Solidification method for radioactive iodine-containing waste and manufacturing method for radioactive iodine-containing solidified body

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 waste containing radioactive iodine and a method for producing a radioactive iodine-containing solidified body. [Background technology]

[0002] Radioactive iodine generated in nuclear fuel reprocessing plants is usually captured by silver adsorbents. A certain amount of iodine adsorbed by the silver adsorbent must be disposed of as radioactive waste. Among radioactive iodines, iodine-129 has an extremely long half-life of approximately 15 million years, meaning there is a possibility of long-term exposure. Therefore, when disposing of waste containing iodine-129, it must be disposed of as a solidified body with a lifespan of more than 100,000 years.

[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 is packed with copper powder and solidified at a temperature of 860°C by hot isostatic pressing (hereinafter sometimes referred to as the "HIP method"). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-62598 Summary of the Invention [Problem to be solved by the invention]

[0005] When solid waste containing radioactive iodine is mixed with metal powder, packed, and treated using the HIP method (hereinafter referred to as "HIP treatment"), if the treatment temperature is low, the radioactive iodine contained in the solid waste may not be trapped.

[0006] The present invention has been made based on the above circumstances, and an object of the present invention is to provide a method for solidifying radioactive iodine-containing waste capable of trapping radioactive iodine, and a method for producing a radioactive iodine-containing solidified body capable of producing a solidified body having the ability to trap radioactive iodine. [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 material obtained in the filling step by hot isostatic pressing, 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, powdered, plate-like, rod-like, lump-like or tubular, and when the metal-containing base material filled in the filling step is granular or powdered, the average particle size of the metal-containing base material is 5 μm or more, 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 radioactive iodine-containing solidified bodies, which includes 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 filled material obtained in the filling step by hot isostatic pressing, and the melting point of the metal contained in the metal-containing base material is 1500°C or higher. Effect of the Invention

[0009] According to the present invention, it is possible to provide a method for solidifying radioactive iodine-containing waste capable of trapping radioactive iodine, and a method for producing a radioactive iodine-containing solidified body capable of producing a solidified body having the ability to trap radioactive iodine. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view showing the concept of an apparatus used in hot isostatic pressing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [1] A method for solidifying radioactive iodine-containing waste according to one embodiment of the present invention includes 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 filler obtained in the filling step by hot isostatic pressing, in which 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-like, rod-like, block-like or cylindrical, and when 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 treatment temperature in the pressurizing step is 1100°C or higher. According to the method for solidifying radioactive iodine-containing waste, the radioactive iodine contained in the radioactive iodine-containing waste can be trapped. The "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, in the solidification method of radioactive iodine-containing waste using hot isostatic pressing (HIP), an alkaline solution or a metal such as copper is added to the waste containing radioactive iodine, and the waste is filled into a metal container, and the entire metal container after filling, which is the object to be treated, is pressurized by the HIP method to obtain a solidified body. However, when an alkaline solution is used, the object to be treated contains moisture, so the solidified body becomes porous and is not sufficient in terms of long-term stability and corrosion resistance. In addition, when a metal material is used, 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, for copper, which has a low melting point of 1085°C, the limit of the treatment temperature during HIP treatment is about 870°C. When such a metal is used, the treatment temperature during HIP treatment becomes low, and a solidified body with low density due to insufficient sintering may be produced. On the other hand, when treatment is performed at a temperature exceeding the melting point of the added metal, insufficient sintering or voids may be produced due to the melting of the added metal. When these solidification methods are used, there is an inconvenience that the radioactive iodine contained in the waste cannot be contained.

