Methods for decommissioning nuclear reactors

Abstract

To provide a nuclear reactor decommissioning method that can enables radiation to be surely shield radiation shielded over a long period of time, and is advantageous to achieving for materializing reduction in a burden of a monitoring work and maintenance work.SOLUTION: A nuclear reactor decommissioning method comprises: a step in which a caisson frame 10 housing a nuclear reactor building 22 is configured: a step in which ground beneath the caisson frame 10 is digged by a caisson shovel 20 to thereby cause the caisson frame 10 to sink to a lower level by weights of the nuclear reactor building 22 and caisson frame 10; a step in which a space 28 beneath the caisson frame 10 is filled with concrete C when the caisson frame 10 is caused to sink thereto; a step in which the inside of the caisson frame 10 is filled with a filler F and a decommissioning work is ended by burring the nuclear reactor building 22 and support ground underground together with the caisson frame 10. As the filler F, the nuclear reactor decommissioning method uses treatment soil having a property of the soil such as solidification treatment soil, fluidization treatment soil or the like improved, or low radiation concrete or high-fluidity concrete.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a method for decommissioning a nuclear reactor. [Background technology]

[0002] A method for decommissioning a nuclear reactor in a nuclear power plant has been disclosed in which the reactor is shut down, cleaned up to the extent that it is no longer dangerous, and abandoned in that state (see Patent Document 1). This decommissioning method involves constructing a water tank to contain the entire reactor building that houses the containment vessel, submerging the reactor vessel in the tank, and maintaining a state in which radiation from the reactor vessel is shielded by water. That is, a water tank is used as the decommissioning structure, and the water tank is composed of a bottom constructed below the ground directly below the reactor building and the surrounding ground, a submerged wall section that rises from around the bottom and surrounds the reactor building and its surroundings, and a ceiling section that connects the upper end of the submerged wall section above the reactor building. Water is then stored inside the tank, submerging the reactor building. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-176939 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned conventional technology, the submerged wall and ceiling parts of the tank that are exposed above ground are exposed to harsh environmental changes such as wind and rain, direct sunlight, and temperature changes over long periods of time, which accelerates deterioration over time and causes cracks and damage, which may result in leakage of water stored in the tank. Furthermore, the ground supporting the reactor building within the water tank will be submerged in water for an extremely long decommissioning period, such as 100 or 200 years or more, which could cause problems such as the ground loosening within the water tank, causing the reactor building to tilt and collapse, destroying the flooded walls. Even if spent fuel and other materials are completely removed from the reactor building and reactor containment vessel during decommissioning, low-level radioactive materials remain in the reactor building. Therefore, the decommissioned structure must maintain its ability to shield radiation from the reactor building over the extremely long decommissioning period. If the flooded wall is destroyed as described above, there is a risk that this ability to shield radiation will be lost. As a result, it will be necessary to monitor the degree of deterioration of the water tank over a long period of time and repair any cracks or damaged areas as necessary.As this will mean an excessive burden of monitoring and maintenance work over the extremely long decommissioning period, some kind of improvement is required. The present invention has been made in consideration of the above points, and an object of the present invention is to provide a method for decommissioning a nuclear reactor that can reliably shield radiation for a long period of time while avoiding the effects of severe environmental changes, and is advantageous in reducing the burden of monitoring and maintenance work. [Means for solving the problem]

[0005] In order to achieve the above-mentioned object, one embodiment of the present invention is characterized in that a caisson body is provided to house a reactor building, the ground below the caisson body is excavated to allow the reactor building to sink into the ground together with the caisson body, filler material is filled inside the caisson body, and after the caisson body is sunk, the filler material or concrete is filled into the space below the excavated caisson body to bury the reactor building together with the caisson body in the ground, and treated soil with improved soil properties is used as the filler material. In addition, one embodiment of the present invention is characterized in that the treated soil is solidification treated soil or liquefaction treated soil. Furthermore, one embodiment of the present invention is characterized in that a caisson body for housing a reactor building is provided, the ground below the caisson body is excavated to allow the reactor building to sink into the ground together with the caisson body, filler material is filled inside the caisson body, and after the caisson body is sunk, the filler material or concrete is filled into the space below the excavated caisson body to bury the reactor building together with the caisson body in the ground, and low-radiation concrete or high-fluidity concrete is used as the filler material. In addition, one embodiment of the present invention is characterized in that the filling material is filled into the inside of the caisson body after the caisson body has been sunk. In addition, one embodiment of the present invention is characterized in that the filling material is filled into the inside of the caisson body before the caisson body is lowered. In addition, one embodiment of the present invention is characterized in that the caisson body includes a bottom wall having cutting edges protruding from the periphery of its underside, side walls rising from the periphery of the bottom wall, and a ceiling wall connecting the upper ends of the side walls, and the bottom wall is integrated with the reactor building. Furthermore, one embodiment of the present invention is characterized in that a recess surrounding the reactor building is excavated in the ground around the reactor building, an excavator that excavates the ground below the caisson body is placed in the recess, and then the bottom wall is provided on the recess. In addition, one embodiment of the present invention is characterized in that the caisson body includes a bottom wall having cutting edges protruding from the periphery of its underside, side walls rising from the periphery of the bottom wall, and a ceiling wall connecting the upper ends of the side walls, and the bottom wall is provided below the ground supporting the reactor building and supports the ground supporting the reactor building together with the reactor building. Furthermore, one embodiment of the present invention is characterized in that a frame-shaped recess is excavated in the ground around the reactor building to surround the reactor building and the supporting ground, the bottom wall is constructed from the frame-shaped recess, and the lower parts of the side walls are constructed at a height from the periphery of the bottom wall to the ground surface. In addition, one embodiment of the present invention is characterized in that after constructing the bottom wall and the lower parts of the side walls, the ground inside the cutting edge is excavated to create a space below the caisson body inside the cutting edge, and the excavation of the ground below the caisson body is carried out by an excavator transported into the space from the frame-shaped recess. [Effects of the Invention]

