Method for dismantling the upper core structure

Abstract

This invention provides a method for dismantling the upper core structure of a reactor while minimizing the effects of radiation and allowing workers to perform their tasks. [Solution] A method for dismantling an upper core structure comprises an upper core plate, an upper core support plate positioned vertically above the upper core plate, and a guide tube having a lower tube and an upper tube and extending vertically from the upper core plate to the upper core support plate, the method comprising the steps of removing the guide tube from a through hole formed in the upper core support plate, and attaching a shielding cover to close the through hole.

Description

Technical Field

[0001] This disclosure relates to a method for disassembling an upper core structure.

Background Art

[0002] Patent Document 1 describes a method for disassembling a nuclear power plant for which decommissioning measures have been determined. In this disassembling method, in order to suppress the period during which the working pool is filled with water, the removal and disassembly work of the in-vessel structures is started before the completion of the removal of the fuel from the fuel storage pool.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, a nuclear reactor has many members with a high radiation dose (dose), including the in-vessel structures which are internal structures. Therefore, when disassembling a nuclear reactor, it often touches members with a high dose, resulting in many restrictions on the working time and working place. Among the in-vessel structures, the upper core structure located at the upper part inside the reactor vessel is also composed of members with various levels of dose. Therefore, it is desired that the operator can perform the disassembly work on the upper core structure while suppressing the influence of radiation.

[0005] This disclosure has been made to solve the above problems, and an object thereof is to provide a method for disassembling an upper core structure that allows an operator to work while suppressing the influence of radiation.

Means for Solving the Problems

[0006] To solve the above problems, the method for dismantling an upper core structure according to the present disclosure comprises an upper core plate, an upper core support plate positioned vertically above the upper core plate, and a guide tube having a lower tube and an upper tube and extending vertically from the upper core plate to the upper core support plate, the method for dismantling an upper core structure comprising the steps of removing the guide tube from a through hole formed in the upper core support plate, and attaching a shielding cover to close the through hole. [Effects of the Invention]

[0007] According to the method for dismantling the upper core structure of this disclosure, workers can perform their tasks while minimizing the effects of radiation. [Brief explanation of the drawing]

[0008] [Figure 1] This is a longitudinal cross-sectional view showing a pressurized water reactor according to the embodiment. [Figure 2] This is a schematic diagram showing a pressurized water reactor, with water stored inside according to the embodiment, with its reactor vessel lid removed, positioned inside a pool within the reactor building. [Figure 3] This is a perspective view showing the upper core structure according to the embodiment. [Figure 4] This is a longitudinal cross-sectional view showing the upper core structure according to the embodiment. [Figure 5] This figure shows the positional relationship of the guide tubes with respect to the upper core support plate according to the embodiment. [Figure 6] This diagram shows the relationship between the upper core support plate and the guide tubes being fixed to the upper core plate according to this embodiment. [Figure 7] This is a flowchart illustrating a method for dismantling the upper core structure according to the embodiment. [Figure 8] This figure shows an example of the upper core support plate after the guide tube according to this embodiment has been removed. [Figure 9] This figure shows an example of attaching the shielding cover according to the embodiment to the through-hole of the upper core support plate. [Figure 10]This is a cross-sectional view showing an example of the configuration of the shielding cover according to the embodiment and its positional relationship with the upper core support plate. [Figure 11] This is a cross-sectional view showing an example of the configuration of the shielding cover and its positional relationship with the upper core support plate according to a modified embodiment. [Modes for carrying out the invention]

[0009] The following describes embodiments for implementing the method of dismantling the upper core structure according to this disclosure, with reference to the attached drawings. However, this disclosure is not limited to these embodiments.

[0010] <Embodiment> (nuclear reactor) Figure 1 is a longitudinal cross-sectional view showing a pressurized water reactor 1, which is the reactor to be dismantled according to this embodiment. The reactor is a pressurized water reactor 1 (PWR) that uses light water as a reactor coolant and neutron moderator to create high-temperature, high-pressure water that does not boil throughout the entire core 7, sends this high-temperature, high-pressure water to a steam generator to generate steam through heat exchange, and sends this steam to a turbine generator to generate electricity.

