Method for managing buffers in radioactive material storage containers, and management system for buffers in radioactive material storage containers

Abstract

To provide a method for managing the buffer material of a radioactive material storage container, and a management system for the buffer material of a radioactive material storage container, which can appropriately manage changes in the strength of the buffer material caused by moisture absorption. [Solution] The method for managing the buffer material of a radioactive material storage container according to this disclosure includes the steps of measuring the moisture content of a test piece of the buffer material and estimating the compressive strength of the buffer material based on the measured moisture content.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for managing a buffer body of a radioactive substance storage container, and a system for managing a buffer body of a radioactive substance storage container.

BACKGROUND ART

[0002] Radioactive waste such as spent fuel generated in nuclear power facilities is stored in a radioactive substance storage container, then transported to a storage facility, a reprocessing facility, or the like, to be stored or reprocessed. The radioactive substance storage container is provided with, for example, buffer bodies at its axial end portions.

[0003] The buffer body absorbs impact by deforming when the radioactive substance storage container is dropped, overturned, or collided. Since the buffer body is required to have a buffer function, wood or foamed resin having excellent compressive strength is used therefor. However, these materials undergo changes in strength properties due to moisture absorption. Therefore, it has been necessary to monitor changes in strength properties resulting from moisture absorption.

[0004] For example, the following Patent Document 1 discloses a protection device for protecting a radioactive substance storage container, the protection device comprising: a hollow support member provided on the outer side of the radioactive substance storage container; a shock absorbing material disposed inside the support member to absorb impact by deforming; and a deterioration monitoring member disposed under the same temperature environment as the shock absorbing material and made of a material having equivalent aging deterioration properties to the shock absorbing material.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0005]

PATENT DOCUMENT 1

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0006] The protective device for radioactive material storage containers described in Patent Document 1 above required test specimens of sufficient size and shape to conduct strength tests on the degradation monitoring members. Therefore, in order to minimize the impact on the function of the buffer, it was necessary to manage changes in the strength characteristics of the buffer caused by moisture absorption using a small amount of test specimens.

[0007] In view of the above issues, this disclosure aims to provide a method for managing the buffer material of a radioactive material storage container, and a management system for the buffer material of a radioactive material storage container, which can appropriately manage changes in the strength of the buffer material due to moisture absorption. [Means for solving the problem]

[0008] To solve the above-mentioned problems and achieve the objectives, the method for managing the buffer material of a radioactive material storage container according to this disclosure includes the steps of: creating a strength characteristic map showing the relationship between the moisture content and compressive strength of the buffer material; taking a test piece of the buffer material from the buffer material and measuring the moisture content of the test piece; and using the strength characteristic map, estimating the compressive strength of the buffer material based on the measured moisture content.

[0009] To solve the above-mentioned problems and achieve the objectives, the buffer management system for a radioactive material storage container according to this disclosure includes: a strength characteristic map storage unit that stores a strength characteristic map showing the relationship between the moisture content and compressive strength of the buffer material; a moisture content acquisition unit that obtains the moisture content of a test piece measured by removing the test piece of buffer material from the buffer; and a compressive strength estimation unit that estimates the compressive strength of the buffer material based on the measured moisture content using the strength characteristic map. [Effects of the Invention]

[0010] According to this disclosure, it is possible to provide a method for managing the buffer material of a radioactive material storage container, and a management system for the buffer material of a radioactive material storage container, which can appropriately manage changes in the strength of the buffer material due to moisture absorption. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a perspective view showing a section of a radioactive material containment container that has been fractured. [Figure 2] Figure 2 is a plan view of a radioactive material containment container fitted with a buffer. [Figure 3] Figure 3 is a front view showing a vertically stored radioactive material container fitted with a buffer. [Figure 4] Figure 4 is a front view showing a horizontal storage of a radioactive material container. [Figure 5] Figure 5 is a cross-sectional view showing the internal structure of the buffer. [Figure 6] Figure 6 is a flowchart showing the flow of the method for estimating the strength of the cushioning material according to this disclosure. [Figure 7] Figure 7 shows an example of an intensity characteristic map related to this disclosure. [Figure 8] Figure 8 is a flowchart showing the flow of the method for managing the buffer material of a radioactive material storage container according to this disclosure. [Figure 9] Figure 9 shows an example of the configuration of a buffer management system for a radioactive material storage container according to this disclosure. [Modes for carrying out the invention]

[0012] Embodiments of this disclosure will be described in detail below with reference to the drawings. However, the present invention is not limited to the embodiments described below.

[0013] (Composition of radioactive material storage containers) First, the radioactive material storage container related to this disclosure will be explained using Figure 1. Figure 1 is a perspective view of a portion of the radioactive material storage container in a cross-section.

[0014] As shown in Figure 1, the cask 11, which serves as a container for radioactive material, comprises a body 12 and a lid 13.

[0015] The body portion 12 has a container main body 21. As shown in Figure 1, the container main body 21 may be formed in a cylindrical shape, with an opening 22 formed at the upper end and the lower end closed. The container main body 21 has a cavity 23 which is a hollow space inside, and a basket 24 is provided inside the cavity 23. The basket 24 is provided with a plurality of cells 25 capable of independently storing radioactive substances (for example, spent fuel assemblies).

[0016] The container main body 21 may be a forged product made of carbon steel having a gamma ray shielding function, but stainless steel can also be used in place of carbon steel. Further, cast products such as spheroidal graphite cast iron and carbon steel cast steel may be used for the container main body 21.

