Radioactive material storage container and method for manufacturing a radioactive material storage container

Abstract

To provide a radioactive material storage container capable of achieving improvement of shielding performance more easily than before and a method for manufacturing the same.SOLUTION: A radioactive material transportation storage cask 12 includes an inner body 2, an outer cylinder 4, a neutron shielding material 5 provided between the inner body 2 and the outer cylinder 4, and a plenum pipe 10 provided inside the neutron shielding material 5 and having a void 11 therein. A method for manufacturing the radioactive material transportation storage cask 12 includes the steps of: providing the plenum pipe 10 for forming the void 11 in which no neutron shielding material 5 exists in a space for casting the neutron shielding material 5 between the inner body 2 and the outer cylinder 4; and casting the neutron shielding material 5 in the space.SELECTED DRAWING: Figure 3

Description

Technical Field

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[0001] The present invention relates to a radioactive substance storage container and a method for manufacturing the same.

Background Art

[0002] Patent Document 1 discloses a transport and storage cask for radioactive substances, comprising a neutron shielding material provided around a basket for containing radioactive substances, and a metallic member of a cylindrical outer cylinder provided outside the neutron shielding material. An air gap secured as an expansion allowance for the neutron shielding material is formed in an annular shape in the cask body such that the neutron shielding material and the metallic member form a double-cylindrical structure. The air gap is divided in the circumferential direction into a plurality of cells by heat transfer fins, and further, the plurality of cells are partially connected through a communication portion, and at least one relief valve that opens to the outside at a pressure equal to or higher than a predetermined pressure with respect to the plurality of connected cells is provided.

Prior Art Documents

[0006] Furthermore, under normal transport or storage conditions for casks, decay heat is generated from the radioactive materials such as spent fuel loaded into the casks. Therefore, even if the decay heat is removed by the heat transfer fins, the neutron shielding material is designed to reach a temperature of approximately 120°C. In contrast, the resin-based and rubber-based materials that form the neutron shielding material generally have a heat resistance temperature of around 120°C to 150°C, although this depends on the properties of the material.

[0007] Therefore, if a cask is used to store radioactive materials for several decades, even under normal transport or storage conditions, water vapor or other gases gradually generated from the neutron shielding material can accumulate, increasing the pressure in the neutron shielding space and potentially compromising the safety of the cask. To address this, a plenum space is provided above the neutron shielding space to keep the internal pressure below a certain level.

[0008] Patent Document 1, mentioned above, describes a cask that can ensure safety even during normal transportation or storage.

[0009] However, if a plenum space is provided above the neutron shielding space to keep the internal pressure of the neutron shielding space below a certain value, taking into account the water vapor or other gases that are gradually generated and accumulated from the neutron shielding material, a decrease in shielding performance in that area is unavoidable because the plenum space is not filled with neutron shielding material, and there is room for improvement.

[0010] The object of the present invention is to provide a radioactive material storage container and a method for manufacturing a radioactive material storage container that can easily achieve improved shielding performance compared to conventional methods. [Means for solving the problem]

[0011] The present invention includes multiple means for solving the above problems, but one example is a container comprising an inner cylinder, an outer cylinder, a neutron shielding material provided between the inner cylinder and the outer cylinder, and a partition provided inside the neutron shielding material and having an air gap on its inner side. [Effects of the Invention]

[0012] According to the present invention, improved shielding performance can be achieved more easily than in the conventional method. Other issues, configurations, and effects will be clarified by the following description of the embodiments. [Brief explanation of the drawing]

[0013] [Figure 1] A perspective view showing the general configuration of a conventional radioactive material storage container. [Figure 2] A cross-sectional view of one side of a conventional radioactive material storage container. [Figure 3] A longitudinal cross-sectional view of a radioactive material storage container according to this embodiment. [Figure 4] Enlarged top view of section X in Figure 3. [Figure 5] Enlarged view of section AA in Figure 4. [Modes for carrying out the invention]

[0014] Examples of the radioactive material storage container and the method for manufacturing the radioactive material storage container of the present invention will be described with reference to Figures 1 to 5.

[0015] In the following embodiments, a radioactive material transport and storage cask will be used as an example of a radioactive material storage container.

