is possible to form a denser structure within the solidified material, and the radioactive iodine can be sufficiently contained. Furthermore, it is preferable that the processing temperature is 0.9 times or less the melting point of the base metal, and more preferably 0.85 times or less. If HIP processing is performed at a temperature exceeding the above range, insufficient sintering or voids may occur due to the melting of the base metal, and the radioactive iodine may not be contained.

[0045] In the pressurization process described above, the applied pressure is preferably between 10 MPa and 200 MPa. If the pressure is less than 10 MPa, the containment of radioactive iodine may not be sufficient. On the other hand, a pressure exceeding 200 MPa places a heavy burden on the equipment and may worsen the operating efficiency of the equipment. The above pressure range is more preferably between 100 MPa and 200 MPa. Within this pressure range, radioactive iodine can be sufficiently contained even at a processing temperature of around 1000°C. The holding time (processing time) is preferably longer than the time required to equalize the temperature of the entire solidified material involved in the HIP treatment, specifically, it is often 1 hour or more, and more preferably 3 hours or more.

[0046] (Recovery process) When the above-mentioned Zircaloyhull is used as the base metal, the method for solidifying the radioactive iodine-containing waste may include a recovery step for recovering the Zircaloyhull. The Zircaloyhull can be recovered, for example, from the fuel cladding that constitutes a nuclear fuel rod. By further including the above recovery step, the method for solidifying the radioactive iodine-containing waste can process the radioactive iodine-containing waste using nuclear fuel rods, fuel cladding, etc., which are radioactive waste processed in the same radioactive waste treatment facility.

[0047] (Other processes) The method for solidifying the radioactive iodine-containing waste may include the following steps in addition to the above-mentioned filling step and pressurization step. Specifically, the method for solidifying the radioactive iodine-containing waste may include, for example, a step of pre-compressing the filling material in an inert gas atmosphere before HIP treatment, a step of vacuum degassing the inside of the metal container, and a post-treatment step of letting it stand in a normal temperature and pressure environment after HIP treatment.

[0048] <Method for producing a radioactive iodine-containing solidified body> Another embodiment of the present invention provides a method for producing a radioactive iodine-containing solidified body, comprising a filling step and a pressurizing step, wherein the melting point of the metal contained in the metal-containing base material is 1500°C or higher.

[0049] (filling process) In the above filling process, radioactive iodine-containing waste and a metal-containing base material are filled into a metal container. Alternatively, the filling may be carried out by adding the metal-containing base material to a metal container containing radioactive iodine-containing waste generated at a reprocessing plant or the like. Through the above filling process, a filled material can be obtained in which radioactive iodine-containing waste and a metal-containing base material are filled into a metal container. The above filling process can be the same as the filling process in one embodiment of the method for solidifying radioactive iodine-containing waste described above.

[0050] (Pressurization process) In the pressurization step, the packing material obtained in the filling step is pressurized by the hot isostatic pressing (HIP) method. This pressurization step allows for the solidification of the radioactive iodine-containing waste and the metal-containing base material. Furthermore, by performing the HIP treatment, a homogeneous radioactive iodine-containing solidified body that has been uniformly heated and pressurized can be produced. In addition, by using a metal with a melting point of 1500°C or higher as the base material metal, a radioactive iodine-containing solidified body with radioactive iodine containment capabilities can be produced. Note that the pressurization step can be the same as the pressurization step in one embodiment of the radioactive iodine-containing waste solidification method described above.

[0051] (Recovery process) The method for producing the radioactive iodine-containing solidified material further comprises a recovery step for recovering at least one metal selected from zirconium and zirconium alloys, wherein the base metal is preferably the metal recovered in the recovery step. By further comprising the recovery step, it is possible to produce a radioactive iodine-containing solidified material with radioactive iodine containment performance while suppressing raw material costs. The at least one metal selected from zirconium and zirconium alloys can be recovered, for example, from fuel cladding. The recovery step may include, for example, removing the fuel cladding from the nuclear fuel rod, cutting the fuel cladding to an arbitrary size, acid-treating the cut pieces, and washing the insoluble residue after acid treatment.

[0052] The above recovery process may include placing the recovered base metal into the metal container. By filling the metal container containing the base metal with radioactive iodine waste, the recovery process and the filling process can be carried out continuously. For example, the recovery process and the filling process can be carried out continuously in the same radioactive waste treatment facility, making it possible to improve the production efficiency of radioactive iodine-containing solidified waste.