[0017] The body portion 12 is provided with an outer cylinder 26 with a predetermined gap left on the outer peripheral surface of the container main body 21. The container main body 21 is provided with a plurality of copper heat transfer fins 27 responsible for heat conduction between the outer peripheral surface and the inner peripheral surface of the outer cylinder 26, arranged in the circumferential direction. In the space surrounded by the outer cylinder 26 and the heat transfer fins 27 of the container main body 21, a resin (neutron shield) 28 containing boron or a boron compound, which is a polymer material rich in hydrogen and has a neutron shielding function, is provided.

[0018] The body portion 12 may be provided with a bottom portion 29 protruding downward from the closed lower end of the container main body 21. A resin (neutron shield) is provided in the space surrounded by the bottom portion 29 and the closed lower end of the container main body 21.

[0019] The body portion 12 is provided with trunnions 30 for lifting the cask 11. The trunnions 30 are cylindrical projections as shown in Figure 1. The trunnions 30 are provided so as to penetrate from the container main body 21 through the outer cylinder 26, and protrude to the outermost periphery in the cask 11.

[0020] The lid portion 13 is provided at the opening 22 of the container body 21 and seals the container body 21 (body portion 12) by closing the opening 22. The lid portion 13 includes a primary lid 31 and a secondary lid 32. The primary lid 31 is formed in a disc shape from carbon steel or stainless steel that shields against gamma rays. The secondary lid 32 covers the primary lid 31 and is visible on the outside of the cask 11, and like the primary lid 31, is formed in a disc shape from carbon steel or stainless steel that shields against gamma rays. A resin (neutron shielding material) 28 may be provided between the primary lid 31 and the secondary lid 32. In addition, the lid portion 13 may be further provided with a tertiary lid or an adapter.

[0021] The primary lid 31 is attached to the container body 21 by being fixed to the first stage portion 22a formed in the opening 22 of the container body 21 with bolts (not shown) made of carbon steel or stainless steel. The secondary lid 32 is attached to the container body 21 by being fixed to the second stage portion 22b formed in the opening 22 of the container body 21 with bolts (not shown) made of carbon steel or stainless steel. Although not shown, metal gaskets are provided between the primary lid 31 and the first stage portion 22a, and between the secondary lid 32 and the second stage portion 22b. The metal gaskets ensure airtightness between the primary lid 31 and the first stage portion 22a, and between the secondary lid 32 and the second stage portion 22b.

[0022] The primary lid 31 is fixed to the first stage portion 22a formed in the opening 22 of the container body 21 by bolts (not shown) made of carbon steel or stainless steel, and is attached to the container body 21. The secondary lid 32 is fixed to the second stage portion 22b formed in the opening 22 of the container body 21 by bolts (not shown) made of carbon steel or stainless steel, and is attached to the container body 21. Although not shown, metal gaskets are provided between the primary lid 31 and the first stage portion 22a, and between the secondary lid 32 and the second stage portion 22b. The metal gaskets ensure airtightness between the primary lid 31 and the first stage portion 22a, and between the secondary lid 32 and the second stage portion 22b.

[0023] The body portion 12 has a cylindrical upper edge 22c that surrounds the secondary lid 32 at the opening 22 of the container body 21 and is visible on the outside of the cask 11. The upper edge 22c has an upper surface that is higher than the surface of the secondary lid 32 and surrounds the secondary lid 32. Multiple bolt holes 33 for attaching the buffer A, which will be described later, are provided on the upper surface of the upper edge 22c at intervals in the circumferential direction.

[0024] (Regarding the buffer material) Next, the buffer according to this disclosure will be described using Figures 2 and 3. Figure 2 is a plan view showing a radioactive material storage container fitted with a buffer. Figure 3 is a front view showing the vertical storage of the radioactive material storage container fitted with a buffer. That is, Figure 2 is a view of the vertical storage state of the radioactive material storage container shown in Figure 3, as seen from above in the vertical direction of Figure 3, along the central axis O. As shown in Figure 2, a plurality of divided block-shaped buffer materials 43 are inserted into the support member 42. Also, as shown in Figure 3, when the cask 11 is stored, its bottom 29 is fitted into a frame 37 installed on the floor F, or the trunnion 30 is secured and fixed to the frame 37, and the cask is stored in a vertical position with its central axis O aligned in the vertical direction.

[0025] The buffer body 41 is attached, for example, to the cask 11 that has been transported to a storage facility, which is a storage location.

[0026] A transport buffer (not shown) is attached to the cask 11 during transport. When transported to the storage facility, the transport buffer is removed, and the cask is stored upright on the floor F with the lid 13 facing upwards, as shown in Figure 3, and a buffer 41 is attached to the lid 13 side.

[0027] The support member 42 has a disc portion 51 and a cylindrical portion 52. The cylindrical portion 52 is provided on the radially outer side of the disc portion 51. The support member 42 is a hollow member formed from a metal plate such as a steel plate, and is formed to be deformable under a predetermined load. The predetermined load is the load that will be applied to the cask 11 when it is assumed to fall, or the load that will be applied to the cask 11 when it hits the floor F if the cask 11 that is standing on the floor F falls over.