[0006] According to one embodiment of the present invention, a caisson body is provided to house the reactor building, and the ground below the caisson body is excavated so that the caisson body integrated with the reactor building sinks into the ground due to its own weight. Filler material is then filled inside the caisson body, and the space below the excavated caisson body is filled with filler material or concrete, thereby burying the reactor building together with the caisson body in the ground. Therefore, unlike conventional technology, the caisson body buried underground is not exposed to harsh environmental changes such as wind, rain, direct sunlight, and temperature changes. Therefore, over an extremely long decommissioning period, such as 100 or 200 years or more, it will be possible to maintain shielding from radiation from radioactive materials remaining in the reactor building and reactor containment vessel by using the caisson body, the filler filled inside the caisson body, and the filler or concrete filled in the space below the caisson body. Furthermore, since it will be possible to maintain shielding from radiation from radioactive materials remaining in the reactor building and reactor containment vessel over the extremely long decommissioning period, this will be advantageous in significantly reducing the burden of monitoring and maintenance work over the extremely long decommissioning period. In addition, by excavating the ground below the caisson body, the caisson body, which is integrated with the reactor building, is allowed to sink into the ground due to its own weight, which ensures that the caisson body sinks reliably, which is advantageous for carrying out decommissioning work safely and reliably. Furthermore, when decommissioning a reactor after the fuel inside the reactor has been properly removed from the reactor building, using treated soil with improved soil properties as filler to fill the inside of the caisson body and the space below the excavated caisson body means that the excavated soil produced when the caisson body is lowered can be used, eliminating the need to prepare new filler.In addition, the amount of work required to dispose of the excavated soil can be reduced by the amount of filler used, which is advantageous in reducing the cost of decommissioning work. In this case, if solidified treated soil mixed with cement-based solidification material is used as the treated soil, the excavated soil produced when the caisson body is lowered can be easily utilized, which is advantageous in reducing the cost of decommissioning work. In addition, if liquefied treated soil is used as the treated soil, its excellent fluidity makes it possible to seal all gaps inside the caisson body, which is advantageous in maintaining the caisson body's ability to shield against radiation from radioactive materials remaining in the reactor building and reactor containment vessel. Furthermore, when decommissioning a nuclear reactor that has experienced a severe accident and has left the fuel inside the reactor intact, using low-radiation concrete as a filler to fill the inside of the caisson body or the space below the excavated caisson body will reduce the radiation of the filler and the caisson body, which is advantageous in reducing the amount of radiation exposure of workers during maintenance. Furthermore, when decommissioning a nuclear reactor after a severe accident has occurred and the fuel inside the reactor remains intact, using high-flow concrete as a filler to fill the inside of the caisson body and the space below the excavated caisson body allows the filler to seal every corner of the gaps inside the caisson body, making the caisson body less radioactive and advantageous in reducing the amount of radiation exposure to workers during maintenance. In addition, the filling material may be filled into the interior of the caisson body after the caisson body has been lowered, or before the caisson body has been lowered. If the filler material is filled inside the caisson body before it is lowered, the weight of the filler material filled inside the caisson body will be added to the weight of the caisson body as it sinks into the ground, allowing the caisson body to sink into the ground more reliably, which is advantageous for carrying out decommissioning work more safely and reliably. In addition, integrating the bottom wall of the caisson body with the reactor building is advantageous in that it allows the bottom wall of the caisson body to be constructed easily, and also ensures that the caisson body and reactor building sink, which is advantageous in that it allows decommissioning work to be carried out safely and reliably. In addition, before installing the bottom wall of the caisson body, if a recess surrounding the reactor building is excavated in the ground around the reactor building and an excavator is brought into this recess, it will be advantageous to use the brought-in excavator to efficiently excavate the ground below the caisson body. Furthermore, if the bottom wall of the caisson body supports the reactor building as well as the ground that supports the reactor building, when decommissioning a reactor that has experienced a severe accident, since the supporting ground will also be contaminated, both the supporting ground and the reactor building can be contained in the caisson body and buried underground, making this an ideal method for decommissioning a reactor that has experienced a severe accident and has left the fuel inside the reactor intact. In addition, if a recess surrounding the reactor building and the supporting ground is excavated in the ground around the reactor building before the caisson body is installed, this is advantageous in that it makes it easier to construct the bottom wall, cutting edge, and lower parts of the side walls of the caisson body, makes it easier to create space below the caisson body, and makes it easier to transport an excavator to excavate the ground below the caisson body, as well as lighting equipment, cameras, etc., into the space below the caisson body. In addition, constructing the lower part of the side wall at a height from the periphery of the bottom wall to the ground surface is advantageous in facilitating the subsequent erection work of the material shaft, man shaft, and compressed air supply pipe, as well as the assembly work of formwork for installing the side walls and ceiling walls. [Brief explanation of the drawings]

[0007] [Figure 1] 1A and 1B are explanatory views of the first embodiment, in which (A) is a plan view showing a state in which a frame-shaped recess is excavated around the reactor building, and (B) is a cross-sectional front view of the same. [Figure 2] (A) is a plan view of the reactor building with the bottom wall formed around it, and (B) is a cross-sectional front view of the same. [Figure 3] (A) is a plan view showing the material shaft, man shaft, and compressed air supply pipe standing upright from the bottom wall, and (B) is a cross-sectional front view of the same. [Figure 4] (A) is a plan view of the caisson body after construction, with material blocks installed in the material shaft and man blocks installed in the man shaft, and (B) is a cross-sectional front view of the same. [Figure 5] This is a cross-sectional front view of the state in which the ground below the reactor building has been excavated with a caisson shovel. [Figure 6] This is a cross-sectional front view of the caisson body sinking as the ground below it is excavated with a caisson shovel. [Figure 7] This is a cross-sectional front view of the caisson body after it has been excavated with a caisson shovel and lowered into the ground until the top surface of the ceiling wall is at the same height as the ground surface. [Figure 8] This is a cross-sectional front view of the caisson body with the space below it filled with filler or concrete. [Figure 9] (A) is a plan view of the caisson body after filling with filler material, removing the various shafts and pipes protruding from the ceiling wall, and closing off the ceiling wall, and (B) is a cross-sectional front view of the same structure. [Figure 10] An explanatory diagram of the second embodiment, (A) is a plan view of the caisson body integrated with the reactor building after construction and filling material has been filled inside the caisson body, and (B) is a cross-sectional front view of the same. [Figure 11] An explanatory diagram of the third embodiment. (A) is a plan view of a caisson body integrated with the reactor building, with a material shaft, man shaft, and compressed air supply pipe erected from the bottom wall, with material blocks installed in the material shaft and man blocks installed in the man shaft. (B) is a cross-sectional front view of the same. [Figure 12]This is a cross-sectional front view of the state in which the ground below the reactor building has been excavated with a caisson shovel. [Figure 13] This is a cross-sectional front view of the caisson body being lowered into the ground by excavating the ground below the caisson body with a caisson shovel. [Figure 14] This is a cross-sectional front view of the caisson body after it has been excavated with a caisson shovel and the upper ends of the side walls have been lowered to a height equal to the thickness of the ceiling wall, below the ground surface. [Figure 15] This is a cross-sectional front view of the caisson body with the space below it filled with filler or concrete. [Figure 16] (A) is a plan view of the state in which a ceiling wall is provided on the side wall, and (B) is a cross-sectional front view of the same. [Figure 17] 10A and 10B are explanatory views of a fourth embodiment, in which (A) is a plan view showing a state in which a frame-shaped recess is excavated around the reactor building, and (B) is a cross-sectional front view of the same. [Figure 18] This is a cross-sectional front view of the bottom wall of the caisson body constructed on the ground supporting the reactor building. [Figure 19] This is a cross-sectional front view of the bottom wall of the caisson body, with a cutting edge and the lower part of the side wall integrally constructed. [Figure 20] (A) is a plan view of the state in which the excavated soil is backfilled at the bottom of the frame-shaped recess so that the cutting edge is in contact with the ground, a space is created below the caisson body, soil is backfilled inside the frame-shaped recess outside the cutting edge, and the supporting ground and reactor building are supported on the bottom wall and inside the lower part of the side wall, and (B) is a front view of the same cross section. [Figure 21] (A) is a plan view showing the material shaft, man shaft, and compressed air supply pipe installed from the bottom wall through the supporting ground, and (B) is a cross-sectional front view of the same. [Figure 22] (A) is a plan view of the caisson body after construction, with material blocks installed in the material shaft and man blocks installed in the man shaft, and (B) is a cross-sectional front view of the same. [Figure 23]This is a cross-sectional front view of the caisson body sinking into the ground due to the weight of the reactor building, the supporting ground, and the caisson body itself as the ground inside the cutting edge is excavated. [Figure 24] This is a cross-sectional front view of the caisson body lowered into the ground until the top surface of the ceiling wall is at the same height as the ground surface. [Figure 25] This is a cross-sectional front view of the caisson body with the space below it filled with filler or concrete. [Figure 26] (A) is a plan view of the caisson body after filling with filler material, removing the various shafts and pipes protruding from the ceiling wall, and closing off the ceiling wall, and (B) is a cross-sectional front view of the same structure. [Figure 27] An explanatory diagram of the fifth embodiment, (A) is a plan view of the caisson body containing the reactor building and supporting ground after it has been constructed, and then filled with filler material, and (B) is a cross-sectional front view of the same. [Figure 28] An explanatory diagram of the sixth embodiment, (A) is a plan view of a caisson body constructed to house the reactor building and supporting ground, with a material shaft, man shaft, and compressed air supply pipe erected from the bottom wall, with material blocks installed in the material shaft and man blocks installed in the man shaft, and (B) is a cross-sectional front view of the same. [Figure 29] This is a cross-sectional front view of the caisson body sinking into the ground due to the weight of the reactor building, the supporting ground, and the caisson body itself as the ground inside the cutting edge is excavated. [Figure 30] This is a cross-sectional front view of the caisson body when it is lowered to a height where the upper end of the side wall is located below the ground surface by the thickness of the ceiling wall. [Figure 31] This is a cross-sectional front view of the caisson body with the space below it filled with filler or concrete. [Figure 32] FIG. 10 is a cross-sectional front view of the state in which a ceiling wall is provided on the side wall. DETAILED DESCRIPTION OF THE INVENTION