[0011] As shown in Figure 2, the pressurized water reactor 1 is located inside the reactor building pool 100. The reactor building pool 100 has a space where cooling water (water) can be stored. The reactor building pool 100 in this embodiment has a first cavity 110 in which the pressurized water reactor 1 is located, and a second cavity 120 located adjacent to the first cavity 110. The first cavity 110 has a first floor surface 111 in which workers can walk. The second cavity 120 has a second floor surface 121 that is recessed from the first floor surface 111. That is, the second floor surface 121 is located vertically Dv below the first floor surface 111. As a result, the second cavity 120 is formed as a space that is recessed vertically Dv lower than the first cavity 110.

[0012] As shown in FIGS. 1 and 2, the pressurized water reactor 1 of the present embodiment includes a reactor vessel 2, a control rod drive device 3, an upper core structure 5, and a lower core structure 6.

[0013] The reactor vessel 2 has a reactor vessel main body 21 and a reactor vessel head 22 (upper vessel head) so that in-vessel structures can be inserted therein. The reactor vessel 2 is disposed inside a hole formed to be recessed with respect to the first floor surface 111. The reactor vessel 2 is disposed in a state where a part thereof (specifically, the reactor vessel head 22) protrudes from the first floor surface 111.

[0014] The upper part of the reactor vessel main body 21 can be opened by removing the reactor vessel head 22. The lower part of the reactor vessel main body 21 has a cylindrical shape closed by a lower vessel head having a hemispherical shape. An inlet nozzle 23 (inlet plenum) for supplying light water (coolant) as primary cooling water (water) and an outlet nozzle 24 (outlet plenum) for discharging the light water are formed in the upper part of the reactor vessel main body 21. Further, a water injection nozzle (water injection plenum) not shown is formed in the reactor vessel main body 21 separately from the inlet nozzle 23 and the outlet nozzle 24.

[0015] The reactor vessel head 22 is attached to the upper part of the reactor vessel main body 21. The reactor vessel head 22 is fixedly attached to the reactor vessel main body 21 so as to be openable and closable by a plurality of stud bolts and nuts (not shown).

[0016] The upper core structure 5 is disposed inside the reactor vessel 2. The upper core structure 5 can be withdrawn from the reactor vessel main body 21 by being moved upward in the vertical direction Dv with respect to the reactor vessel main body 21. As shown in FIGS. 3 and 4, the upper core structure 5 of the present embodiment includes an upper core plate 51, an upper core support plate 52, upper core support columns 53, guide tubes 55, fixing bolts 58 (see FIG. 6), and connecting bolts 59 (see FIG. 6).

[0017] Note that the upper core structure 5 does not have only the structure described above. As a configuration not shown in the figure, the upper core structure 5 has other configurations such as a mixer, a thermocouple lead-out pipe, and a reinforcing beam.

[0018] The upper core plate 51 is arranged at a distance below the upper core support plate 52 in the vertical direction Dv. The upper core plate 51 has a disk shape and a large number of through holes are formed therein. Guide tubes 55 and water level gauge support pipes are inserted through the through holes of the upper core plate 51.

[0019] The upper core support plate 52 is arranged at a distance above the upper core plate 51 in the vertical direction Dv. As shown in FIGS. 1 and 2, the upper core support plate 52 is fixed to the reactor vessel body 21 above the inlet nozzle 23 and the outlet nozzle 24 in the vertical direction Dv inside. As shown in FIGS. 3 and 4, the upper core support plate 52 is formed in a disk shape larger than the upper core plate 51. In the upper core support plate 52, a large number of through holes are formed at the same position as the upper core plate 51 when viewed from the vertical direction Dv. Also, the through holes in the present embodiment are circular when viewed from the vertical direction Dv. That is, the through hole is a hole formed so as to penetrate the upper core support plate 52 in a cylindrical shape. Note that the shape of the through hole when viewed from the vertical direction Dv is not limited to being circular. Guide tubes 55 are inserted through the through holes of the upper core support plate 52. Among the plurality of through holes, the holes through which the guide tubes 55 are not inserted are closed by a detachable lid (dotted line portion in FIG. 5). Also, as shown in FIG. 2, the upper surface of the upper core support plate 52 in the present embodiment is arranged at the same height as the first floor surface 111 in the vertical direction Dv in a state where the upper core structure 5 is arranged in the reactor vessel 2.