[0028] The disc portion 51 is positioned above the lid portion 13 of the cask 11 and along the surface of the tertiary lid or adapter that is visible on the outside of the lid portion 13. The disc portion 51 is positioned to cover the surface of the secondary lid 32 and the surface of the upper edge 22c of the container body 21. The disc portion 51 is fixed to the container body 21 by bolts (not shown) that are attached to bolt holes 33 provided in the upper edge 22c. If a tertiary lid is provided as the lid portion, the disc portion 51 is positioned along the surface of the tertiary lid that is visible on the outside of the lid portion 13 and is positioned to cover the surface of the tertiary lid and the surface of the upper edge 22c of the container body 21.

[0029] The cylindrical portion 52 is positioned to surround the outside of the upper edge 22c of the body portion 12. That is, the cylindrical portion 52 is positioned radially outward from the lid portion 13 and protrudes radially outward from the body portion 12.

[0030] The cushioning material 43 is placed inside and supported by the support member 42. The cushioning material 43 deforms under a predetermined load. The cushioning material 43 is made of wood, porous material, foam material, or polymer compound.

[0031] (Regarding horizontal storage) Next, the horizontal storage of radioactive material containers related to this disclosure will be explained using Figure 4. Figure 4 is a front view showing the horizontal storage of radioactive material containers.

[0032] As shown in Figure 4, the radioactive material storage container 11 may also be stored in a horizontal position with its central axis O aligned horizontally. In this case, a buffer 41 is attached to one side of the radioactive material storage container 11 along the central axis O (the lid 13 side), and a buffer 46 is attached to the other side along the central axis O (the bottom 29 side). The buffer 46 has a configuration almost identical to that of the buffer 41 described above.

[0033] (Internal structure of the buffer) Next, the internal structure of the buffer according to this disclosure will be explained using Figure 5. Figure 5 is a cross-sectional view showing the internal structure of the buffer.

[0034] As shown in Figure 5, the buffer body 41 has a support member 42 and a buffer material 43, with the buffer material 43 being arranged inside the hollow support member 42. The buffer material 43 may be composed of a combination of a first buffer material 43a, a second buffer material 43b, and a third buffer material 43c. The first buffer material 43a, the second buffer material 43b, and the third buffer material 43c are formed by combining multiple blocks of wood. The buffer body 41 is not limited to the three-part configuration of the first buffer material 43a, the second buffer material 43b, and the third buffer material 43c, but may be divided into two or four or more parts. Furthermore, the buffer body 41 may be configured with each of the first buffer material 43a, the second buffer material 43b, and the third buffer material 43c as a single integrated unit, or as a divided unit with each of them separated into multiple parts.

[0035] The first cushioning material 43a is provided along the circumferential direction on the outside of the circumferential side member 53 and the outside of the end face side member 54 of the support member 42. The first cushioning material 43a is composed of multiple blocks divided in at least one of the circumferential and radial directions of the cushioning body 41.

[0036] The second cushioning material 43b is provided adjacent to the first cushioning material 43a along the circumferential direction on the outside of the circumferential side member 53 and on the upper side of the end face side member 54 of the support member 42. The second cushioning material 43b is composed of multiple blocks divided in at least one of the circumferential and radial directions of the cushioning body 41.

[0037] The third buffer material 43c is provided along the inner circumference of the second buffer material 43b and adjacent to the second buffer material 43b. The third buffer material 43c is composed of multiple blocks divided in at least one of the circumferential and radial directions of the buffer body 41.

[0038] The first material constituting the first buffer material 43a may be the material with the highest compressive strength among all the materials that make up the buffer material 43, for example, oak may be used. The second material constituting the second buffer material 43b may be the material with lower compressive strength than the first material, for example, red cedar may be used. The third material constituting the third buffer material 43c may be the material with lower compressive strength than the second material, for example, balsa may be used. Here, compressive strength, which can also be called compressive strength, is the value obtained by dividing the compressive load at which compressive failure occurs when an axial compressive load is applied to a test piece of the buffer material 43 by the cross-sectional area of ​​the test piece.

[0039] Furthermore, the cushioning material 43 is not limited to blocks of wood. The cushioning material 43 may be, for example, a foamed material or a polymer compound, in addition to wood.

[0040] (Regarding methods for estimating the strength of cushioning materials) Before filling the buffer body 41 with the buffer material 43 described above, the moisture content of the buffer material 43 is measured and controlled. However, the buffer body 41 contains buffer material 43 of multiple densities and is heated by the decay heat generated from radioactive materials during operation. In addition, a temperature gradient is created inside the buffer body 41. Therefore, the moisture content of the buffer material 43 inside the buffer body 41 during operation may not be the same as at the time of filling, and it is thought that this changes the compressive strength of the buffer material 43. For this reason, in the method for estimating the strength of the buffer material 43 according to this disclosure, the compressive strength of the buffer material 43 is estimated taking into account the effect of the moisture content of the buffer material 43.

[0041] The method for estimating the strength of the cushioning material 43 according to this disclosure will be explained using Figure 6. Figure 6 is a flowchart showing the flow of the method for estimating the strength of the cushioning material according to this disclosure. The method for estimating the strength of the cushioning material 43 according to this disclosure will be explained in accordance with the flow shown in Figure 6.

[0042] First, a strength characteristic map is created showing the compressive strength of the cushioning material 43 in relation to its moisture absorption state (step S101). For example, a test specimen is periodically removed from the cushioning material 43, and the moisture content is measured and recorded from a portion of it. The moisture content can be calculated by measuring the weight of the test specimen after drying and the weight of the test specimen before drying. That is, the moisture content is expressed by the following formula (1).