[0008] (First embodiment) First, the first embodiment will be described with reference to FIGS. In the first to third embodiments, the decommissioning work of a nuclear reactor will be described when the fuel inside the reactor has been appropriately removed from the reactor building. As shown in FIG. 4, the caisson body 10 is made of reinforced concrete and has a rectangular parallelepiped shape. The caisson body 10 comprises a square frame-shaped bottom wall 12 in plan view that extends above the ground surrounding the reactor building 22 and is integrated with the reactor building 22, side walls 14 that rise from the four sides of the bottom wall 12 and surround the reactor building 22, and a square ceiling wall 16 that connects the upper ends of the side walls 14, and the caisson body 10 houses the reactor building 22. Cutting edges 1204, each with a cutting edge 1204A at its lower end, protrude in a rectangular frame shape from the four sides of the underside of the bottom wall 12, and the caisson body 10, which is integrated with the reactor building 22, sinks into the ground due to the weight of the reactor building 22 and the weight of the caisson body 10 as the ground inside the cutting edge 1204A is excavated. In this embodiment, for example, the bottom wall 12 of the caisson body 10 is a square with sides of approximately 50 m, and the height of the caisson body 10 is approximately 50 m, the bottom wall 24 of the reactor building 22 is a square with sides of approximately 40 m, and the height of the reactor building 22 is approximately 40 m. In addition, if the reactor building 22 is cylindrical, the caisson body 10 may be rectangular, cylindrical, or may have a shape other than rectangular or cylindrical, and various conventionally known structures can be adopted for the shape of the caisson body 10.

[0009] In more detail, as shown in Figure 1, before constructing the caisson body 10, a square frame-shaped recess 28A extending along the perimeter of the reactor building 22 is excavated using an excavator such as a backhoe. This frame-shaped recess 28A is a space for smoothly carrying out excavation work of the ground below the caisson body 10. Once the frame-shaped recess 28A has been excavated, an excavator for excavating the ground below the caisson body 10, lighting equipment and cameras, which will be described later, and the like are carried into the frame-shaped recess 28A. Thereafter, a formwork is assembled around the caisson body 10 on the frame-shaped recess 28A, and concrete is poured into the formwork as shown in FIG. 2 to construct the bottom wall 12 and the cutting edge 1204 having the cutting edge 1204A. At this time, the bottom wall 23 of the reactor building 22 and the bottom wall 12 of the caisson body 10 are integrated with a plurality of anchor bolts 23A.

[0010] In addition, material shaft insertion holes 1210 are provided in a pair of opposing corners of the bottom wall 12, and man shaft insertion holes 1212 are provided in the remaining pair of opposing corners, and further inside these are provided compressed air supply pipe insertion holes 1214 and cable insertion holes (not shown) for inserting power cables and signal cables that supply power to the lighting fixture. Once the bottom wall 12 equipped with the cutting edge 1204A and cutting edge 1204 has been constructed, the formwork is dismantled and the formwork located inside the cutting edge 1204 is transported outside the caisson body 10 through the material shaft insertion hole 1210 and the man shaft insertion hole 1212. Furthermore, the formwork positioned outside the cutting edge 1204 is carried out of the frame-shaped recess 28A, and the outside of the cutting edge 1204 is backfilled with the excavated earth and sand.

[0011] In this embodiment, the excavator that excavates the ground below the caisson body 10 is a caisson shovel 20, so once the bottom wall 12 and cutting edge 1204 have been constructed, multiple rails 32 are attached around the underside of the bottom wall 12, as shown in Figure 3, and the caisson shovel 20 is arranged to be able to run on these rails 32. The caisson shovel 20 is configured to include a running part that runs along the rails 32, and a bendable arm that is rotatably attached to the running part and has a bucket at its tip. In addition, a plurality of lighting fixtures (not shown) are attached to the underside of the bottom wall 12, and a camera (not shown) is attached so as to be horizontally rotatable and vertically swingable. Also, as shown in Figure 3, once the bottom wall 12 and cutting edge 1204 have been constructed, a material shaft 34A is erected by passing through each of the pair of material shaft insertion holes 1210, a man shaft 36A is erected by passing through each of the pair of man shaft insertion holes 1212, and a compressed air supply pipe 38 is erected by passing through each compressed air supply pipe insertion hole 1214.

[0012] In addition, a ground remote control room (not shown) is provided on the ground away from the reactor building 22 to remotely operate the caisson shovel 20, camera, etc. The ground remote control room is equipped with a shovel operating device (not shown) for remotely operating the caisson shovel 20, and a display device (not shown) for displaying images of the area around the caisson shovel 20 based on image data supplied from a camera (not shown). The shovel operating device is connected to the caisson shovel 20 via a signal cable for supplying an operating signal. The display device is connected to the camera via a signal cable for transmitting image data. Hereinafter, these signal cables will be collectively referred to as the signal cable.

[0013] Once the bottom wall 12 of the caisson body 10 integrated with the reactor building 22 has been constructed in this manner, formwork for constructing the side walls 14 and ceiling wall 16 is provided around and above the bottom wall 12, and concrete is poured into this formwork to construct the side walls 14 and ceiling wall 16 as shown in Figure 4. The side walls 14 rise from the four sides of the bottom wall 12 and surround the periphery of the reactor building 22 , and the ceiling wall 16 connects the upper ends of the side walls 14 above the reactor building 22 . This integrates the bottom wall 12, side walls 14, and ceiling wall 16 to construct the caisson body 10 housing the reactor building 22. When constructing the ceiling wall 16, the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 are protruded upward from the ceiling wall 16, and after the ceiling wall 16 is constructed, a material lock 34B is provided at the upper end of the material shaft 34A, and a man lock 36B is provided at the upper end of the man shaft 36A.

[0014] Once the caisson body 10 that houses the reactor building 22 and is integrated with the reactor building 22 has been constructed in this manner, the inside of the frame-shaped recess 28A is illuminated with lighting fixtures, the excavation site is photographed with a camera, and the image of the area around the caisson shovel 20 displayed on a display device is monitored in the ground remote control room, and the caisson shovel 20 is remotely controlled with a shovel operating device, thereby first excavating the ground 24 directly below the reactor building 22 that supports the reactor building 22, as shown in Figure 4. A bucket (not shown) is suspended by a wire from the material lock 34B and material shaft 34A into the frame-shaped recess 28A, and the excavated soil is dumped into the bucket by a caisson shovel 20 via remote control.The soil dumped into the bucket rises up the material shaft 34 along with the bucket and is efficiently transported from the material lock 34B to the outside of the caisson body 10.

[0015] As shown in Figures 4 and 5, by excavating the ground 24 directly below the reactor building 22, a space 28 is created below the caisson body 10, which connects the space below the reactor building 22 with a frame-shaped recess 28A. Thereafter, as shown in Figures 5 and 6, the caisson shovel 20 excavates the ground below the caisson body 10, inside the cutting edge 1204A, and the weight of the reactor building 22 and the caisson body 10 causes the caisson body 10, integrated with the reactor building 22, to sink into the ground. In addition, the ground below the caisson body 10 may be excavated by an excavator such as a backhoe instead of the caisson shovel 20, or alternatively, the caisson shovel 20 may be used in combination with an excavator such as a backhoe. In this case, the excavation of the ground using the caisson shovel 20 or backhoe is not limited to remote control, and may be optionally performed by a worker within the space 28. The excavation of the ground below the caisson shovel 20 is carried out while monitoring the inclination of the caisson body 10 so that the caisson body 10 remains horizontal as it sinks into the ground.