[0016] Figure 1 is a perspective view showing the general configuration of a conventional radioactive material storage container, Figure 2 is a one-sided longitudinal section of a conventional radioactive material storage container, Figure 3 is a longitudinal section of a radioactive material storage container according to this embodiment, Figure 4 is an enlarged top view of section X in Figure 3, and Figure 5 is an enlarged view of section AA in Figure 4.

[0017] In the drawings used in this specification, the same or corresponding components are denoted by the same or similar reference numerals, and repeated explanations of these components may be omitted.

[0018] The conventional radioactive material transport and storage cask 1 shown in FIGS. 1 and 2 is a container for accommodating spent fuel assemblies, and has an inner cylinder 2, a basket 3, an outer cylinder 4, a neutron shielding material 5, heat transfer fins 6, an upper trunnion 7, a lower trunnion 8, and the like. In the radioactive material transport and storage cask 1, a void (hereinafter referred to as "plenum space 9") is formed on the upper end side of the neutron shielding material 5.

[0019] The radioactive material transport and storage cask 1 is formed in a bottomed cylindrical shape, and a basket 3 is inserted into a cylindrical inner cylinder 2 made of carbon steel for ensuring a gamma-ray shielding function and structural strength. The spent fuel assembly is loaded as a radioactive material inside the basket 3.

[0020] As shown in FIGS. 1 and 2, a neutron shielding material 5 mainly made of a material such as resin or rubber is disposed covering the outer cylinder 4 on the outer peripheral portion of the inner cylinder 2 and the inner peripheral portion of the cylindrical outer cylinder 4 made of carbon steel or stainless steel, thereby forming a shielding layer.

[0021] Also, as shown in FIGS. 1 and 2, an upper trunnion 7 and a lower trunnion 8 used as handles when transporting the radioactive material transport and storage cask 1 are provided on the outer peripheral surface of the outer cylinder 4. Further, as shown in FIG. 2, a plenum space 9 is provided between the neutron shielding material 5 and the outer cylinder 4 as an expansion allowance for the neutron shielding material 5.

[0022] In contrast, the radioactive material transport and storage cask 12 of this embodiment, shown in Figures 3 to 5, is a container for housing spent fuel assemblies and has an inner shell 2, a basket 3, an outer cylinder 4, a neutron shielding material 5, heat transfer fins 6, an upper trunnion 7, a lower trunnion 8, etc. It is the same as the radioactive material transport and storage cask 1 in that it is a bottomed cylindrical container with a plenum space 9 formed on the upper end side of the neutron shielding material 5. The difference is that it is provided with at least one plenum tube 10 located inside the neutron shielding material 5 and having an air gap 11 inside it.

[0023] The plenum tube 10 is a partition that forms a void 11 inside the neutron shielding material 5, similar to the plenum space 9. It is installed along the long axis of the radioactive material transport and storage cask 12 to an arbitrary height using a tube or the like. The plenum tube 10 is provided with sufficient length to account for the drop in liquid level when the neutron shielding material 5 is cast.

[0024] Furthermore, the plenum tube 10 does not need to have a round cross-section; it can be square, or even a polygon with sides other than a square, or any other shape. Also, the plenum tube 10 used to form the partition does not need to be a tube; it may be made up of multiple plates or the like.

[0025] Furthermore, although the plenum tube 10 is shown as extending in the longitudinal direction of the radioactive material transport and storage cask 12, it is not limited to an axially extending configuration, as long as a void 11 that replaces the plenum space 9 can be formed inside the neutron shielding material 5. For example, a circumferentially extending donut-shaped tube can be provided between the inner shell 2 and the outer cylinder 4, either in place of or in addition to the axially extending tube, to form a void.

[0026] It is desirable that the inside of the plenum tube 10 be under a constant pressure equivalent to that of the space where the neutron shielding material 5 is installed, with its upper end connected to the plenum space 9 and its lower end in contact with the neutron shielding material 5.

[0027] The material of the plenum tube 10 can be any material commonly used in the nuclear field, and is not particularly limited.

[0028] The position of the plenum tube 10 in the cross-sectional direction within the neutron shielding material 5 is not particularly limited. While Figures 3 to 5 illustrate an example where it is in contact with the outer surface of the inner shell 2, it is not necessary to have this arrangement. It is not limited to arrangements where it is not in contact with the inner shell 2 or the outer cylinder 4, or arrangements where it is in contact with the inner surface of the outer cylinder 4.