[0043] Moisture content = Weight of water / Total dry weight × 100 = (Weight before drying - Total dry weight) / Total dry weight × 100 (%) ... (1)

[0044] Regarding the total dry weight, there are various drying methods, and drying conditions and methods are described in JIS and other standards, but for example, it is the weight after drying at 105°C until there is no further change in weight. Since the moisture content is calculated based on the weight of the cushioning material 43, if it contains a lot of water, the moisture content may be 100% or more. The moisture content mentioned above is based on the dry weight, but depending on the material, a wet weight (before drying) may also be used.

[0045] Furthermore, compressive strength tests will be conducted on the test specimens whose moisture content has been measured.

[0046] The compressive strength test evaluates the compressive strength of the test specimen by uniaxial compression testing. During the manufacture of the cushioning material 43, uniaxial compression testing is performed using the same type of test specimen, and the measured value is set as the reference value. In uniaxial compression testing, for example, the test specimen is placed inside a restraining ring, and only the test specimen is compressed from both ends uniaxially. During compression, the compressive force is measured using a load cell or the like, and the amount of compression is measured using a displacement meter or the like. The maximum compressive stress is determined from these and the compressive strength is calculated. The restraining ring serves as a boundary condition for the test specimen, so it may be made from cushioning material that has not deteriorated over time. Furthermore, since the uniaxial compression test is performed under stable conditions, it may be performed at room temperature. Alternatively, the test may be performed at a high temperature equivalent to the temperature environment of the cushioning body 41.

[0047] By repeatedly measuring the moisture content of test specimens of the cushioning material 43 and conducting compressive strength tests, a strength characteristic map showing the relationship between the moisture content and compressive strength of the cushioning material 43 is created. It should be noted that, since wood is known to decrease compressive strength by approximately 6% for every 1% increase in moisture content, this value may also be used to create the strength characteristic map.

[0048] An example of a strength characteristic map relating to this disclosure will be explained using Figure 7. Figure 7 is a diagram showing an example of a strength characteristic map relating to this disclosure. As shown in Figure 7, the compressive strength of the buffer material 43 tends to decrease with increasing moisture content. By creating a strength characteristic map showing the relationship between moisture content and compressive strength using the buffer material 43, the compressive strength of the buffer material 43 can be accurately estimated based on the moisture content. Furthermore, if the compressive strength of the buffer material 43 changes due to repeated moisture absorption and drying of the buffer material 43, a strength characteristic map may be created with the number of moisture absorption and drying cycles as a variable. That is, the strength characteristic map may change according to the number of moisture absorption and drying cycles, or in other words, the moisture absorption history. For example, the strength characteristic map may show a decrease in strength as the number of moisture absorption cycles increases. Furthermore, if the moisture content and compressive strength of the buffer material 43 change due to the thermal history of the buffer material 43, a strength characteristic map may be created with the temperature related to the thermal history or the time related to the thermal history as variables. That is, the strength characteristic map may change according to the thermal history. For example, a buffer material with a thermal history of being used at high temperatures for a long period of time can be used as a strength characteristic map, showing a decrease in moisture content and an increase in strength.

[0049] Returning to Figure 6, the method for estimating the strength of the cushioning material according to this disclosure will be explained. After step 101 described above, the moisture content of the cushioning material 43 is measured (step S102). The moisture content of the cushioning material 43 may be measured by removing a test specimen from the cushioning material 43 and using the same method as when creating the strength characteristic map described above. In addition, the moisture content of the cushioning material 43 may be measured non-contact or by inserting a probe without removing a test specimen from the cushioning material 43.

[0050] Next, the moisture absorption history of the cushioning material 43 is evaluated (step S103). The moisture absorption history may be evaluated by determining the number of times the cushioning material 43 has absorbed and dried moisture based on the recorded moisture content measurements.

[0051] Next, the compressive strength of the buffer material 43 is estimated based on the measured moisture content using the strength characteristic map (step S104). For example, if the strength characteristic map shown in Figure 7 is used, the compressive strength of the buffer material 43 can be estimated by fitting the moisture content to the strength characteristic map. Alternatively, if the strength characteristic map changes depending on the moisture absorption history, in other words, the number of moisture absorption and drying cycles, the strength characteristic map to be used for estimating the compressive strength may be selected based on the number of moisture absorption and drying cycles, and the compressive strength may be estimated using that strength characteristic map. Furthermore, the strength characteristic map may be created as a multivariate function with moisture content, the number of moisture absorption and drying cycles, the temperature related to the thermal history, and the time related to the thermal history as explanatory variables, and compressive strength as the dependent variable.

[0052] Next, the performance of the buffer 41 is estimated (step S105). Specifically, the compressive strength is used to estimate the load and deformation generated in the buffer 41 in relation to the impact energy that the buffer 41 should absorb. Here, the impact energy to be absorbed is the kinetic energy of the cask when the cask falls, for example, half of that value if the impact is absorbed by the upper and lower buffers, or the kinetic energy of the flying object when it collides with the cask.

[0053] Furthermore, any remaining test pieces removed from the cushioning material 43 may be returned to the inside of the cushioning body 41 and used for subsequent measurements.

[0054] As described above, the method for estimating the strength of the buffer material 41 allows for the appropriate evaluation of the compressive strength of the buffer material 43, which varies depending on the moisture content. Therefore, a method for estimating the strength of the buffer material 41 can be provided that allows for the appropriate estimation of the buffer material 41's strength.