[0013] On the other hand, the inventors have found that the radioactive iodine contained in the radioactive iodine-containing waste can be trapped by performing HIP treatment using a metal-containing base material containing a metal having a melting point of 1500°C or more (hereinafter, the metal contained in the metal-containing base material may be referred to as the "base metal") and radioactive iodine-containing waste. The reason for this is not clear, but it is thought to be as follows. By performing HIP treatment at high temperature using a metal that does not melt even in a high-temperature environment, the particles of the base metal form strong bonds. In other words, depending on the treatment temperature, the bond distance between the base metal particles becomes closer, and a denser structure can be formed. In this way, the dense structure formed by the base metal particles restricts the free movement of the radioactive iodine particles, and it is thought that the radioactive iodine can be trapped in the solidification method for radioactive iodine-containing waste.

[0014] The shape of the metal-containing base material is granular, powdery, plate-like, rod-like, lump-like or cylindrical. When the metal-containing base material filled in the filling step is granular or powdery (hereinafter also referred to as granular, etc.), the average particle size of the metal-containing base material is 5 μm or more. Furthermore, the processing temperature in the pressurizing step is 1100° C. or more. When 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 components adsorbed on the surface of the metal-containing base material before and during the HIP treatment, or oxygen generated by decomposition of the surface oxide of the metal-containing base material. This may make it impossible to confine the radioactive iodine contained in the waste. On the other hand, even if the metal-containing base material is granular or the like, by making the average particle size 5 μm or more, the specific surface area can be reduced, thereby preventing insufficient sintering and the generation of voids due to components adsorbed on the surface of the metal-containing base material or oxygen generated by decomposition of the surface oxide of the metal-containing base material, and forming a dense structure. At this time, if the processing temperature of the pressurizing step (HIP processing) is 1100 ° C or more, the base metal is sintered stably, so that radioactive iodine can be easily trapped. When the melting point of the base metal is 1500 ° C, the temperature of 80% is 1200 ° C, but by using the above-mentioned metal-containing base material in a granular shape or the like with an average particle size of 5 μm or more, the processing temperature can be lowered to less than 1200 ° C. In the present invention, the "average particle size" means the particle size at which the cumulative volume is 50% in the particle size distribution measured by the laser diffraction scattering method.

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

[0016] [3] In the method for solidifying radioactive iodine-containing waste described in [1] or [2] above, it is preferable that the metal-containing base material contains at least one metal selected from zirconium and zirconium alloys. Zirconium and zirconium alloys are inexpensive, have high melting points, and are highly practical metals. In the method for solidifying radioactive iodine-containing waste, if zirconium or a zirconium alloy is used as the base metal, it becomes possible to carry out the HIP treatment under high temperature conditions, for example, at 1000°C or higher, and the structure formed between metal particles can be made denser, and the radioactive iodine can be firmly trapped.

[0017] [4] In the method for solidifying radioactive iodine-containing waste described in [3] above, the metal contained in the metal-containing base material can be waste material from spent fuel cladding tubes. For example, waste material from spent fuel cladding tubes made of zircaloy (hereinafter referred to as "zircaloy hull") can be used as the zirconium alloy. In general, zircaloy hull needs to be disposed of as long-half-life, low-heat radioactive waste. In the method for solidifying radioactive iodine-containing waste, by using zircaloy hull, which is radioactive waste, the structure formed between metal particles can be made denser, as with the zirconium alloy described above, while firmly confining radioactive iodine, and further, the cost of disposing of radioactive iodine-containing waste can be reduced.

[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. The term "low-melting-point alloy" refers to an alloy whose eutectic temperature is lower than the melting point of the base metal or the melting point of the main metal of the metal container, whichever is lower. The term "main metal" refers to a metal material that is contained in an amount of 50% by weight or more of the entire metal container. By performing HIP treatment at a high temperature, the main metal of the metal container (hereinafter sometimes referred to as "container metal") and zirconium may form a low-melting-point alloy. If the HIP treatment is continued in the presence of a low-melting-point alloy, the metal container may melt even at a temperature lower than the melting point of the container metal. This may cause the metal container to become unable to maintain its shape. In this method for solidifying radioactive iodine-containing waste, the container metal is made of a metal that does not form a low-melting point alloy with zirconium, making it possible to carry out HIP treatment under higher temperature conditions without deforming the metal container, and making it possible to make the structure formed between the base metal particles denser.