[0053] According to the method for producing the radioactive iodine-containing solidified body, it is possible to produce a radioactive iodine-containing solidified body that has the ability to contain radioactive iodine.

[0054] <Other Embodiments> Furthermore, the method for solidifying radioactive iodine-containing waste and the method for producing a radioactive iodine-containing solidified body according to the present invention are not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and a part of the configuration of one embodiment may be replaced with the configuration of another embodiment or with well-known technology. In addition, a part of the configuration of one embodiment may be deleted. Furthermore, well-known technology may be added to the configuration of one embodiment. [Examples]

[0055] The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples.

[0056] [Example 1] A zirconium powder with an average particle size of 5.5 μm was used as the base metal to create the metal-containing base material. In addition, a mixture of silver iodide and alumina was used as a simulated radioactive iodine waste. In this example, approximately 300 g of a mixture was prepared, consisting of 97% by weight of zirconium powder and 3% by weight of a simulated silver waste adsorbent made from silver iodide and alumina.

[0057] Next, the resulting mixture was filled into a cylindrical metal container with an inner diameter of 38 mm, a length of 40 mm, and a wall thickness of 1 mm to obtain a filler. The metal container was made by machining the inside of a niobium steel bar.

[0058] After vacuum degassing the inside of the metal container, the container was sealed and subjected to HIP treatment. The HIP treatment conditions were a temperature of 1200°C, a pressure of 175 MPa, a holding time of 3 hours, and argon as the pressure medium. After the holding time had elapsed, post-treatment was performed and the metal container was removed to obtain a solidified body according to the radioactive iodine waste solidification method of Example 1.

[0059] [Example 2] A packing material was obtained in the same manner as in Example 1. The obtained packing material was subjected to HIP treatment and post-treatment under the same conditions except that the temperature was set to 1500°C. This yielded a solidified body according to the radioactive iodine waste solidification method of Example 2.

[0060] [Example 3] A packing material was obtained in the same manner as in Example 1. The obtained packing material was subjected to HIP treatment and post-treatment under the same conditions, except that the temperature was set to 1700°C. This yielded a solidified body according to the radioactive iodine waste solidification method of Example 3.

[0061] [Example 4] A 1 mm thick zirconium alloy (Zircaloy-4) plate was cut and used as the base metal to create the metal-containing base material. A mixture of silver iodide and alumina was used as simulated radioactive iodine waste. In this example, approximately 300 g of a mixture was prepared, consisting of 97% by weight of zirconium alloy and 3% by weight of a simulated silver waste adsorbent made from silver iodide and alumina. The resulting mixture was packed into a cylindrical metal container with an inner diameter of 38 mm, a length of 40 mm, and a wall thickness of 1 mm to obtain a filler. The metal container was made by molding stainless steel (SUS304). The obtained filler was subjected to HIP treatment and post-treatment under the same conditions as in Example 1, except that the temperature was set to 1000°C. This yielded a solidified body related to the radioactive iodine waste solidification method of Example 4.

[0062] [Reference example 1] A packing material was obtained in the same manner as in Example 4. The obtained packing material was subjected to HIP treatment and post-treatment under the same conditions except that the pressure was set to 10 MPa. This yielded a solidified body related to the radioactive iodine waste solidification method of Reference Example 1.

[0063] (Evaluation of solidified material) The solidified bodies produced by the radioactive iodine waste solidification methods of Examples 1 to 4 and Reference Example 1 were evaluated for their iodine containment performance by observing the cross-section of the solidified body with a scanning electron microscope and observing the external appearance of the container with the naked eye. The evaluation criteria were as follows: For the observation of the cross-section of the solidified body with a scanning electron microscope, a score of + was given if the iodine was surrounded by the base metal particles, and a score of - was given if the iodine was not surrounded by the base metal particles. For the observation of the external appearance of the container with the naked eye, a score of + was given if the metal container maintained its shape independently of the internal base material, i.e., if there was no deformation or perforation of the metal container, and a score of - was given if part of the metal container did not maintain its shape.

[0064] (Overall evaluation of containment performance) Based on the individual evaluations described above, an overall evaluation of the containment performance of radioactive iodine was conducted. Case A was defined as when the iodine was surrounded by the base metal particles and also held in place by the container; Case B was defined as when the iodine was surrounded by the base metal particles but not held in place by part of the metal container; and Case C was defined as when the iodine was not surrounded by the base metal particles.