[0055] (Regarding the management methods for buffer materials in radioactive material storage containers) Next, the method for managing the buffer 41 of the radioactive material storage container 11 according to this disclosure will be explained using Figure 8. Figure 8 is a flowchart showing the flow of the method for managing the buffer of the radioactive material storage container according to this disclosure. The method for managing the buffer 41 of the radioactive material storage container 11 according to this disclosure will be explained in accordance with the flow shown in Figure 8.

[0056] First, the strength of the cushioning material 43 is estimated using the method for estimating the strength of the cushioning material 43 described above (step S201). Next, it is determined whether the strength of the cushioning material 43 is outside the range of variation set during the design (step S202).

[0057] If the strength of the cushioning material 43 is outside the range of variation set during the design (Step S202: Yes), the cushioning performance of the cushioning body 41 is re-evaluated using the estimated compressive strength (Step S203). Next, it is determined whether the re-evaluated performance of the cushioning body 41 is outside the range of the performance of the cushioning body 41 set during the design (Step S204). Whether it is outside the range of the performance of the cushioning material 43 set during the design is determined by whether the deformation and load are below a predetermined value.

[0058] If the re-evaluated performance of the buffer 41 is outside the range of the performance of the buffer 41 set at the time of design (Step S204: Yes), the compressive strength of the buffer 41 after drying is estimated (Step S205). Next, it is determined whether the strength of the buffer 41 after drying is outside the range of variation set at the time of design (Step S206).

[0059] If the strength of the buffer 41 after drying is outside the range of variation set during the design (Step S206: Yes), the buffer performance of the buffer 41 is re-evaluated using the strength after drying (Step S207). Next, it is determined whether the buffer performance of the buffer 41 after drying is outside the range of the performance of the buffer 41 set during the design (Step S208).

[0060] If the cushioning performance of the cushioning body 41 after drying is outside the range of the performance set for the cushioning body 41 at the time of design (Step S208: Yes), it is determined whether replacing a portion of the cushioning material 43 will bring it within the range of the performance set for the cushioning body 41 at the time of design (Step S209). If replacing a portion of the cushioning material 43 will bring it within the range of the performance set for the cushioning body 41 at the time of design (Step S209: Yes), a portion of the cushioning material 43 is replaced (Step S210).

[0061] Furthermore, in step S202, if the strength of the cushioning material 43 is not outside the range of variation set during the design (step S202: No), it is determined that the cushioning body 41 can be used for continued purposes (step S211).

[0062] Furthermore, in step S204, if the performance of the re-evaluated buffer 41 is not outside the range of the performance of the buffer 41 set at the time of design (step S204: No), it is determined that the buffer 41 can be used for continued purposes (step S211).

[0063] Furthermore, in step S206, if the strength of the cushioning material 43 is not outside the range of variation set during the design (step S206: No), it is determined that the cushioning body 41 can be dried and used for continued purposes (step S212).

[0064] Furthermore, in step S208, if the cushioning performance of the cushioning material 41 after drying is not outside the range of the performance of the cushioning material 41 set at the time of design (step S208: No), it is determined that the cushioning material 41 can be dried and used for continued purposes (step S212).

[0065] Furthermore, in step S209, if replacing a portion of the cushioning material 43 brings the performance within the range set for the cushioning body 41 during the design phase (step S209: No), it is determined that the entire cushioning material 43 can be replaced and the system can continue to be used (step S213).

[0066] This makes it possible to maintain the buffering performance of the buffer 41 in an appropriate state. Therefore, it is possible to provide a method for managing the buffer of a radioactive material storage container that can appropriately determine deterioration caused by moisture absorption.

[0067] Note that each step in the method for managing the buffer 41 of the radioactive material storage container 11 shown in Figure 8 may be skipped as appropriate.

[0068] Because the amount of heat generated by the radioactive material stored in each individual radioactive material container 11 differs, and the amount of heat transferred to the buffer material 43 also differs, the moisture content of the buffer material 43 varies from container to container. In other words, since the buffer material 43 is heated by the decay heat generated from the radioactive material while sealed and in operation, the temperature and moisture content of the buffer material 43 will vary. Therefore, by selecting which buffer material 43 to replace according to its moisture content and replacing only some of the buffer material 43, maintenance costs can be reduced.

[0069] (Management system for buffers in radioactive material storage containers) The management system 100 for the buffer body 41 of the radioactive material storage container 11 according to this disclosure will be explained with reference to Figure 9. Figure 9 is a diagram showing an example configuration of the management system for the buffer body of the radioactive material storage container according to this disclosure. As shown in Figure 9, the management system 100 for the buffer body 41 of the radioactive material storage container 11 according to this disclosure comprises a communication unit 110, a storage unit 120, a control unit 130, an input unit 140, and a display unit 150. These configurations will be explained in order below.

[0070] The communication unit 110 is responsible for sending and receiving various types of information with external devices. The communication unit 110 may be implemented by, for example, an antenna for a wireless LAN (Local Area Network) as defined in IEEE 802.11, a Wi-Fi® module, or an antenna for a fifth-generation mobile communication system (5G) or a sixth-generation mobile communication system (6G).

[0071] The memory unit 120 is a storage device that stores various types of information. The memory unit 120 comprises a main memory and an auxiliary storage device. The main memory may be implemented using semiconductor memory elements such as RAM (Random Access Memory), ROM (Read Only Memory), or flash memory. The auxiliary storage device may be implemented using a hard disk, SSD (Solid State Drive), or optical disc, for example.

[0072] As shown in Figure 9, the memory unit 120 includes an intensity characteristic map memory unit 121. These components will be described in detail below.