[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 above-mentioned low melting point alloy with zirconium. By using niobium or titanium as the container metal, for example, HIP treatment at a high temperature of 1200°C or more is possible, and the structure formed between the base metal particles can be made denser. In addition, it is possible to improve thermal diffusion within the solidified body and make the internal composition uniform.

[0020] [7] In the method for solidifying radioactive iodine-containing waste according to any one of [1] to [6] above, it is preferable that the treatment temperature in the pressurizing step is lower than the eutectic temperature of 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 treatment temperature of the HIP treatment lower than the eutectic temperature of 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] A method for producing radioactive iodine-containing solidified bodies according to one 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 hot isostatic pressing, and the melting point of the metal contained in the metal-containing base material is 1500°C or higher.

[0022] According to the method for producing a radioactive iodine-containing solidified body, a radioactive iodine-containing solidified body having a radioactive iodine-confining ability can be produced. 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 treatment temperature, the bond distance between the base metal particles becomes closer, and a more dense structure can be formed. In the radioactive iodine-containing solidified body produced in this way, it becomes difficult for the radioactive iodine to move freely outside the solidified body. Therefore, the radioactive iodine-containing solidified body obtained by the method for producing a radioactive iodine-containing solidified body is considered to have a radioactive iodine-confining ability. In addition, by performing HIP treatment, a homogeneous radioactive iodine-containing solidified body can be produced.

[0023] [9] The method for producing a radioactive iodine-containing solidified body described in [8] above may further include a recovery step of recovering at least one metal selected from zirconium and a zirconium alloy, and the metal-containing base material may contain the metal recovered in the recovery step. In the method for producing a radioactive iodine-containing solidified body, for example, by using a zirconium alloy recovered from a spent fuel cladding tube, which is a waste, it is possible to produce a radioactive iodine-containing solidified body that has the ability to contain radioactive iodine while reducing raw material costs and improving the efficiency of the production process. In addition, it is possible to carry out the recovery step and the filling step continuously, thereby improving production efficiency.

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

[0025] <Method of solidifying radioactive iodine-containing waste> A method for solidifying radioactive iodine-containing waste according to one embodiment of the present invention includes a filling step and a pressurizing step, and the melting point of the metal contained in the metal-containing base material is 1500° C. or higher.

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

[0027] (metal-containing base material) The above-mentioned 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, particularly refers to a material that makes the solidified body dense and confines radioactive iodine. The above-mentioned base material may also contain alumina, silica gel, etc. By containing such components, radioactive iodine can be sufficiently confined.

[0028] (Base metal) The metal contained in the metal-containing base material (base metal) is a metal having a melting point of 1500°C or more. The metal-containing base material contains a metal with a high melting point, which makes it possible to increase the processing temperature of the HIP treatment, and the particles of the base metal form strong bonds with each other and the bond distance shrinks. This makes it possible to form a dense structure in the solidified body, and radioactive iodine can be trapped. 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 two or more of them mixed together. It is also preferable to reuse the metals that are generated during the dismantling of nuclear facilities and have radioactive materials below the regulated value.

[0029] The base metal preferably has 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 obtained solidified body can maintain a high containment performance for a long period of time without corrosion even in a deep geological layer (geological disposal) environment with almost no dissolved oxygen. Examples of such metals include zirconium, titanium, hafnium, vanadium, niobium, tantalum, palladium, rhodium, iridium, ruthenium, osmium, and alloys thereof.

[0030] For the base metal, zirconium or a zirconium alloy can be suitably used from the viewpoints of price, stability, melting point, practicality, etc. Examples of the zirconium alloy include zircaloy-2, zircaloy-4, zirconium-niobium alloy, and zirconium-cobalt alloy. A mixture of a plurality of types of zirconium alloys may be used. When such zirconium or a zirconium alloy is used for the base metal, it becomes possible to carry out the HIP treatment under high temperature conditions, for example, at 1000° C. or higher, and the structure formed between the particles of the base metal can be made denser, and the radioactive iodine can be firmly trapped.