[0016] In the event that groundwater springs up in the space 28 below the caisson body 10 when the caisson body 10 sinks into the ground, in this embodiment, a compressed air supply pipe 38 is provided, and compressed air appropriate to the groundwater pressure is supplied from the compressed air supply pipe 38 to the space 28 below the caisson body 10. In other words, this embodiment employs a pneumatic caisson construction method in which compressed air is supplied to the space 28 below the caisson body 10 to prevent the inflow of groundwater while excavating, thereby lowering the caisson body 10; however, it goes without saying that the open caisson method may also be employed in cases where the ground has little groundwater. This prevents groundwater from seeping into the space 28, which is advantageous for smoothly excavating the ground below the caisson body 10. In this embodiment, a material lock 34B and a material shaft 34A are provided, which is advantageous in that excavated soil can be transported outside the caisson body 10 without any problems, even when compressed air corresponding to the groundwater pressure is supplied to the space 28 below the caisson body 10. In addition, the provision of man shafts 36A and man locks 36B is advantageous in enabling workers to smoothly enter and exit space 28 even when compressed air corresponding to the groundwater pressure is supplied to space 28 below the caisson body 10. It should be noted that various conventionally known structures can be employed for the material lock 34B and the man lock 36B. In addition, an openable / closable entrance / exit for workers may be provided on the side wall 14 of the caisson body 10.In this case, if groundwater springs up on the side wall 14 of the caisson body 10 when the caisson body 10 and the reactor building 22 sink into the ground, a pipe (not shown) may be provided that penetrates the ceiling wall 16 of the caisson body 10, and compressed air corresponding to the groundwater pressure may be supplied from the pipe (not shown) into the inside of the caisson body 10 to increase the pressure inside the caisson body 10 and prevent groundwater from entering the inside of the caisson body 10.

[0017] As shown in Figure 7, once the caisson body 10 has been lowered into the ground until the upper surface of the ceiling wall 16 of the caisson body 10 is at approximately the same height as the ground surface G, as shown in Figure 8, the space 28 below the caisson body 10 where the caisson shovel 20 is located is filled with filler F or highly fluid concrete C, as described below, from the material rock 34B and material shaft 34A to stabilize the caisson body 10. Next, the material lock 34B is removed from the material shaft 34A, and the portion of the material shaft 34A that protrudes from the ceiling wall 16 is removed. Additionally, the man lock 36B is removed from the man shaft 36A, the portion of the man shaft 36A protruding from the ceiling wall 16 is removed, and the portion of the compressed air supply pipe 38 protruding from the ceiling wall 16 is removed. Then, for example, a portion of the material shaft 34 located in the ceiling wall 16 is removed, and filler material F is filled into the inside of the caisson body 10 from this removed area as shown in Figure 9(B), or filler material filling holes are pre-made in the ceiling wall 16, and filler material F is filled into the inside of the caisson body 10 from these filler material filling holes. Of course, filler material F is also filled inside the material shaft 34A, man shaft 36A, and compressed air supply pipe 38. In this case, treated soil with improved soil properties can be used as the filler F. If solidified treated soil mixed with cement-based solidification material is used as the treated soil, the excavated soil produced when sinking the caisson body 10 can be easily utilized, eliminating the need to prepare new filler material F. In addition, the disposal work for the excavated soil can be reduced by the amount of filler material F used, which is advantageous in reducing the cost of decommissioning work. Furthermore, when fluidized treated soil is used as the treated soil, in addition to the above-mentioned effects, it has excellent fluidity, making it possible to seal all gaps inside the caisson body 10, which is advantageous in maintaining the caisson body 10 in a state where it is shielded from radiation from radioactive materials remaining in the reactor building 22 and the reactor containment vessel. Once the filler material F has been filled inside the caisson body 10, the holes in the ceiling wall 16 through which the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 were inserted are blocked with filler material F or concrete C, as shown in Figure 9(A). In this way, the reactor building 22 is buried in the ground together with the caisson body 10, completing the decommissioning work. It is optional to pour concrete to a predetermined thickness on the ground surface G and the upper surface of the ceiling wall 16. If the caisson body 10 is lowered into the ground until the upper surface of the ceiling wall 16 is at approximately the same height as the ground surface G, it is advantageous for maintaining a state in which radiation from radioactive materials remaining in the reactor building 22 and the reactor containment vessel is shielded. However, the caisson body 10 may be lowered into the ground until the upper surface of the ceiling wall 16 is lower than the ground surface G, or, in cases such as when concrete is poured to a predetermined thickness on the ground surface G and the upper surface of the ceiling wall 16, the lowering of the caisson body 10 into the ground may be stopped at a point where the upper surface of the ceiling wall 16 is higher than the ground surface G.

[0018] According to this embodiment, a caisson body 10 is provided that is integrated with the reactor building 22 and that houses the reactor building 22, and by excavating the ground below the caisson body 10, the caisson body integrated with the reactor building 22 is caused to sink into the ground due to its own weight.After it has sunk into the ground, filler material F or concrete C is filled into the space 28 below the excavated caisson body 10, and filler material F is filled inside the caisson body 10, so that the bottom wall 12 and side wall 14 of the caisson body 10 are buried in the ground together with the reactor building 22. Therefore, the filler F or concrete C filled in the bottom wall 12 and side wall 14 of the caisson body 10, the space 28 below the excavated caisson body 10, and the filler F filled inside the caisson body 10 are not exposed to harsh environmental changes such as wind, rain, direct sunlight, or temperature changes, and the caisson body 10, the filler F or concrete C filled in the space 28 below the caisson body 10, and the filler F filled inside the caisson body 10 can continue to shield radiation from radioactive materials remaining in the reactor building 22 and the reactor containment vessel over an extremely long decommissioning period, for example, 100 or 200 years or more. Furthermore, since it will be possible to maintain a state in which radiation from radioactive materials remaining in the reactor building 22 and the reactor containment vessel is shielded over an extremely long decommissioning period, this will be advantageous in significantly reducing the burden of monitoring and maintenance work over an extremely long decommissioning period. Furthermore, by excavating the ground below the caisson body 10, the caisson body integrated with the reactor building 22 is caused to sink into the ground by its own weight, which ensures that the caisson body 10 and the reactor building 22 sink, which is advantageous for carrying out decommissioning work safely and reliably.

[0019] (Second embodiment) Next, a second embodiment will be described with reference to FIG. In the following description of the embodiment, the same parts and members as those in the first embodiment will be denoted by the same reference numerals, and the description thereof will be omitted, with the focus being on the differences. In the second embodiment, as shown in FIG. 10(A), a filler filling hole 1620 is provided in the ceiling wall 16. Then, as shown in Figure 10(B), once the caisson body 10 is constructed in which the caisson body 10 and the reactor building 22 are integrally connected, unlike the first embodiment, filler material F is filled into the inside of the caisson body 10 through the filler filling hole 1620, and the filler filling hole 1620 is blocked with filler material F or concrete C.

[0020] After filling the inside of the caisson body 10 with filler material F, as in the first embodiment, the excavation area is illuminated with lighting fixtures as shown in Figures 4 to 7, and the excavation area is photographed with a camera while the ground below the reactor building 22 is excavated with a caisson shovel 20, and the excavated soil is transported outside the caisson body 10 using a bucket that moves up and down inside the material shaft 34, allowing the caisson body 10 to sink into the ground. After the caisson body 10 has sunk into the ground, as shown in Figures 8 and 9, filler material F or concrete C is filled into the space 28 below the caisson body 10 from the material shaft 34, and the holes in the ceiling wall hole 16 through which the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 were inserted are blocked with filler material F or concrete C, completing the decommissioning work.

[0021] The second embodiment differs from the first embodiment in that the work of filling the inside of the caisson body 10 with the filler material F is carried out before the caisson body 10 is lowered. According to the second embodiment, when the caisson body 10 sinks into the ground, the weight of the caisson body 10 is added to the weight of the filler material F filled inside the caisson body 10, which allows the caisson body 10 to sink into the ground more reliably, which is advantageous in carrying out decommissioning work more safely and reliably.