[0020] As shown in Figures 3 and 4, the multiple upper core support columns 53 are connected to the upper core support plate 52 and the upper core plate 51. The multiple upper core support columns 53 extend in a straight line in the vertical direction Dv. The upper ends of the upper core support columns 53 are fixed to the upper core support plate 52. The lower ends of the upper core support columns 53 are fixed to the upper core plate 51. When viewed from the vertical direction Dv, the multiple upper core support columns 53 are offset from the guide tube 55 so as not to overlap.

[0021] The guide tube 55 is fixed to the upper core support plate 52 by being inserted through a through-hole in the upper core support plate 52. The guide tube 55 guides the vertical movement Dv of the control rod cluster. The guide tube 55 is designed to allow the control cluster to be inserted inside. The guide tube 55 is made of, for example, stainless steel. The guide tube 55 is inserted from above in the vertical direction Dv into the through-hole in the upper core support plate 52 and the through-hole in the upper core plate 51. In other words, the guide tube 55 is movable upward in the vertical direction Dv relative to the upper core support plate 52 and the upper core plate 51 by releasing its fixation. Specifically, the lower end of the guide tube 55 is connected to the upper core plate 51. In other words, the upper end of the guide tube 55 is positioned above in the vertical direction Dv relative to the upper core support plate 52. As shown in Figure 6, the guide tube 55 of this embodiment includes an upper tube 551, a lower tube 555, a fixing bolt 58, and a connecting bolt 59.

[0022] The upper tube 551 extends linearly in the vertical direction Dv. The upper tube 551 is positioned above the upper core support plate 52 in the vertical direction Dv. The lower end of the upper tube 551 is fixed to the lower tube 555. In this embodiment, the upper tube 551 has an upper tube body 553 and an upper flange 554.

[0023] The upper tube body 553 is a tubular member extending in the vertical direction Dv. The upper tube body 553 is designed so that a control rod cluster can be inserted through it. The upper flange 554 is formed at the lower end of the upper tube body 553. The upper flange 554 extends from the upper tube body 553 in the horizontal direction Dh, which is perpendicular to the vertical direction Dv.

[0024] The lower tube 555 is positioned below the upper tube 551 in the vertical direction Dv. The lower tube 555 extends in the vertical direction Dv while being inserted through a through-hole in the upper core support plate 52. The upper end of the lower tube 555 is fixed to the upper core support plate 52. The upper end of the lower tube 555 is also fixed to the lower end of the upper tube 551 at a position different from that of the upper core support plate 52. The lower tube 555 in this embodiment has a lower tube body 557 and a lower flange 558.

[0025] The lower tube body 557 is a tubular member extending in the vertical direction Dv. The control rod cluster can be inserted through the lower tube body 557. The lower flange 558 is formed at the upper end of the lower tube body 557. The lower flange 558 extends horizontally Dh from the lower tube body 557. The lower flange 558 is positioned in contact with the upper flange 554. Specifically, the upper surface of the lower flange 558 (the surface facing upward in the vertical direction Dv) is in contact with the lower surface of the upper flange 554 (the surface facing downward in the vertical direction Dv). The lower surface of the lower flange 558 (the surface facing downward in the vertical direction Dv) is in contact with the upper surface of the upper core support plate 52.

[0026] Furthermore, the lower tube body 557 is fitted to the upper core plate 51. The lower tube body 557's movement in the horizontal direction Dh is restricted by its fitting to the upper core plate 51. The lower tube body 557 is configured to be released when it is separated from the upper core plate 51 in the vertical direction Dv. As an example, the lower tube body 557 and the upper core plate 51 have holes and pins formed in them that fit together.