[0031] Zirconium alloys are used in nuclear fuel rods, particularly fuel cladding tubes that constitute nuclear fuel rods, and are collected from the fuel cladding tubes in the nuclear fuel reprocessing process and disposed of as long-lived low-radioactive waste. In the method for solidifying radioactive iodine-containing waste, zircaloy-hull can be suitably used as the base metal from the viewpoint of reducing raw material costs. When zircaloy-hull is used as the base metal, only zircaloy-hull may be used as the base metal, or zirconium or the zirconium alloy may be mixed.

[0032] The shape of the metal-containing base material containing the base metal is granular, powdery, plate-like, rod-like, chunky or cylindrical. The shape of the metal-containing base material may be a combination of granular, powdery, plate-like, rod-like, chunky or cylindrical. In addition, when there are a plurality of metal-containing base materials, all of the metal-containing base materials may have the same shape, or some of the metal-containing base materials may have different shapes from other parts of the metal-containing base materials. That is, the metal-containing base materials of different shapes may be mixed. The granular or powdery metal-containing base material may be one obtained by crushing the metal-containing base material of a rod-like or plate-like shape by a crusher or the like. In addition, when the zircaloy hull is used as the base metal, a fuel cladding tube containing a zirconium alloy can be used as the metal-containing base material, and cut pieces thereof or residues after acid treatment can be used. When a cut piece of a fuel cladding tube is used, the length of the cut piece is preferably about 1 mm to 50 mm.

[0033] The metal-containing base material containing the base metal may be granular or powdered. By the metal-containing base material being granular or powdered, the radioactive iodine-containing waste and the metal-containing base material can be easily mixed uniformly in the filling step. In addition, when the metal-containing base material is granular or powdered, 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 equal to or greater than the lower limit, it is possible to prevent insufficient sintering and the generation of voids due to components adsorbed on the surface of the metal-containing base material and oxygen generated by decomposition of the surface oxide 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 grinder or the like.

[0034] (Radioactive iodine-containing waste) The radioactive iodine-containing waste is not particularly limited, but is typically an iodine adsorbent that adsorbs radioactive iodine. As the iodine adsorbent, silver alumina, silver zeolite, silver silica gel, cerium oxide, activated carbon, etc. are used. The radioactive iodine-containing waste is preferably granular or powdery. Powdered radioactive iodine-containing waste can usually be obtained by pulverizing granular radioactive iodine-containing waste. By using powdered radioactive iodine-containing waste, a packing material in which the particles are more uniformly dispersed in the packing step can be obtained, and uniform solidification can be performed by the subsequent HIP treatment. In order to obtain the powdered radioactive iodine-containing waste, a pulverizer, a classifier, etc. are used. Examples of the pulverization method include a method using a mortar, a ball mill, a sand mill, a vibration ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling airflow type jet mill, etc.

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

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

[0037] (container metal) It is preferable that the main metal (container metal) of the metal container is a metal that does not form a low melting point alloy with the base metal. The main metal refers to a metal material that is contained in 50% by weight or more of the entire metal container. The metal container may be a single component of the main metal, or may contain components other than the main metal. The main metal may be laminated on a base material, and the base material is not particularly limited as long as it is not altered by 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. An appropriate metal can be selected as the main metal of the metal container by referring to the phase equilibrium 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 is a metal that does not form a low melting point alloy with zirconium.

[0038] When the metal-containing base material contains at least one metal selected from zirconium and zirconium alloys, the main metal (container metal) of the metal container is preferably at least one metal selected from niobium and titanium. Niobium and titanium do not form the above-mentioned low melting point alloy with zirconium. The eutectic temperature in a two-phase system of niobium and zirconium is about 1750°C, and the eutectic temperature in a two-phase system of titanium and zirconium is about 1550°C. That is, in the method for solidifying radioactive iodine-containing waste, when the base metal is zirconium, zirconium alloy, or zircaloy hull, by using niobium or titanium as the container metal, it becomes possible to perform HIP treatment under high temperature conditions, for example, 1500°C or higher, while maintaining the shape of the metal container, and it becomes possible to form a dense structure in the solidified body.