[0022] (Third embodiment) Next, a third embodiment will be described with reference to FIGS. The third embodiment differs from the first and second embodiments in that the caisson body 10 does not have a ceiling wall 16 before it is lowered, and a ceiling wall 16 is provided after the caisson body 10 is lowered. As shown in Figure 11, the caisson body 10 has a square bottom wall 12 and side walls 14 erected from the four sides of the bottom wall 12, and does not have a ceiling wall 16, so that the top of the bottom wall 12 and the top of the reactor building 22 are both open inside the side walls 14. In more detail, as shown in Figures 1 and 2, as in the first embodiment, before constructing the caisson body 10, a frame-shaped recess 28A extending in the shape of a square frame along the perimeter of the reactor building 22 is excavated, and an excavator, lighting equipment, camera, etc. are carried into the frame-shaped recess 28A. After these items are carried in, a formwork is assembled on the frame-shaped recess 28A, and as shown in Figure 2, concrete is poured to construct the bottom wall 12, cutting edge 1204, and cutting edge 1204A, and the bottom wall 24 of the reactor building 22 and the bottom wall 12 of the caisson body 10 are integrated with a plurality of anchor bolts 24A. Once the caisson body 10 integrated with the bottom wall 24 of the reactor building 22 has been constructed, a rail 32, a caisson shovel 20, a plurality of lighting fixtures (not shown), and a camera are attached to the underside of the bottom wall 12.

[0023] As shown in Figures 2(A) and 11, the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 protrude upward from the material shaft insertion hole 1210, man shaft insertion hole 1212, and compressed air supply pipe insertion hole 1214 in the bottom wall 12, and material locks 34B and man locks 36B are provided at the upper ends of the material shaft 34A and man shaft 36A, and cables are inserted through cable insertion holes (not shown). Then, a formwork is set up around the bottom wall 12, concrete is poured into this formwork, side walls 14 are constructed that rise from the four sides of the bottom wall 12 and surround the reactor building 22, and a caisson body 10 that houses the reactor building 22 and is integrated with the reactor building 22 is constructed.

[0024] Once the caisson body 10 integrated with the reactor building 22 has been constructed, as shown in Figures 11 and 12, the excavation area is illuminated with lighting fixtures and photographed with a camera, and in the ground remote control room, the image of the area around the caisson shovel 20 displayed on a display device is monitored, and the caisson shovel 20 is remotely controlled using a shovel operating device to excavate the ground 24 below the reactor building 22 that supports the reactor building 22. A bucket (not shown) is suspended by a wire from the material lock 34B and material shaft 34A into the frame-shaped recess 28A, and the excavated soil is dumped into the bucket by a caisson shovel 20 via remote control.The soil dumped into the bucket rises up the material shaft 34 along with the bucket and is efficiently transported from the material lock 34B to the outside of the caisson body 10.

[0025] As shown in FIG. 12, by excavating the ground 42 directly below the reactor building 22, a space 28 is created below the caisson body 10, in which the space below the reactor building 22 and the frame-shaped recess 28 communicate with each other. Thereafter, as shown in Figures 12 and 13, the caisson shovel 20 excavates the ground below the caisson body 10 inside the cutting edge 1204A, causing it to sink into the ground due to the weight of the reactor building 22 and the weight of the caisson body 10. As in the first embodiment, if groundwater springs up in the space 28 below the caisson body 10 when the caisson body 10 sinks into the ground, installing a compressed air supply pipe 38 will allow compressed air appropriate to the groundwater pressure to be supplied from the compressed air supply pipe 38 to the space 28 below the caisson body 10, which will be advantageous in facilitating smooth excavation of the ground below the caisson body 10. Furthermore, as in the first embodiment, providing a material lock 34B and a material shaft 34A is advantageous in enabling the excavated soil to be transported outside the caisson body 10 without any problems, even when compressed air is being supplied to the space 28 below the caisson body 10. Furthermore, as in the first embodiment, providing a man shaft 36A and a man lock 36B is advantageous in enabling workers to smoothly enter and exit the space 28 below the caisson body 10, even when compressed air is being supplied to the space 28.

[0026] As shown in Figure 14, once the upper end of the side wall 14 has been sunk into the ground by the thickness of the ceiling wall 16 below the ground surface G, as shown in Figure 15, filler material F or concrete C is poured into the space 28 below the caisson body 10 from the material rock 34B and material shaft 34A to stabilize the caisson body 10. Next, as shown in FIG. 16, a formwork is assembled above the reactor building 22, concrete is poured into the formwork, and a ceiling wall 16 is provided to connect the upper ends of the side walls 14. Next, as in the first embodiment, as shown in Figure 9, the material lock 34B, the portion of the material shaft 34A protruding from the ceiling wall 16, the man lock 36B, the portion of the man shaft 36A protruding from the ceiling wall 16, etc. are removed, and the portion of the compressed air supply pipe 38 protruding from the ceiling wall 16 is removed.As in the first and second embodiments, filler material F is filled inside the caisson body 10, and the material shaft insertion hole 1210, the man shaft insertion hole 1212, and the compressed air supply pipe insertion hole 1214 are blocked with filler material F or concrete C, completing the decommissioning work.

[0027] According to the third embodiment, when the caisson structure 26 sinks, the space inside the side wall 14 is always open upward, allowing workers to enter and exit the inside of the caisson body 10, which is advantageous for performing various types of work.

[0028] In this embodiment, a method for decommissioning a nuclear reactor in which the fuel inside the reactor has been properly removed from the reactor building has been described, but the present invention is of course also applicable to a method for decommissioning a nuclear reactor in which a severe accident has occurred and the fuel inside the reactor has been left as it is.

[0029] (Fourth embodiment) Next, a fourth embodiment will be described with reference to FIGS. In the following fourth to sixth embodiments, a method for decommissioning a nuclear reactor will be described for the case where a severe accident occurs at a nuclear power plant, the nuclear reactor installed in the reactor building goes into a meltdown state, the molten fuel breaks through the bottom wall of the reactor building and reaches the ground below the bottom wall, and the ground is contaminated with radioactive materials. As shown in FIG. 17, a reactor building 22 is supported on a supporting ground 24 that is contaminated with radioactive materials. As shown in FIG. 22, the caisson body 10 has a rectangular parallelepiped shape. The caisson body 10 comprises a square bottom wall 12 in plan view that supports the reactor building 22 together with the supporting ground 24 of the reactor building 22, side walls 14 that rise from the four sides of the bottom wall 12 and surround the supporting ground 24 and the reactor building 22, and a square ceiling wall 16 that connects the upper ends of the side walls 14 above the reactor building 22, and the caisson body 10 accommodates the supporting ground 24 and the reactor building 22. As in the first to third embodiments, cutting edges 1204 with cutting edges 1204A at the lower end protrude in a rectangular frame shape from the four sides of the underside of the bottom wall 12, and the caisson body 10 containing the reactor building 22 and supporting ground 24 sinks into the ground due to the weight of the reactor building 22, the weight of the supporting ground 24, and the weight of the caisson body 10 as the ground inside the cutting edges 1204A is excavated.

[0030] In this embodiment, for example, as in the first to third embodiments, the bottom wall 12 of the caisson body 10 is a square with sides of approximately 50 m, and the height of the caisson body 10 is approximately 50 m, the bottom wall 24 of the reactor building 22 is a square with sides of approximately 40 m, and the height of the reactor building 22 is approximately 40 m. In addition, if the reactor building 22 is cylindrical, the caisson body 10 may be rectangular, cylindrical, or may have a shape other than rectangular or cylindrical, and various conventionally known structures can be adopted for the shape of the caisson body 10. In addition, the depth of the supporting ground 24 accommodated in the caisson body 10 and the length of one side of the bottom wall 12 of the caisson body 10 are appropriately determined based on the degree of contamination of the supporting ground 24 by radioactive materials.