[0027] As shown in Figure 6, the fixing bolts 58 secure the guide tube 55 to the upper core support plate 52. Multiple fixing bolts (for example, four) are arranged for each guide tube 55. The fixing bolts 58 directly secure the lower tube 555 to the upper core support plate 52. Specifically, the fixing bolts 58 secure the lower flange 558 to the upper core support plate 52 from above in the vertical direction Dv. Furthermore, the fixing bolts 58 are secured to the lower flange 558 and the upper core support plate 52 in a non-rotatable manner. In addition, the fixing bolts 58 do not directly secure the upper tube 551 to the upper core support plate 52.

[0028] The connecting bolts 59 secure the upper tube 551 and the lower tube 555. Multiple connecting bolts 59 are arranged for each guide tube 55. The connecting bolts 59 directly secure the upper tube 551 and the lower tube 555. Specifically, the connecting bolts 59 secure the upper flange 554 to the lower flange 558 from above in the vertical direction Dv. Furthermore, the connecting bolts 59 are secured to the upper flange 554 and the lower flange 558 in a non-rotatable manner. In addition, when viewed from above in the vertical direction Dv, the connecting bolts 59 are positioned away from (not overlapping with) the fixing bolts 58. Each connecting bolt 59 is positioned alongside the nearest fixing bolt 58 in a location that allows simultaneous access by a worker from the upper core support plate 52 without having to move. In other words, the connecting bolts 59 are positioned very close to the fixing bolts 58.

[0029] As shown in Figures 1 and 2, the lower core structure 6 is located inside the reactor vessel 2. Many of the components of the lower core structure 6 are positioned vertically Dv below the upper core structure 5. The lower core structure 6 can be removed from the reactor vessel body 21 by moving it vertically Dv above the reactor vessel body 21. The lower core structure 6 is separable from the upper core structure 5 inside the reactor vessel body 21.

[0030] (Method for dismantling the upper core structure) The following describes a method for dismantling the upper core structure 5 according to the embodiment of this disclosure. Figure 7 is a flowchart showing the method for dismantling the upper core structure 5 according to the embodiment of this disclosure.

[0031] The dismantling method for the upper core structure 5 involves removing the guide tubes 55 and closing the through-holes 521 in the upper core support plate 52. The dismantling method for the upper core structure 5 is carried out in a state where it is possible to contact the internal components of the reactor vessel body 21 installed in the pool 100 inside the reactor building. In other words, the dismantling method for the upper core structure 5 is carried out with the reactor vessel lid 22 removed from the reactor vessel body 21. Furthermore, the dismantling method for the upper core structure 5 is carried out with water stored inside the reactor vessel body 21. Specifically, water is stored inside the reactor vessel body 21 up to the position where the upper core support plate 52 is located. In other words, the upper surface of the upper core support plate 52 is located above the liquid surface of the cooling water in the vertical direction Dv. The lower surface of the upper core support plate 52 may be in contact with the cooling water. In addition, the dismantling method for the upper core structure 5 is carried out with the control rod cluster removed from inside the guide tubes 55.

[0032] In the dismantling method for the upper core structure 5 shown in Figure 7, the first step is to remove the guide tube 55 (step S1). In step S1, the guide tube 55 is removed from the through-hole 521 of the upper core support plate 52. In this step, the fixing bolts 58 are removed first. The fixing bolts 58 are removed while the upper core structure 5 is located inside the reactor vessel body 21. In other words, the upper core structure 5 is located inside the reactor vessel 2, which contains water. Therefore, the upper core plate 51 and the lower tube 555 are located in water. The removal of the fixing bolts 58 is performed on the upper core support plate 52, which is located in an airy environment above the water.