[0073] The strength characteristic map storage unit 121 stores information regarding the strength characteristic map relating to the compressive strength of the cushioning material 43. For example, as shown in Figure 7, the strength characteristic map storage unit 121 may store the relationship between moisture content and compressive strength as tabular data, or it may store a program that expresses the relationship between moisture content and compressive strength as a mathematical formula. Alternatively, it may store a program that expresses the strength characteristic map as a multivariate function with moisture content, the number of moisture absorption and drying cycles, the temperature related to the thermal history, and the time related to the thermal history as explanatory variables, and compressive strength as the dependent variable.

[0074] The control unit 130 is a controller that manages the management system 100 and performs various calculations and other operations. The control unit 130 is implemented by a CPU (Central Processing Unit) or MPU (Micro Processing Unit), etc., which executes various programs stored in the memory unit 120 using RAM as the working area. Alternatively, the control unit 130 may be implemented by an integrated circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array).

[0075] The control unit 130 includes an acquisition unit 131, a compression strength estimation unit 132, a buffer performance evaluation unit 133, a treatment determination unit 134, and a display control unit 135, as functions realized by the execution of a program stored in the memory unit 120 and the circuit configuration. The control unit 130 may perform these processes with a single CPU, or it may have multiple CPUs and perform these processes in parallel with the multiple CPUs.

[0076] The acquisition unit 131 acquires various types of information. For example, the acquisition unit 131 acquires the moisture content of the test specimen input to the input unit 140, which will be described later. The acquisition unit 131 may also acquire the number of times the cushioning material 43 has absorbed and dried moisture, in other words, its moisture absorption history, the temperature related to its thermal history, and the time related to its thermal history, which are input to the input unit 140.

[0077] The compressive strength estimation unit 132 estimates the compressive strength of the cushioning material 43. The compressive strength estimation unit 132 estimates the compressive strength of the cushioning material 43 by, for example, fitting the moisture content to the strength characteristic map stored in the strength characteristic map storage unit 121. Alternatively, if the strength characteristic map changes depending on the moisture absorption history, in other words, the number of moisture absorption and drying cycles, the compressive strength estimation unit 132 may select the strength characteristic map to be used for estimating the compressive strength based on the number of moisture absorption and drying cycles, and estimate the compressive strength using that strength characteristic map. Alternatively, the compressive strength may be estimated using the strength characteristic map as a multivariate function with moisture content and the number of moisture absorption and drying cycles as explanatory variables and compressive strength as the dependent variable.

[0078] Furthermore, the compressive strength estimation unit 132 estimates the compressive strength of the cushioning material 43 after drying. For example, the compressive strength estimation unit 132 estimates the compressive strength of the cushioning material 43 after drying by applying a moisture content of 0% to the strength characteristic map stored in the strength characteristic map storage unit 121. Alternatively, if the strength characteristic map changes depending on the moisture absorption history, in other words, the number of times moisture absorption and drying occurs, the compressive strength estimation unit 132 may select a strength characteristic map to be used to estimate the compressive strength after drying based on the number of times moisture absorption and drying occurs, and estimate the compressive strength of the cushioning material after drying using that strength characteristic map.

[0079] The cushioning performance evaluation unit 133 evaluates the performance of the cushioning material 43. Specifically, the cushioning performance evaluation unit 133 uses the compressive strength estimated by the compressive strength estimation unit 132 to estimate the amount of deformation of the cushioning body 41 in response to the load applied to the cushioning body 41.

[0080] Furthermore, the cushioning performance evaluation unit 133 evaluates the performance of the cushioning material 43 after drying. The cushioning performance evaluation unit 133 uses the compressive strength of the cushioning material 43 after drying, estimated by the compressive strength estimation unit 132, to estimate the amount of deformation of the cushioning body 41 in response to the load applied to the cushioning body 41.

[0081] The treatment determination unit 134 determines what treatment to be taken on the cushioning material 43. First, the treatment determination unit 134 determines whether the strength of the cushioning body 41 is outside the range of variation set during the design phase. If the strength of the cushioning body 41 is outside the range of variation set during the design phase, the treatment determination unit 134 determines that the cushioning performance of the cushioning body 41 should be re-evaluated using the strength after drying. If the strength of the cushioning material 43 is not outside the range of variation set during the design phase, the treatment determination unit 134 determines that the cushioning body 41 can be used continuously.

[0082] Furthermore, the treatment determination unit 134 determines whether the cushioning performance of the dried buffer 41 is outside the range of the performance set for the buffer 41 during the design phase. If the cushioning performance of the dried buffer 41 is outside the range of the performance set for the buffer 41 during the design phase, the treatment determination unit 134 determines whether replacing a portion of the cushioning material 43 will bring it within the range of the performance set for the buffer 41 during the design phase. If replacing a portion of the cushioning material 43 will bring it within the range of the performance set for the buffer 41 during the design phase, the treatment determination unit 134 determines that the action of replacing a portion of the cushioning material 43 should be taken. If the cushioning performance of the dried buffer 41 is not outside the range of the performance set for the buffer 41 during the design phase, the treatment determination unit 134 determines that the buffer 41 can be dried and continued to be used.

[0083] Furthermore, the treatment determination unit 134 determines that if replacing a portion of the cushioning material 43 brings the cushioning body 41 within the performance range set during the design phase, it is possible to replace all of the cushioning material 43 and continue using it.