[0031] In more detail, as shown in Figure 17, before constructing the caisson body 10, a square frame-shaped recess 28A extending along the perimeter of the reactor building 22 is excavated using an excavator such as a backhoe. The depth of the frame-shaped recess 28A to be excavated is greater than the thickness of the supporting ground 24 to be contained in the caisson body 10, which is determined based on the degree of contamination with radioactive materials, plus the thickness of the bottom wall 12 and the dimension of the cutting edge 1204 from the underside of the bottom wall 12 to the cutting edge 1204A, in order to install a formwork for constructing the cutting edge 1204 having the cutting edge 1204A. When the frame-shaped recess 28A is excavated, a rectangular columnar ground having an approximately square cross section, including the supporting ground 24, is created from the bottom of the frame-shaped recess 28A to the underside of the reactor building 22. A solidifying material is injected into this rectangular columnar ground to solidify the rectangular columnar ground supporting the reactor building 22 and increase the strength of the rectangular columnar ground. Such ground improvement may be omitted, but if ground improvement is performed, it will be advantageous for the reactor building 22 to be firmly supported by the rectangular columnar ground below the reactor building 22, even in the unlikely event of a major earthquake occurring in the state shown in Figure 17(B). Although not shown, an undercutting method may be used to drive multiple piles into the ground outside the frame-shaped recess down to solid ground so as to surround the frame-shaped recess, thereby preventing the collapse of the wall outside the frame-shaped recess due to uneven settlement of the ground outside the frame-shaped recess.

[0032] Next, as shown in FIG. 18, the bottom wall 12 of the caisson body 10 is constructed on the prismatic ground located below the contaminated supporting ground 24. Various conventionally known construction methods such as the pipe roof method and the shield method can be used to construct the bottom wall 12 of the caisson body 10. For example, in the pipe roofing method, a plurality of steel pipes are driven horizontally into a column-like pattern at equal intervals from the frame-shaped recess 28A into the ground located below the contaminated ground. Adjacent cast steel pipes are connected together via joints, and concrete is poured inside the steel pipes, thereby constructing the bottom wall 12 of the caisson body 10 over the entire area of the ground surrounded by the frame-shaped recess 28A. In addition, in the shield method, a plurality of tunnels are constructed horizontally in parallel with each other from the frame-shaped recess 28A in the ground located below the contaminated ground using a shield machine. Next, adjacent tunnels are connected to each other, and concrete is poured into each tunnel to construct the reinforced concrete base 12 of the caisson body 10 over the entire area of the ground surrounded by the frame-shaped recess 28A.

[0033] Once the bottom wall 12 of the caisson body 10 has been constructed, as shown in Figure 19, the cutting edge 1204 having the cutting edge 1204A and the lower part of the side wall 14 are constructed integrally with the bottom wall 12 at a height from the periphery of the bottom wall 12 to the ground surface G. To construct them, a formwork is set up in the frame-shaped recess 28A, reinforcing bars are placed, and concrete is poured into the formwork to construct a cutting edge 1204 made of reinforced concrete and integrated with the bottom wall 12, which has a cutting edge 1204A, and the lower part of a side wall 14 made of reinforced concrete and integrated with the bottom wall 12 and the cutting edge 1204, which reaches the ground surface G, and after construction, the formwork is removed. Then, the excavated earth and sand are backfilled into the bottom of the frame-shaped recess 28A so that the cutting edge 1204 is in contact with the ground.

[0034] Next, as shown in FIG. 20, a space 28 is provided below the bottom wall 12 of the caisson body 10 and inside the cutting edge 1204, that is, a space 28 is provided below the caisson body 10. This space 28 can be created by, for example, leaving a portion of the cutting edge 1204 when constructing the cutting edge 1204, and then using an excavator such as a backhoe to excavate the ground inside the cutting edge 1204 below the bottom wall 12, passing through the area where the cutting edge 1204 is not constructed from the frame-shaped recess 28, or by using an excavator to excavate the area below a portion of the cutting edge 1204 extending in a rectangular frame shape from the frame-shaped recess 28.Various conventionally known methods can be used to create this space 28. Then, an excavator for excavating the ground below the caisson body 10, lighting equipment, cameras, etc. are brought into the space 28. In this embodiment, a caisson shovel 20 is used as an excavator for excavating the ground below the caisson body 10, so the caisson shovel 20 and rails 32 for the caisson shovel 20 to travel on are brought into the space 28. Furthermore, if the excavator used to create the space 28 is used as the excavator to excavate the ground below the caisson body 10, the excavator can be left in the space 28, or if a caisson shovel 20 and an excavator are used together, the excavator can be left in place and the caisson shovel 20 and rails 32 can be transported into the space 28. After bringing in an excavator, lighting equipment, a camera, etc. into the space 28, as shown in Figure 20, the excavated soil and sand are backfilled inside the frame-shaped recess 28 on the outside of the cutting edge 120 and on the outside of the lower part of the side wall 14.

[0035] Next, as shown in Figure 21, material shafts 34A are erected inside the side wall 14 and at a pair of opposing corners of the bottom wall 12 around the reactor building 22, and man shafts 36A are erected at the remaining pair of opposing corners.Furthermore, compressed air supply pipes 38 are erected inside these, and cable insertion pipes (not shown) are erected to pass power cables and signal cables that supply power to lighting fixtures. The lower portions of the material shaft 34A, man shaft 36A, compressed air supply pipe 38, and cable insertion pipe (not shown) pass through the supporting ground 24, penetrate the bottom wall 12, and reach the space 28, and the upper ends of the material shaft 34A, man shaft 36A, compressed air supply pipe 38, and cable insertion pipe (not shown) are located above the ceiling wall 16.

[0036] In this embodiment, the excavator that excavates the ground below the caisson body 10 is a caisson shovel 20, so a worker enters the space 28 from the man shaft 36A, attaches multiple rails 32 around the underside of the bottom wall 12, and sets up the caisson shovel 20 so that it can run on these rails 32. The caisson shovel 20 is configured to include a running part that runs along the rails 32, and a bendable arm that is rotatably attached to the running part and has a bucket at its tip. In addition, a plurality of lighting fixtures (not shown) are attached to the underside of the bottom wall 12, and a camera (not shown) is attached so as to be horizontally rotatable and vertically swingable.

[0037] In addition, a ground remote control room (not shown) is provided on the ground away from the reactor building 22 to remotely operate the caisson shovel 20, camera, etc. The ground remote control room is equipped with a shovel operating device (not shown) for remotely operating the caisson shovel 20, and a display device (not shown) for displaying images of the area around the caisson shovel 20 based on image data supplied from a camera (not shown). The shovel operating device is connected to the caisson shovel 20 via a signal cable for supplying an operating signal. The display device is connected to the camera via a signal cable for transmitting image data. Hereinafter, these signal cables will be collectively referred to as the signal cable.

[0038] Next, formwork is provided around the reactor building 22 and supporting ground 24 to construct the side walls 14 and ceiling wall 16, and concrete is poured into the formwork to construct the reinforced concrete side walls 14 and ceiling wall 16 integrated with the lower part of the already constructed side walls 14, as shown in Figure 22. The side walls 14 surround the reactor building 22 and the supporting ground 24 , and the ceiling wall 16 connects the upper ends of the side walls 14 above the reactor building 22 and the supporting ground 24 . This integrates the bottom wall 12, side walls 14, and ceiling wall 16 to construct the caisson body 10 that houses the reactor building 22 and supporting ground 24. When constructing the ceiling wall 16, the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 are protruded upward from the ceiling wall 16, and after the ceiling wall 16 is constructed, a material lock 34B is provided at the upper end of the material shaft 34A, and a man lock 36B is provided at the upper end of the man shaft 36A.

[0039] Once the caisson body 10 containing the reactor building 22 and supporting ground 24 has been constructed in this manner, the interior of the space 28 is illuminated with lighting fixtures, the excavation site is photographed with a camera, and the image of the area around the caisson shovel 20 displayed on a display device is monitored in the ground remote control room, while the caisson shovel 20 is remotely controlled with a shovel operating device, and the ground directly below the caisson body 10 is excavated inside the cutting edge 1204A. As the ground inside the cutting edge 1204A is excavated, the weight of the reactor building 22, the weight of the supporting ground 24, and the weight of the caisson body 10 causes the caisson body 10 to sink into the ground, as shown in Figure 23. A bucket (not shown) is suspended by a wire from the material lock 34B and material shaft 34A into the frame-shaped recess 28A, and the excavated soil is dumped into the bucket by a caisson shovel 20 via remote control.The soil dumped into the bucket rises up the material shaft 34 along with the bucket and is efficiently transported from the material lock 34B to the outside of the caisson body 10.