[0029] Furthermore, there are no particular limitations on the number of plenum tubes 10 that can be arranged. However, as a result of the inventors' studies, it was found that it is desirable for the cross-sectional area of ​​the void 11 in the long axis direction of the radioactive material transport storage cask 12 to be 10% or less of the cross-sectional area of ​​the neutron shielding material 5, so that the dose equivalent rate at a distance of 1 m from the surface of the radioactive material transport storage cask 12 is below the legal standard of 100 μSv / h.

[0030] By installing such a plenum tube 10, it becomes possible to reduce or eliminate the plenum space 9 above the neutron shielding material 5, making it possible to cast the neutron shielding material 5 further up to the upper side of the radioactive material transport and storage cask 12. This makes it possible to easily improve the shielding function at the upper side of the radioactive material transport and storage cask 12 compared to conventional methods.

[0031] Next, a method for manufacturing the radioactive material transport and storage cask 12 according to this embodiment will be described.

[0032] First, in the case of a radioactive material transport and storage cask 1 that does not have a plenum tube 10 as a partition as in the present invention, a sufficient casting height of the neutron shielding material 5 is required on the side of the radioactive material transport and storage cask 1 in order to not reduce the shielding function of the upper side.

[0033] As shown in Figure 2, a plenum space 9 of a certain volume is required above the neutron shielding material 5 to allow for expansion of the neutron shielding material 5 and for gases generated from the neutron shielding material 5 to escape. When casting the neutron shielding material 5, a drop in the liquid level occurs due to hardening caused by a decrease in temperature, so it was necessary to cast the neutron shielding material 5 multiple times.

[0034] In contrast, in the present invention, a plenum tube 10 is provided in the space between the inner shell 2 and the outer cylinder 4 where the neutron shielding material 5 is cast, to form a gap 11 where the neutron shielding material 5 is not present, and then the neutron shielding material 5 is cast into the space.

[0035] In the process of casting the neutron shielding material 5, it is desirable to provide a cover on the lower end of the plenum tube 10 and remove the cover after the casting is complete. There are no particular limitations on the method of providing the cover on the lower end during casting and then removing it.

[0036] This process allows the plenum space 9, which was located above the neutron shielding material 5 cast into the side of the radioactive material transport and storage cask 12, to be reduced by the plenum tube 10. This makes it possible to increase the casting height of the neutron shielding material 5 compared to the conventional manufacturing height, and to allow for a wider tolerance in the casting height. As a result, it becomes possible to cast the neutron shielding material 5 in one or fewer steps.

[0037] Next, the effects of this embodiment will be described.

[0038] The radioactive material transport and storage cask 12 of this embodiment described above comprises an inner shell 2, an outer cylinder 4, a neutron shielding material 5 provided between the inner shell 2 and the outer cylinder 4, and a plenum tube 10 provided inside the neutron shielding material 5 and having an air gap 11 on its inner side.

[0039] Furthermore, the method for manufacturing the radioactive material transport storage cask 12 includes the steps of: providing a plenum tube 10 to form a gap 11 in which the neutron shielding material 5 is not present in the space between the inner shell 2 and the outer cylinder 4 into which the neutron shielding material 5 is cast; and casting the neutron shielding material 5 into the space.

[0040] Therefore, in addition to the plenum space 9 on the upper side, a void 11 can be secured as an air gap to allow water vapor and gas to escape, which are generated when the temperature of the neutron shielding material rises due to the decay heat of the radioactive material stored in the neutron shielding material casting section and the cask. This suppresses pressure due to thermal expansion of the neutron shielding material 5, reduces stress generated by internal pressure, and allows for a structure that does not require special design for the generated stress, thereby simplifying the structure of the storage container. Furthermore, since the plenum space 9 is secured in advance by a partition, the neutron shielding material 5 can be cast up to the top of the container, exceeding 80-90% of the total space of the neutron shielding material 5 on the side of the body, which is the height of the neutron shielding material 5 after casting in conventional containers. This improves the shielding function compared to conventional containers.

[0041] In particular, when casting the neutron shielding material 5, the liquid level of the neutron shielding material 5 decreases due to hardening after casting. However, since the casting height of the neutron shielding material 5 affects the shielding performance, it is difficult to increase the margin of the plenum space 9 above the neutron shielding material 5. Therefore, because it is difficult to ensure sufficient tolerance from the design value, it was necessary to cast the neutron shielding material in multiple stages.