[0084] The display control unit 135 controls the information to be displayed on the display unit 150. For example, the display control unit 135 causes the display unit 150 to display the estimated result of the compressive strength estimated by the compressive strength estimation unit 132. The display control unit 135 also causes the display unit 150 to display the evaluation result of the performance of the buffer material 41 estimated by the buffer performance evaluation unit 133. Furthermore, the display control unit 135 causes the display unit 150 to display the treatment for the buffer material determined by the treatment determination unit 134.

[0085] The input unit 140 accepts various operation information. The input unit 140 may be implemented by an input device such as a keyboard, mouse, or touch panel. The user inputs various operation information and operation information for displaying a GUI (Graphical User Interface) that shows various information via the input unit 140.

[0086] The display unit 150 is a display device that displays various types of information. The display unit 150 may, for example, display a GUI that accepts operation information and input of various types of information from an administrator, or display estimated values ​​of compressive strength and evaluation results of the performance of the buffer 41. The display unit 150 may be implemented by, for example, a liquid crystal display, an organic EL (Electro Luminescence) display, a micro LED (Light Emitting Diode) display, etc.

[0087] In this way, the buffering performance of the buffer 41 according to this disclosure can be maintained in an appropriate state. Therefore, a management system 100 for the buffer 41 of a radioactive material storage container 11 can be provided that can appropriately determine deterioration caused by moisture absorption.

[0088] (Structure and effect) A method for managing the buffer 41 of a radioactive material storage container 11 according to the first embodiment includes the steps of measuring the moisture content of a test piece of the buffer material 43 and estimating the compressive strength of the buffer material 43 based on the measured moisture content.

[0089] This configuration allows for the appropriate estimation of the compressive strength of the buffer material 43 based on its moisture content. Therefore, changes in compressive strength due to moisture absorption of the buffer material 43 can be appropriately estimated. Consequently, a method for managing the buffer material 41 of a radioactive material storage container 11 can be provided that appropriately manages changes in the strength of the buffer material 41 due to moisture absorption.

[0090] The method for managing the buffer material 41 of the radioactive material storage container 11 according to the second embodiment is the method for managing the buffer material 41 of the radioactive material storage container 11 according to the first embodiment, wherein in the step of estimating the compressive strength, the compressive strength of the buffer material is estimated using a strength characteristic map that shows the relationship between the moisture content of the buffer material 43 and the compressive strength.

[0091] With this configuration, the compressive strength of the buffer material 43 can be estimated using a strength characteristic map that shows the relationship between the moisture content of the buffer material 43 and its compressive strength. Therefore, it is possible to provide a method for managing the buffer material 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer material 41 caused by moisture absorption.

[0092] A method for managing the buffer material 41 of a radioactive material storage container 11 according to the third embodiment is a method for managing the buffer material 41 of a radioactive material storage container 11 according to the first or second embodiment, further comprising the step of creating a strength characteristic map showing the relationship between the water content and compressive strength of the buffer material 43.

[0093] This configuration allows for the creation of a strength characteristic map showing the relationship between the moisture content and compressive strength of the buffer material 43, and enables the estimation of the compressive strength using this strength characteristic map. Therefore, it is possible to provide a method for managing the buffer material 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer material 41 caused by moisture absorption.

[0094] A method for managing the buffer 41 of a radioactive material storage container 11 according to the fourth embodiment is a method for managing the buffer 41 of a radioactive material storage container 11 according to any one of the first to third embodiments, further comprising the steps of: determining whether the compressive strength of the buffer material 43 is outside the range of variation set at the time of design; and determining that the buffer material 43 can be used continuously if the compressive strength of the buffer material 43 is within the range of variation set at the time of design.

[0095] With this configuration, if the compressive strength of the buffer material 43 is within the range of variation set during the design phase, it can be determined that the buffer material 43 can be used continuously. Therefore, it is possible to provide a method for managing the buffer material 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer material 41 caused by moisture absorption.

[0096] A method for managing the buffer material 41 of a radioactive material storage container 11 according to the fifth embodiment is a method for managing the buffer material 41 of a radioactive material storage container 11 according to any one of the first to fourth embodiments, further comprising the steps of: estimating the compressive strength of the buffer material 43 when the buffer material 43 is dried; determining whether the compressive strength of the buffer material 43 when dried is outside the range of variation set at the time of design; and determining that the buffer material 43 can be dried and continued to be used if the compressive strength of the buffer material 43 when dried is outside the range of variation set at the time of design.

[0097] With this configuration, if the compressive strength of the buffer material 43 after drying is outside the range of variation set during the design phase, it can be determined that the buffer material 43 can be dried and continued to be used. Therefore, it is possible to provide a method for managing the buffer material 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer material 41 caused by moisture absorption.

[0098] A method for managing the buffer material 41 of a radioactive material storage container 11 according to the sixth embodiment is a method for managing the buffer material 41 of a radioactive material storage container 11 according to any one of the first to fifth embodiments, further comprising the steps of: re-evaluating the buffering performance of the buffer material 41 using the compressive strength of the buffer material 43 after drying; determining whether the re-evaluated performance of the buffer material 41 is outside the range of the performance of the buffer material 41 set at the time of design; and, if the re-evaluated performance of the buffer material 41 is outside the range of the performance of the buffer material 41 set at the time of design, determining that the buffer material 43 can be dried and used for continued purposes.

[0099] With this configuration, if the re-evaluated performance of the buffer 41 falls outside the range of the performance set for the buffer 41 at the time of design, it is possible to determine that the buffer material 43 can be dried and used for continued purposes. Therefore, it is possible to provide a method for managing the buffer 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer 41 caused by moisture absorption.