[0040] In addition, the ground below the caisson body 10 may be excavated by an excavator such as a backhoe instead of the caisson shovel 20, or alternatively, the caisson shovel 20 may be used in combination with an excavator such as a backhoe. In this case, the excavation of the ground using the caisson shovel 20 or backhoe is not limited to remote control, and may be optionally performed by a worker within the space 28. The excavation of the ground below the caisson shovel 20 is carried out while monitoring the inclination of the caisson body 10 so that the caisson body 10 remains horizontal as it sinks into the ground.

[0041] In the event that groundwater springs up in the space 28 below the caisson body 10 when the caisson body 10 sinks into the ground, in this embodiment, a compressed air supply pipe 38 is provided, and compressed air appropriate to the groundwater pressure is supplied from the compressed air supply pipe 38 to the space 28 below the caisson body 10. In other words, this embodiment employs a pneumatic caisson construction method in which compressed air is supplied to the space 28 below the caisson body 10 to prevent the inflow of groundwater while excavating, thereby lowering the caisson body 10; however, it goes without saying that the open caisson method may also be employed in cases where the ground has little groundwater. This prevents groundwater from seeping into the space 28, which is advantageous for smoothly excavating the ground below the caisson body 10. In this embodiment, a material lock 34B and a material shaft 34A are provided, which is advantageous in that excavated soil can be transported outside the caisson body 10 without any problems, even when compressed air corresponding to the groundwater pressure is supplied to the space 28 below the caisson body 10.

[0042] In addition, the provision of man shafts 36A and man locks 36B is advantageous in enabling workers to smoothly enter and exit space 28 even when compressed air corresponding to the groundwater pressure is supplied to space 28 below the caisson body 10. It should be noted that various conventionally known structures can be employed for the material lock 34B and the man lock 36B. In addition, an opening and closing entrance for workers may be provided on the side wall 14 of the caisson body 10.In this case, if groundwater springs up on the side wall 14 of the caisson body 10 when the caisson body 10 and the reactor building 22 sink into the ground, a pipe (not shown) may be provided that penetrates the ceiling wall 16 of the caisson body 10, and compressed air corresponding to the groundwater pressure may be supplied from the pipe (not shown) into the inside of the caisson body 10 to increase the pressure inside the caisson body 10 and prevent groundwater from entering the inside of the caisson body 10.

[0043] As shown in Figure 24, once the caisson body 10 has been lowered into the ground until the upper surface of the ceiling wall 16 of the caisson body 10 is at approximately the same height as the ground surface G, as shown in Figure 25, the space 28 below the caisson body 10 where the caisson shovel 20 is located is filled with filler F or highly fluid concrete C, as described below, from the material rock 34B and material shaft 34A to stabilize the caisson body 10. Next, the material lock 34B is removed from the material shaft 34A, and the portion of the material shaft 34A that protrudes from the ceiling wall 16 is removed. Additionally, the man lock 36B is removed from the man shaft 36A, the portion of the man shaft 36A protruding from the ceiling wall 16 is removed, and the portion of the compressed air supply pipe 38 protruding from the ceiling wall 16 is removed.

[0044] Then, for example, a portion of the material shaft 34 located in the ceiling wall 16 is removed, and filler material F is filled into the inside of the caisson body 10 from this removed area as shown in Figure 26(B), or filler material filling holes are pre-made in the ceiling wall 16, and filler material F is filled into the inside of the caisson body 10 from these filler material filling holes. Of course, filler material F is also filled inside the material shaft 34A, man shaft 36A, and compressed air supply pipe 38. In this case, if low-radiation concrete is used as the filler F, the filler F and the caisson body 10 can be made low-radiation, which is advantageous in reducing the radiation exposure of workers during maintenance. In addition, if high-flow concrete is used as the filler F, the gaps inside the caisson body 10 can be completely sealed with the filler F, making the caisson body 10 less radioactive and advantageous in reducing the amount of radiation exposure of workers during maintenance. Once the filling material F is filled inside the caisson body 10, the holes in the ceiling wall 16 through which the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 were inserted are also blocked with the filling material F, as shown in Figure 26(A). In this way, the reactor building 22 and supporting ground 24 are buried underground together with the caisson body 10, completing the decommissioning work. It is optional to pour concrete to a predetermined thickness on the ground surface G and the upper surface of the ceiling wall 16. If the caisson body 10 is lowered into the ground until the upper surface of the ceiling wall 16 is at approximately the same height as the ground surface G, it is advantageous for maintaining a state in which radiation from radioactive materials remaining in the reactor building 22 and the reactor containment vessel is shielded. However, the caisson body 10 may be lowered into the ground until the upper surface of the ceiling wall 16 is lower than the ground surface G, or, in cases such as when concrete is poured to a predetermined thickness on the ground surface G and the upper surface of the ceiling wall 16, the lowering of the caisson body 10 into the ground may be stopped at a point where the upper surface of the ceiling wall 16 is higher than the ground surface G.

[0045] According to this embodiment, a caisson body 10 is provided that houses the reactor building 22 and supporting ground 24, and the ground below the caisson body 10 is excavated, causing the caisson body housing the reactor building 22 and supporting ground 24 to sink into the ground due to the weight of the reactor building 22, the weight of the supporting ground 24, and the weight of the caisson body 10. After sinking into the ground, the space 28 below the excavated caisson body 10 is filled with filler F or concrete C, and the inside of the caisson body 10 is filled with filler F, so that the bottom wall 12 and side wall 14 of the caisson body 10 are buried in the ground together with the reactor building 22 and supporting ground 24. Therefore, the filler F or concrete C filled in the bottom wall 12 and side wall 14 of the caisson body 10, the space 28 below the excavated caisson body 10, and the filler F filled inside the caisson body 10 will not be exposed to harsh environmental changes such as wind, rain, direct sunlight, or temperature changes, and it will be possible to maintain a state in which the caisson body 10, the filler F or concrete C filled in the space 28 below the caisson body 10, and the filler F filled inside the caisson body 10 shield radiation from radioactive materials remaining in the reactor building 22, the reactor containment vessel, and the supporting ground 24 over an extremely long decommissioning period, for example, 100 or 200 years or more. In addition, it will be possible to maintain a state of shielding from radiation from radioactive materials remaining in the reactor building 22, the reactor containment vessel, and the supporting ground 24 over the extremely long decommissioning period, which will be advantageous in significantly reducing the burden of monitoring and maintenance work over the extremely long decommissioning period.

[0046] In addition, by excavating the ground below the caisson body 10, the caisson body 10 containing the reactor building 22 and supporting ground 24 is caused to sink into the ground by the weight of the reactor building 22, the weight of the supporting ground 24, and the weight of the caisson body 10, which ensures that the caisson body 10 sinks reliably, which is advantageous for carrying out decommissioning work safely and reliably. Of course, this embodiment is also applicable to a method for decommissioning a nuclear reactor when the fuel inside the reactor has been properly removed from the reactor building 22. However, in this invention, the supporting ground 24 contaminated with radioactive materials is contained in the caisson body 10 together with the reactor building 22 and buried underground, so this is suitable as a method for decommissioning a nuclear reactor when a severe accident occurs and the fuel inside the reactor is left as it is.

[0047] (Fifth embodiment) Next, a fifth embodiment will be described with reference to FIG. In the following description of the embodiment, the same parts and members as those in the fourth embodiment will be denoted by the same reference numerals, and the description thereof will be omitted, with the focus being on the differences. In the fifth embodiment, as shown in FIG. 27(A), a filler filling hole 1620 is provided in the ceiling wall 16. Then, as shown in Figure 27 (B), once the caisson body 10 containing the reactor building 22 and supporting ground 24 has been constructed, unlike the fourth embodiment, filler material F is filled into the inside of the caisson body 10 through the filler filling hole 1620, and the filler filling hole 1620 is also blocked with filler material F. The filler F is the same as in the fourth embodiment, and is low-radiative concrete or high-fluidity concrete.