[0039] (Mixing amount) The amount of the metal-containing base material is not particularly limited as long as it is within a range that can confine the radioactive iodine. From the viewpoint of sufficiently confining the radioactive iodine, the amount of the metal-containing base material is preferably in the range of 50% by weight or more and 99% by weight or less, and more preferably in the range of 97% by weight or more and 99% by weight or less, based on the total weight of the metal-containing base material and the radioactive iodine waste.

[0040] The amount of the base metal mixed in the metal-containing base material is not particularly limited as long as it is within a range that allows radioactive iodine to be trapped. From the viewpoint of sufficiently trapping radioactive iodine, the amount of the base metal mixed is preferably in the range of 50% by weight to 100% by weight, more preferably in the range of 60% by weight to 100% by weight, based on the total weight of the metal-containing base material.

[0041] (Pressure process) In the pressurizing step, the filled material obtained in the filling step is pressurized by hot isostatic pressing, which makes it possible to solidify the radioactive iodine-containing waste and the metal-containing base material.

[0042] (Hot isostatic pressing method) The hot isostatic pressing (HIP) method carried out in the pressurizing step is a method of treating the filler, which is the object to be treated, by simultaneously applying a temperature of 100°C or more and an isotropic pressure of 10 MPa or more. An inert gas such as argon gas or nitrogen gas is used as the pressure medium. Argon gas is preferably used because it can increase the heat transfer efficiency by convection. By using an inert gas as the pressure medium, it is possible to apply an isotropic pressure to the object to be treated by utilizing the gas pressure, and deformation of the filler, which is the object to be treated, can be prevented.

[0043] An example of an apparatus used in the HIP method is shown in Figure 1. 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 compressed at a high temperature and with a uniform pressure by heating with an internal heater 14.

[0044] The treatment temperature in the pressurizing step is preferably 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 treatment temperature can be set by referring to a phase equilibrium diagram in a two-phase system of the metal container and the base metal. By setting the treatment temperature as described above, the radioactive iodine-containing waste can be solidified without deforming the metal container during the treatment process. The treatment temperature in the pressurizing step is 1100°C or higher, preferably 1150°C or higher, and more preferably 1200°C or higher. By setting the treatment temperature in the above range, it is possible to form a denser structure in the solidified body, and the radioactive iodine can be sufficiently trapped. In addition, the treatment temperature is preferably 0.9 times the melting point of the base metal or lower, and more preferably 0.85 times the melting point of the base metal or lower. If the HIP treatment is performed at a temperature exceeding the above treatment temperature range, insufficient sintering or voids may occur due to the melting of the base metal, and the radioactive iodine may not be trapped.

[0045] In the above pressurizing step, the pressure applied is preferably 10 MPa or more and 200 MPa or less. If the pressure is less than 10 MPa, the radioactive iodine may not be sufficiently contained. On the other hand, if the pressure exceeds 200 MPa, the burden on the device is large and the operating efficiency of the device may be deteriorated. The above pressure range is more preferably 100 MPa or more and 200 MPa or less. If the pressure range is as high as 1000°C, the radioactive iodine can be sufficiently contained. The holding time (treatment time) is preferably at least the time required to homogenize the temperature of the entire solidified body involved in the HIP treatment, specifically, 1 hour or more, and more preferably 3 hours or more.

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

[0047] (Other processes) The method for solidifying radioactive iodine-containing waste may include the following steps in addition to the filling step and the pressurizing step: For example, the method for solidifying radioactive iodine-containing waste may include a step of precompressing the filled 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 leaving the material in a room temperature and pressure environment after HIP treatment.