[0033] In this state, the fixing bolts 58 that secure the lower tube 555 to the upper core support plate 52 are removed. More specifically, firstly, the anti-rotation mechanism of the fixing bolts 58 is removed from above in the vertical direction Dv. At this time, in order to release the anti-rotation mechanism, the welded portion or the head of the fixing bolt 58 may be cut off with a cutter or grinder. After that, the fixing bolts 58 are removed from the lower tube 555 and the upper core support plate 52 by pulling them out in the vertical direction Dv. As a result, the guide tube 55 is released from its fixation to the upper core support plate 52 and becomes movable in the vertical direction Dv above the upper core support plate 52 (and upper core plate 51).

[0034] Furthermore, when removing the fixing bolt 58, the connecting bolt 59 that connects the upper tube 551 and the lower tube 555 is loosened while maintaining the connection between the upper tube 551 and the lower tube 555. Specifically, the anti-rotation mechanism of the connecting bolt 59 is removed from above in the vertical direction Dv. This work is performed by workers making contact with and removing the connecting bolts 59 located near the outer circumference of the upper core support plate 52. For connecting bolts 59 that cannot be accessed from near the outer circumference of the upper core support plate 52, a work platform is used. Furthermore, the connecting bolts 59 are loosened to a point where they cannot be completely removed. It is preferable that the connecting bolts 59 are loosened to a point where any sticking or other issues are released, allowing workers to easily remove them. Also, even when the connecting bolts 59 are loosened, the fixing of the upper tube 551 and the lower tube 555 by the connecting bolts 59 is not released. In other words, the connecting bolts 59 are only loosened, not removed.

[0035] After the fixing bolts 58 are removed, the upper tube 551 is pulled up and removed from the upper core support plate 52. Specifically, the upper tube 551 is pulled up and removed from the upper core support plate 52. After the upper tube 551 is removed, the lower tube 555 is pulled up and removed.

[0036] In this embodiment, the method for removing the guide tube 55 is not limited to a specific procedure. For example, the upper tube 551 and the lower tube 555 may be removed separately as described above, or they may be removed simultaneously.

[0037] After the guide tube 55 is removed, a step (step S2) is performed in which a shielding cover 200 (see Figures 9 and 10) is installed to close the through-hole 521 of the upper core support plate 52. In this step, the through-hole 521 of the upper core support plate 52 that is exposed when step S1 is completed is closed. That is, when step S1 is completed, as shown in Figure 8, the space below the upper core support plate 52 in the vertical direction Dv is accessible from the through-hole 521 of the upper core support plate 52. In step S2, as shown in Figure 9, the shielding cover 200 is installed to close the exposed through-hole 521 of the upper core support plate 52 from above in the vertical direction Dv.

[0038] When installing the shielding cover 200, the worker makes contact with it from above the first floor surface 111. For the multiple through-holes 521, the worker installs the shielding cover 200 starting from the through-holes 521 located closer to the outer circumference of the upper core support plate 52 when viewed from above in the vertical direction Dv. As an example of the order in which the shielding cover 200 is installed, all the through-holes 521 located on the outer circumference side of the upper core support plate 52 are closed first, and then the through-holes 521 located on the inner circumference side of the upper core support plate 52 are closed. Alternatively, as another example, the through-holes 521 located closer to the outer circumference of the upper core support plate 52 may be closed first, and then the through-holes 521 (regardless of whether they are on the outer or inner circumference side) located closer to the worker may be closed.

[0039] Figure 10 shows the configuration of the shielding cover 200 to be installed. The shielding cover 200 is configured to be supported by the upper core support plate 52 with a portion of it inserted through a through hole 521 in the upper core support plate 52. The shielding cover 200 is also made of a material capable of shielding a certain amount of radiation. For example, the shielding cover 200 is made of a metal such as iron or lead. The shielding cover 200 in this embodiment has an insertion part 202, a cover part 204, and a handle part 206.

[0040] The insertion portion 202 is the part that is inserted into the through-hole 521 of the upper core support plate 52, in a state where it is installed to close the through-hole 521 of the upper core support plate 52. That is, the insertion portion 202 is formed to be insertable into the through-hole 521 of the upper core support plate 52. The insertion portion 202 is formed in a cylindrical shape extending in the vertical direction Dv. In this embodiment, the insertion portion 202 is formed with a diameter smaller than the diameter of the through-hole 521 of the upper core support plate 52 in the horizontal direction Dh. Furthermore, it is preferable that the insertion portion 202 has approximately the same diameter as the through-hole 521 when viewed from the vertical direction Dv. That is, it is preferable that the insertion portion 202 is formed with a diameter that allows insertion with a slight gap between it and the through-hole 521 along its outer circumference.