[0048] After filling the inside of the caisson body 10 with filler material F, as in the fourth embodiment, the excavation area is illuminated with lighting fixtures as shown in Figures 23 and 24, and the excavation area is photographed with a camera while the ground below the reactor building 22 is excavated with a caisson shovel 20, and the excavated soil is transported outside the caisson body 10 using a bucket that moves up and down inside the material shaft 34, allowing the caisson body 10 to sink into the ground. After the caisson body 10 has sunk into the ground, as shown in Figure 25, filler material F or concrete C is filled into the space 28 below the caisson body 10 from the material shaft 34, and as shown in Figure 26, the holes in the ceiling wall 16 through which the material shaft 34A, man shaft 36A, and compressed air supply pipe 38 were inserted are blocked with filler material F or concrete C, completing the decommissioning work.

[0049] The fifth embodiment differs from the fourth embodiment in that the work of filling the inside of the caisson body 10 with the filler material F is carried out before the caisson body 10 is lowered. According to the fifth embodiment, when the caisson body 10 sinks into the ground, the weight of the reactor building 22, the weight of the supporting ground 24, the weight of the caisson body 10, and the weight of the filler material F filled inside the caisson body 10 are added, so that the caisson body 10 can sink into the ground more reliably, which is advantageous in carrying out decommissioning work more safely and reliably.

[0050] (Sixth embodiment) Next, a sixth embodiment will be described with reference to FIGS. The sixth embodiment differs from the first and fifth embodiments in that the caisson body 10 does not have a ceiling wall 16 before it is lowered, and a ceiling wall 16 is provided after the caisson body 10 is lowered. As shown in Figure 28, the caisson body 10 has a square bottom wall 12 and side walls 14 erected from the four sides of the bottom wall 12, and since it does not have a ceiling wall 16 compared to the caisson body 10 of the fourth embodiment shown in Figure 22, the reactor building 22 and supporting ground 24 are open above the inside of the side walls 14.

[0051] Once the caisson body 10 containing the reactor building 22 and supporting ground 24 has been constructed, the excavation site is illuminated with lighting fixtures and photographed with a camera, and in the ground remote control room, the image of the area around the caisson shovel 20 displayed on a display device is monitored while the caisson shovel 20 is remotely controlled using a shovel operating device, thereby excavating the ground directly below the caisson body 10 and inside the cutting edge 1204A. As the ground inside the cutting edge 1204A is excavated, the weight of the reactor building 22, the weight of the supporting ground 24, and the weight of the caisson body 10 causes the caisson body 10 to sink into the ground, as shown in Figure 29. A bucket (not shown) is suspended by a wire from the material lock 34B and material shaft 34A into the frame-shaped recess 28A, and the excavated soil is dumped into the bucket by a caisson shovel 20 via remote control.The soil dumped into the bucket rises up the material shaft 34 along with the bucket and is efficiently transported from the material lock 34B to the outside of the caisson body 10. As in the fourth embodiment, the sixth embodiment also employs a pneumatic caisson construction method in which compressed air is supplied to the space 28 below the caisson body 10 to prevent groundwater from flowing in while excavating, thereby lowering the caisson body 10. However, of course, in cases where the ground has little groundwater, the open caisson method may also be employed.

[0052] As shown in Figure 30, once the upper end of the side wall 14 has been sunk into the ground by the thickness of the ceiling wall 16 below the ground surface G, as shown in Figure 31, the space 28 below the caisson body 10 is filled with filler F or concrete C from the material rock 34B and material shaft 34A to stabilize the caisson body 10. Next, as shown in FIG. 32, a formwork is assembled above the reactor building 22, concrete is poured into the formwork, and a ceiling wall 16 is provided to connect the upper ends of the side walls 14. Next, as in the fourth embodiment, as shown in Figure 26, the material lock 34B, the portion of the material shaft 34A protruding from the ceiling wall 16, the man lock 36B, the portion of the man shaft 36A protruding from the ceiling wall 16, etc. are removed, the portion of the compressed air supply pipe 38 protruding from the ceiling wall 16 is removed, filler material F is filled inside the caisson body 10, the material shaft insertion hole 1210, the man shaft insertion hole 1212, and the compressed air supply pipe insertion hole 1214 are blocked with filler material F, and the decommissioning work is completed.

[0053] According to the sixth embodiment, when the caisson structure 26 sinks, the space inside the side wall 14 is always open upward, allowing workers to enter and exit the inside of the caisson body 10, which is advantageous for performing various types of work. [Explanation of symbols]

[0054] 10 Caisson body 12 Bottom wall 1204 Blade mouth 1204A cutting edge 1210 Material shaft insertion hole 1212 Man shaft insertion hole 1214 Compressed air supply pipe insertion hole 14 Side wall 16 Ceiling Wall 1620 Hole for filling with filler material 20 Caisson Excavator 22 Reactor Building 23 Bottom wall 23A anchor bolt 24 Supporting ground 28A Frame-shaped recess 32 Rail 34A Material Shaft 34B Material Lock 36A Manschaft 36B Manlock 38 Compressed air supply pipe F Filling material G ground surface C. Concrete

Claims

1. A caisson structure was installed to house the reactor building. The ground below the caisson body is excavated to allow the reactor building to sink into the ground together with the caisson body, Filling the inside of the caisson body with a filler material, After the caisson body is lowered, the filler material or concrete is filled into the space below the excavated caisson body, and the reactor building is buried together with the caisson body in the ground; Treated soil with improved soil properties is used as the filler. A method for decommissioning a nuclear reactor.

2. The treated soil is solidified treated soil or liquefied treated soil.

2. The method for decommissioning a nuclear reactor according to claim 1.

3. A caisson structure was installed to house the reactor building. The ground below the caisson body is excavated to allow the reactor building to sink into the ground together with the caisson body, Filling the inside of the caisson body with a filler material, After the caisson body is lowered, the filler material or concrete is filled into the space below the excavated caisson body, and the reactor building is buried together with the caisson body in the ground; Low-radiation concrete or high-fluidity concrete is used as the filler. A method for decommissioning a nuclear reactor.

4. The filling of the filler material into the inside of the caisson body is carried out after the caisson body is sunk.

4. The method for decommissioning a nuclear reactor according to claim 1, wherein the decommissioning step is carried out in a manner similar to that described above.

5. The filling material is filled into the inside of the caisson body before the caisson body is lowered.

4. The method for decommissioning a nuclear reactor according to claim 1, wherein the decommissioning step is carried out in a manner similar to that described above.

6. The caisson body is configured to include a bottom wall having cutting edges protruding from the periphery of its lower surface, side walls rising from the periphery of the bottom wall, and a ceiling wall connecting the upper ends of the side walls, the bottom wall is integrated with the reactor building; 6. The method for decommissioning a nuclear reactor according to any one of claims 1 to 5.

7. excavating a recess surrounding the reactor building in the ground around the reactor building; After placing an excavator that excavates the ground below the caisson body in the recess, the bottom wall is provided on the recess.

7. The method for decommissioning a nuclear reactor according to claim 6.

8. The caisson body is configured to include a bottom wall having cutting edges protruding from the periphery of its lower surface, side walls rising from the periphery of the bottom wall, and a ceiling wall connecting the upper ends of the side walls, the bottom wall is provided below the ground supporting the reactor building and supports the ground supporting the reactor building together with the reactor building.

6. The method for decommissioning a nuclear reactor according to any one of claims 1 to 5.

9. excavating a frame-shaped recess in the ground around the reactor building, the frame-shaped recess surrounding the reactor building and the supporting ground; The bottom wall is constructed from the frame-shaped recess, and the lower part of the side wall is constructed at a height from the periphery of the bottom wall to the ground surface.

9. The method for decommissioning a nuclear reactor according to claim 8.

10. After constructing the bottom wall and the lower part of the side wall, excavate the ground inside the cutting edge to create a space below the caisson body inside the cutting edge, The excavation of the ground below the caisson body is carried out by an excavator carried into the space from the frame-shaped recess. The method for decommissioning a nuclear reactor according to claim 9.

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