[0042] However, in the present invention, by setting the plenum space 9 and the gap 11, it is easy to secure a sufficient volume of the plenum, and it is possible to cast the neutron shielding material 5 up to the top of the cask. Therefore, even if the liquid level drops due to the hardening of the neutron shielding material 5 after casting, the minimum casting height required for shielding can be satisfied, and the number of casting steps for the neutron shielding material can be reduced.

[0043] Furthermore, because the partition is a pipe, it is possible to easily form a partition to secure the air gap 11.

[0044] Furthermore, because the plenum tube 10 has a round or square cross-sectional shape, it is easier to procure tubes that make up the void 11.

[0045] Furthermore, by ensuring that the cross-sectional area of ​​the void 11 in the long axis direction of the radioactive material transport and storage cask 12 is 10% or less of the cross-sectional area of ​​the neutron shielding material 5, the dose equivalent rate at a distance of 1 m from the surface of the radioactive material transport and storage cask 12 is less than 100 μSv / h, thereby reliably preventing a decrease in shielding capacity caused by thinning of the neutron shielding material 5 due to the installation of the plenum tube 10.

[0046] Furthermore, because the plenum tube 10 extends in the longitudinal direction of the radioactive material transport and storage cask 12, a plenum space for the escape of water vapor and gas can be easily secured on the lower end side of the neutron shielding material 5, and it is easy to connect the void 11 to the plenum space 9 on the upper end side, and it is also easy to secure the volume of the void 11, thus making it easier and more reliable to obtain the function of ensuring safety.

[0047] Furthermore, in the process of casting the neutron shielding material 5, a lid is provided on the lower end side of the plenum tube 10, and the lid is removed after the casting is complete. This prevents the neutron shielding material 5 from flowing into the plenum tube 10 during casting, ensuring the formation of the void 11.

[0048] <Other> It should be noted that the present invention is not limited to the embodiments described above, and various modifications and applications are possible. The embodiments described above are explained in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described. [Explanation of symbols]

[0049] 1…Radioactive material transport and storage cask 2...Inner body (inner cylinder of the container) 3...Basketball 4…Outer cylinder 5… Neutron shielding material 6… Heat transfer fins 7… Upper trunnion 8…Lower trunnion 9…Plenum space 10… Plenum tube (partition) 11...Void 12…Radioactive material transport and storage casks (containers for storing radioactive materials)

Claims

1. The inner cylinder of the container, Outer cylinder and A neutron shielding material is provided between the inner cylinder and the outer cylinder of the container, A plenum space provided above the neutron shielding material, A partition is provided, at least a portion of which is located inside the neutron shielding material, and which has an air gap inside it, The upper end of the partition protrudes upward from the upper end of the neutron shielding material. A container for storing radioactive materials.

2. In the radioactive material storage container according to Claim 1, The plenum space and the void are connected. A container for storing radioactive materials.

3. In the radioactive material storage container according to claim 1, The partition is a plenum tube. A container for storing radioactive materials.

4. In the radioactive material storage container according to claim 3, The plenum tube has a round or square cross-sectional shape. A container for storing radioactive materials.

5. In the radioactive material storage container according to claim 1, The cross-sectional area of ​​the void in the longitudinal direction of the radioactive material storage container is 10% or less of the cross-sectional area of ​​the neutron shielding material, such that the dose equivalent rate at a distance of 1 m from the surface of the radioactive material storage container is less than 100 μSv / h. A container for storing radioactive materials.

6. In the radioactive material storage container according to claim 3, The plenum tube extends in the longitudinal direction of the radioactive material storage container. A container for storing radioactive materials.

7. In the radioactive material storage container according to Claim 3, The lower end of the plenum tube is in contact with the neutron shielding material. A container for storing radioactive materials.

8. A method for manufacturing a radioactive material storage container, A step of providing a partition in the space between the inner and outer cylinders of the container where the neutron shielding material is cast, to create an empty space where the neutron shielding material is not present, The process includes the step of casting the neutron shielding material into the space, In the process of casting the neutron shielding material, a lid is provided on the lower end side of the partition, and the lid is removed after the casting is complete. A method for manufacturing a radioactive material storage container.

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