[0041] The lid portion 204 is positioned above the upper core support plate 52 and is installed to close the through-hole 521 of the upper core support plate 52. The lid portion 204 is designed to prevent the shielding lid 200 from falling downward through the through-hole 521. The lid portion 204 is formed with an enlarged diameter in the horizontal direction Dh compared to the insertion portion 202. In other words, the lid portion 204 has a larger diameter in the horizontal direction Dh than the diameter of the through-hole 521. Furthermore, the lid portion 204 is designed to cover the entire through-hole 521 when viewed from the vertical direction Dv. The lower surface of the lid portion 204 is positioned in contact with the upper surface of the upper core support plate 52.

[0042] Furthermore, the lid portion 204 is formed with a thickness in the vertical direction Dv that is thinner than that of the insertion portion 202. In other words, the insertion portion 202 is formed with a thickness in the vertical direction Dv that is thinner than that of the lid portion 204. The lid portion 204 is formed with a thickness that is sufficient to support the entire shielding lid 200, but is thinner than that of the insertion portion 202. Moreover, the shielding lid 200 is formed with a thickness in the vertical direction Dv that is equal to the thickness of the upper core support plate 52. That is, the shielding lid 200 is formed so that the thickness from the lower end of the insertion portion 202 to the upper end of the lid portion 204 is equal to the thickness of the upper core support plate 52.

[0043] The handle portion 206 allows workers to easily move the shielding cover 200. The handle portion 206 is formed in a position that overlaps with the cover portion 204 when viewed from the vertical direction Dv. The handle portion 206 is formed in a shape that workers can grasp. Specifically, the handle portion 206 is formed in an arc shape that connects two points on the upper surface of the cover portion 204 and protrudes upward from the cover portion 204. Workers can move the shielding cover 200 by grasping the handle portion 206.

[0044] After shielding covers 200 are installed over all exposed through-holes 521, a process (step S3) is carried out in which workers enter the area above the upper core support plate 52 and the shielding covers 200. One specific example of the work performed in this step is the process of releasing the fixing between the upper core support column 53 and the upper core support plate 52.

[0045] (Effects and Benefits) In the dismantling method for the upper core structure 5 according to this embodiment, firstly, the guide tube 55 is removed from the through hole 521 of the upper core support plate 52. The exposed through hole 521 is then closed by a shielding cover 200. With this process, the through hole 521 of the upper core support plate 52 is closed by the shielding cover 200 after the guide tube 55 has been removed. By installing the shielding cover 200, the diffusion of radiation through the through hole 521 is suppressed. In addition, by installing the shielding cover 200, access to the lower part of the upper core support plate 52 from the through hole 521 can be suppressed. Therefore, working time can be secured in the process after the worker has pulled out the guide tube 55. Furthermore, by closing the through hole 521, movement on the upper core support plate 52 becomes easier, improving work efficiency. Thus, according to the dismantling method for the upper core structure 5 according to this embodiment, workers can work while suppressing the effects of radiation.

[0046] Furthermore, this embodiment includes a step in which, after the shielding cover 200 is installed, a worker enters the area above the upper core support plate 52 and the shielding cover 200. This step ensures that workers have sufficient time to work on the upper core support plate 52 when entering the area above the upper core support plate 52 after the guide tube 55 has been removed, thereby improving work efficiency. As a result, workers can work while minimizing the effects of radiation.