[0065] The evaluation results are shown in Table 1.

[0066] [Table 1]

[0067] Table 1 shows that the solidification methods for radioactive iodine-containing waste in Examples 1 and 2, which used a metal-containing matrix material containing a metal with a melting point of 1500°C or higher, were able to contain the iodine. The container shape was not affected, and the resulting solidified material possessed iodine containment capabilities, indicating its suitability for long-term storage.

[0068] The solidification method for radioactive iodine-containing waste in Example 3, which involved processing at a temperature approximately 0.92 times the melting point of the base metal, was able to contain the iodine. Although deformation of the container, presumably due to the processing temperature, was observed in the solidified body, the iodine was sufficiently contained, making long-term storage of the resulting solidified body possible. Similarly, in the solidification method for radioactive iodine-containing waste in Example 4, which used stainless steel as the container metal, deformation of the container was observed, but iodine containment was possible, and the resulting solidified body can be stored for a long period. On the other hand, because a low-melting-point alloy is formed between zirconium and the iron contained in stainless steel, it becomes difficult to raise the temperature of the HIP treatment. For this reason, when HIP treatment was performed under low-pressure conditions, iodine could not be contained (see Reference Example 1).

[0069] As described above, the method for solidifying radioactive iodine-containing waste is capable of containing iodine, and the solidified material produced by this method for manufacturing radioactive iodine-containing solidified material has been shown to have radioactive iodine containment capabilities. [Industrial applicability]

[0070] The method for solidifying radioactive iodine-containing waste according to the present invention is useful as a method for treating radioactive iodine-containing waste. Furthermore, the method for producing a radioactive iodine-containing solidified body according to the present invention can be suitably used as a method for producing a radioactive iodine-containing solidified body for geological disposal. [Explanation of Symbols]

[0071] 1 HIP device 11 Metal-containing base material 12. Radioactive iodine-containing waste 13 Metal containers 14 Heater 15 Pressure vessel 16 Gas injection tube

Claims

1. A filling process in which radioactive iodine-containing waste and metal-containing base material are filled into a metal container, A pressurizing step is performed in which the filler obtained in the above filling step is pressurized by a hot isostatic pressurizing method. It is equipped with, The metal contained in the above-mentioned metal-containing base material has a melting point of 1500°C or higher, and the shape of the above-mentioned metal-containing base material is granular, powdery, plate-shaped, rod-shaped, lump-shaped, or cylindrical. If the metal-containing base material filled in the above filling process is granular or powdered, the average particle size of the metal-containing base material is 5 μm or more. A method for solidifying radioactive iodine-containing waste, wherein the processing temperature in the pressurization step is 1100°C or higher.

2. The method for solidifying radioactive iodine-containing waste according to claim 1, wherein the corrosion rate of the metal contained in the above-mentioned metal-containing base material in a low-oxygen environment is 0.1 μm / year or less.

3. The method for solidifying radioactive iodine-containing waste according to claim 1, wherein the metal-containing base material contains at least one metal selected from zirconium and zirconium alloys.

4. The method for solidifying radioactive iodine-containing waste according to claim 3, wherein the metal contained in the above-mentioned metal-containing base material is waste material from used fuel cladding tubes.

5. The method for solidifying radioactive iodine-containing waste according to claim 3, wherein the main metal of the metal container is a metal that does not form a low-melting-point alloy with zirconium.

6. The method for solidifying radioactive iodine-containing waste according to claim 3, wherein the main metal of the metal container is at least one metal selected from niobium and titanium.

7. A method for solidifying radioactive iodine-containing waste according to any one of claims 1 to 6, wherein the processing temperature in the pressurization step is lower than the eutectic temperature between the main metal of the metal container and the metal contained in the metal-containing base material.

8. A filling process in which radioactive iodine-containing waste and metal-containing base material are filled into a metal container, A pressurizing step is performed in which the filler obtained in the above filling step is pressurized by a hot isostatic pressurizing method. It is equipped with, A method for producing a radioactive iodine-containing solidified body, wherein the melting point of the metal contained in the above-mentioned metal-containing base material is 1500°C or higher.

9. The process further comprises a recovery step for recovering at least one metal selected from zirconium and zirconium alloys, The method for producing a radioactive iodine-containing solidified body according to claim 8, wherein the metal-containing base material contains the metal recovered in the recovery step.

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