[0048] <Manufacturing method of radioactive iodine-containing solidified bodies> A method for producing a radioactive iodine-containing solidified material according to another embodiment of the present invention includes a filling step and a pressurizing step, and the melting point of the metal contained in the metal-containing base material is 1500° C. or higher.

[0049] (filling process) In the above-mentioned filling step, the radioactive iodine-containing waste and the metal-containing base material are filled into the metal container. Alternatively, the metal container containing the radioactive iodine-containing waste generated in a reprocessing plant or the like may be filled with the metal container by introducing the metal-containing base material. By the above-mentioned filling step, a filled material in which the radioactive iodine-containing waste and the metal-containing base material are filled into the metal container can be obtained. Note that the above-mentioned filling step can be the same as the filling step in one embodiment of the above-mentioned method for solidifying radioactive iodine-containing waste.

[0050] (Pressure process) In the pressurizing step, the filling material obtained in the filling step is pressurized by hot isostatic pressing (HIP). The radioactive iodine-containing waste and the metal-containing base material can be solidified by the pressurizing step. Furthermore, by carrying out the HIP treatment, a homogeneous radioactive iodine-containing solidified body that has been uniformly heated and pressurized can be produced. Furthermore, by using a metal having a melting point of 1500°C or higher as the base metal, a radioactive iodine-containing solidified body that has the ability to contain radioactive iodine can be produced. The pressurizing step can be the same as the pressurizing step in one embodiment of the above-mentioned method for solidifying radioactive iodine-containing waste.

[0051] (Recovery process) The method for producing a radioactive iodine-containing solidified body may further include a recovery step of recovering at least one metal selected from zirconium and zirconium alloys, and the base metal may be the metal recovered in the recovery step. By further including the recovery step, a radioactive iodine-containing solidified body having radioactive iodine containment performance can be produced while suppressing raw material costs. At least one metal selected from zirconium and zirconium alloys can be recovered, for example, from a fuel cladding tube. The recovery step may include, for example, removing the fuel cladding tube from a nuclear fuel rod, cutting the fuel cladding tube to an arbitrary size, acid-treating the cut pieces, and washing insoluble residues after the acid treatment.

[0052] The recovery step may include putting the recovered base metal into the metal container. The recovery step and the filling step can be carried out continuously by filling the metal container containing the base metal with radioactive iodine waste. For example, the recovery step and the filling step can be carried out continuously in the same radioactive waste treatment facility, which makes it possible to improve the production efficiency of radioactive iodine-containing solidified bodies.

[0053] According to the method for producing a radioactive iodine-containing solidified body, a radioactive iodine-containing solidified body having the ability to contain radioactive iodine can be produced.

[0054] <Other embodiments> The method for solidifying radioactive iodine-containing waste and the method for producing radioactive iodine-containing solidified bodies according to the present invention are not limited to the above-mentioned embodiments, and various modifications may be made without departing from the scope of the present invention. For example, the configuration of one embodiment may be added to the configuration of another embodiment, and part of the configuration of one embodiment may be replaced with the configuration of another embodiment or a well-known technique. Furthermore, part of the configuration of one embodiment may be deleted. Also, a well-known technique 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, but the present invention is not limited to the following examples.

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

[0057] The mixture was then 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 packing. The metal container was made by cutting out the inside of a niobium steel bar and shaping it.

[0058] After the inside of the metal container was evacuated, the metal 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 taken out, thereby obtaining a solidified body according to the method for solidifying radioactive iodine waste of Example 1.

[0059] [Example 2] A packing was obtained in the same manner as in Example 1. The obtained packing was subjected to HIP treatment and post-treatment under the same conditions, except that the temperature was 1500° C. As a result, a solidified body according to the method for solidifying radioactive iodine waste in Example 2 was obtained.

[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 1700° C. As a result, a solidified body according to the method for solidifying radioactive iodine waste in Example 3 was obtained.