[0100] A method for managing the buffer 41 of a radioactive material storage container 11 according to the seventh embodiment is a method for managing the buffer 41 of a radioactive material storage container 11 according to any one of the first to sixth embodiments, further comprising the steps of: determining whether replacing the buffer material 43 will bring the performance of the buffer 41 within the range set at the time of design; and, if replacing the buffer material 43 will bring the performance of the buffer 41 within the range set at the time of design, replacing the buffer material 43 and determining that it can be used for continued purposes.

[0101] With this configuration, if replacing the buffer material 43 brings the buffer body 41 within the performance range set at the time of design, it is possible to determine that the buffer material 43 can be replaced and the container can continue to be used. Therefore, it is possible to provide a method for managing the buffer body 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer body 41 caused by moisture absorption.

[0102] The method for managing the buffer body 41 of the radioactive material storage container 11 according to the eighth embodiment is a method for managing the buffer body 41 of the radioactive material storage container 11 according to any one of the first to seventh embodiments, wherein the buffer body 41 is composed of a combination of multiple block-shaped buffer materials.

[0103] With this configuration, if the compressive strength of any of the buffer material 43, which is composed of multiple blocks, falls below the standard, the buffer material 43 can be replaced to allow for continued use. Therefore, it is possible to provide a method for managing the buffer material 41 of a radioactive material storage container 11 that can appropriately manage changes in the strength of the buffer material 41 caused by moisture absorption.

[0104] The management system 100 for the buffer body 41 of the radioactive material storage container 11 according to the first embodiment includes an acquisition unit 131 for acquiring the moisture content of the buffer material 43, and a compressive strength estimation unit 132 for estimating the compressive strength of the buffer material 43 based on the measured moisture content.

[0105] This configuration allows for the appropriate estimation of the compressive strength of the buffer material 43 based on its moisture content. Therefore, the buffering performance of the buffer body 41 can be maintained in an appropriate state. Consequently, a management system 100 for the buffer body 41 of a radioactive material storage container 11 can be provided that appropriately manages changes in the strength of the buffer body 41 caused by moisture absorption.

[0106] Although embodiments of the present disclosure have been described above, the embodiments are not limited to those described herein. Furthermore, the aforementioned components include those that can be easily conceived by those skilled in the art, those that are substantially the same, and those that fall within the so-called equivalent range. Moreover, the aforementioned components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the gist of the embodiments described above. [Explanation of Symbols]

[0107] 11. Cask (container for radioactive material) 12 Torso 13 Lid 21 Container body 22 Opening 22a First stage section 22b Second stage section 22c Upper edge 23 Cavity 24 Basketball 25 cells 26 Outer cylinder 27 Heat transfer fins 28. Resin (neutron shielding material) 29 Bottom 30 Trunnion 31 Primary lid 32 Secondary lid 33 bolt holes 37. Stand 41,46 Buffer 42 Support member 43 Cushioning material 43a 1st buffer material 43b Second buffer material 43c 3rd buffer material 51 Disc section 52 Cylindrical section 53 Side member 54 End face side member F floor O Center axis

Claims

1. A step to measure the moisture content of the cushioning material test piece, The step of estimating the compressive strength of the cushioning material based on the measured moisture content, Methods for managing buffer materials in radioactive material storage containers.

2. In the step of estimating the compressive strength, The compressive strength of a cushioning material is estimated using a strength characteristic map that shows the relationship between the moisture content of the cushioning material and its compressive strength. A method for managing the buffer material of a radioactive material storage container according to claim 1.

3. The further step includes creating a strength characteristics map showing the relationship between the moisture content and compressive strength of the cushioning material. A method for managing the buffer material of a radioactive material storage container according to claim 2.

4. A step to determine whether the compressive strength of the cushioning material is outside the range of variation set during the design, The steps include determining that the cushioning material can be used for continued purposes if its compressive strength is within the range of variation set during the design phase, and further including, A method for managing the buffer material of a radioactive material storage container according to claim 3.

5. A step to estimate the compressive strength of the cushioning material after it has been dried, A step to determine whether the compressive strength of the cushioning material when dried is outside the range of variation set during the design, The process further includes the step of determining whether the cushioning material can be dried and used for continued purposes if the compressive strength of the cushioning material after drying falls outside the range of variation set during the design phase. A method for managing the buffer material of a radioactive material storage container according to claim 4.

6. A step to re-evaluate the cushioning performance of the cushioning material using the compressive strength of the cushioning material after drying, A step to determine whether the re-evaluated performance of the buffer is outside the range of buffer performance set at the time of design, The further step includes determining whether the cushioning material can be dried and used for continued purposes if the re-evaluated cushioning performance falls outside the range of cushioning performance set at the time of design. A method for managing the buffer material of a radioactive material storage container according to claim 5.

7. A step to determine whether replacing the cushioning material brings the cushioning performance within the range set during the design phase, The step of determining that the cushioning material can be replaced and continued use is possible if replacing the cushioning material brings the cushioning material within the range of performance set at the time of design is necessary. A method for managing the buffer material of a radioactive material storage container according to claim 6.

8. The aforementioned buffer is composed of multiple block-shaped buffer materials combined together. A method for managing the buffer material of a radioactive material storage container according to any one of claims 1 to 7.

9. A unit for obtaining the moisture content of a test piece of cushioning material, Includes a compressive strength estimation unit that estimates the compressive strength of the cushioning material based on the measured moisture content, A management system for buffers in containers holding radioactive materials.

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