[0061] [Example 4] A plate of zirconium alloy (Zircaloy-4) with a thickness of 1 mm was cut out and used as the base metal, which was used as the metal-containing base material. In addition, a mixture of silver iodide and alumina was used as the simulated waste of radioactive iodine waste. In this embodiment, about 300 g of a mixture was prepared by mixing 97% by weight of zirconium alloy and 3% by weight of the simulated waste silver adsorbent made from silver iodide and alumina. The 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 packing. The metal container was made by molding stainless steel (SUS304). The obtained packing 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. As a result, a solidified body related to the solidification method of radioactive iodine waste of Example 4 was obtained.

[0062] [Reference example 1] A packing was obtained in the same manner as in Example 4. The obtained packing was subjected to HIP treatment and post-treatment under the same conditions except that the pressure was 10 MPa. As a result, a solidified body according to the method for solidifying radioactive iodine waste in Reference Example 1 was obtained.

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

[0064] (Overall evaluation of containment performance) Based on the individual evaluations above, a comprehensive evaluation of the containment performance of radioactive iodine was performed. The case where iodine was surrounded by the base metal particles and the container was also held was designated A, the case where iodine was surrounded by the base metal particles but was not held by part of the metal container was designated B, and the case where iodine was not surrounded by the base metal particles was designated C.

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

[0066] [Table 1]

[0067] From Table 1, it can be seen that the methods for solidifying radioactive iodine-containing waste in Examples 1 and 2, which used a metal-containing base material containing a metal with a melting point of 1500°C or higher, were able to confine iodine. The container shape was not affected, and the obtained solidified body had iodine-confining properties, making it suitable for long-term storage.

[0068] The method for solidifying radioactive iodine-containing waste in Example 3, which was treated at a temperature about 0.92 times the melting point of the base metal, was able to confine iodine. Although deformation of the container was confirmed in the solidified body, which was presumably caused by the treatment temperature, the obtained solidified body can be stored for a long time because iodine was sufficiently trapped. In addition, the method for solidifying radioactive iodine-containing waste in Example 4, which used stainless steel as the container metal, was able to confine iodine, and the obtained solidified body can be stored for a long time, although deformation of the container was confirmed. On the other hand, since a low-melting-point alloy is generated by zirconium and the iron contained in the stainless steel, it becomes difficult to increase the temperature of the HIP treatment. For this reason, when the HIP treatment was performed under low pressure conditions, iodine could not be confined (see Reference Example 1).

[0069] As described above, it has been demonstrated that the method for solidifying waste containing radioactive iodine is capable of trapping iodine, and that the solidified bodies produced by the method for producing radioactive iodine-containing solidified bodies have the ability to trap radioactive iodine. [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. Also, the method for producing radioactive iodine-containing solidified bodies according to the present invention can be suitably used as a method for producing radioactive iodine-containing solidified bodies for geological disposal. [Explanation of symbols]

[0071] 1 HIP device 11 Metal-containing base material 12 Waste containing radioactive iodine 13 Metal container 14 Heater 15 Pressure Vessels 16 Gas injection pipe

Claims

1. A step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material; a pressing step of pressing the filled material obtained in the filling step by hot isostatic pressing; Equipped with The melting point of the metal contained in the metal-containing base material is 1500° C. or higher, and the shape of the metal-containing base material is granular, powdery, plate-like, rod-like, lump-like, or cylindrical; When 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 more; A method for solidifying radioactive iodine-containing waste, wherein the treatment temperature in the pressurizing step is 1100°C or higher.

2. 2. The method for solidifying radioactive iodine-containing waste according to claim 1, wherein 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.

3. 2. 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 the group consisting of zirconium and zirconium alloys.

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

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

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

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

8. A step of filling a metal container with radioactive iodine-containing waste and a metal-containing base material; a pressing step of pressing the filled material obtained in the filling step by hot isostatic pressing; Equipped with A method for producing a radioactive iodine-containing solidified body, wherein the melting point of the metal contained in the metal-containing base material is 1500°C or higher.

9. The method further comprises 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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