[0047] Furthermore, the shielding cover 200 installed in this embodiment has an insertion portion 202 formed to be insertable into the through hole 521, and a cover portion 204 positioned above the insertion portion 202 and formed with a diameter larger than the diameter of the through hole 521. The shielding cover 200 with this configuration can be positioned with the insertion portion 202 inserted into the through hole 521. The cover portion 204 is formed to completely close the through hole 521 and be supported by the upper core support plate 52. Therefore, the shielding cover 200 can suppress the diffusion of radiation through the through hole 521 without falling downward from the through hole 521. In addition, because the insertion portion 202 is formed with a diameter smaller than the through hole 521, workers can easily install the shielding cover 200. Furthermore, because the insertion portion 202 is formed with approximately the same diameter as the through hole 521, the amount of radiation diffused from the gap formed between the insertion portion 202 and the through hole 521 in the horizontal direction Dh can be further suppressed. In other words, by making the insertion portion 202 insertable into the through hole 521 and having approximately the same diameter as the through hole 521, the diffusion of radiation through the through hole 521 can be further suppressed.

[0048] Furthermore, the insertion portion 202 attached in this embodiment is formed with a thickness in the vertical direction Dv greater than that of the cover portion 204. With a shielding cover 200 of this configuration, the portion of the shielding cover 200 that protrudes above the upper core support plate 52 can be reduced while suppressing the diffusion of radiation through the through-hole 521. Therefore, the height difference between the upper core support plate 52 and the shielding cover 200 can be reduced, making it easier for workers to move above the upper core support plate 52. Thus, workers can work while minimizing the effects of radiation, without being obstructed when moving above the upper core support plate 52.

[0049] Furthermore, the shielding cover 200 installed in this embodiment is formed so that its thickness in the vertical direction Dv is equal to the thickness of the upper core support plate 52. With a shielding cover 200 of this configuration, radiation diffusion can be suppressed to the same extent as the upper core support plate 52. In other words, by closing the through-hole 521 with such a shielding cover 200, the radiation dose at the location where the shielding cover 200 is installed can be made equal to that of the upper core support plate 52. Therefore, workers can work while minimizing the effects of radiation.

[0050] <Modified examples of embodiments> Next, with reference to Figure 11, an embodiment for carrying out the method for dismantling the upper core structure 5 according to a modified example of the embodiment will be described. The only difference in the method for dismantling the upper core structure 5 according to the modified example is the structure of the shielding cover that is attached in step S2 of the embodiment.

[0051] The modified shield cover 200A does not have the handle portion 206 according to the embodiment. Furthermore, the modified shield cover 200A has a mounting portion 208. The mounting portion 208 is designed to allow the attachment of a jig that facilitates the movement of the shield cover 200A. The mounting portion 208 is a hole formed in the cover portion 204 with screw threads. In this modified example, the mounting portion 208 is a hole with two screw threads. A jig, such as the handle portion 206 according to the embodiment, can be fixed to the mounting portion 208 by bolts. Attaching the jig to the mounting portion 208 facilitates the movement of the shield cover 200A, while removing the jig eliminates the portion that protrudes upward from the cover portion 204. In other words, the top surface of the shield cover 200A in this modified example is flat when the jig is removed. This configuration allows workers to safely handle the upper core structure 5 while being less obstructed when moving above the upper core support plate 52.

[0052] (Other embodiments) Although embodiments of this disclosure have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and may include design changes and the like that do not depart from the gist of this disclosure.

[0053] Furthermore, the reactor having the guide tube 55 to be dismantled is not limited to pressurized water reactors 1, but may have an upper core structure 5. For example, the reactor may be a boiling water reactor.

[0054] Furthermore, the installation of the shielding covers 200 and 200A is not limited to being carried out only after all of the guide tubes 55 have been removed. For example, each time a guide tube 55 is removed, the exposed through-hole 521 may be closed with a shielding cover 200 or 200A.

[0055] <Note> The method for dismantling the upper core structure 5 described in the embodiments and modified examples can be understood, for example, as follows.

[0056] (1) A method for dismantling an upper core structure 5 according to the first embodiment, comprising an upper core plate 51, an upper core support plate 52 positioned above the upper core plate 51 in a vertical direction Dv, and a guide tube 55 having a lower tube 555 and an upper tube 551 and extending in the vertical direction Dv so as to penetrate the upper core plate 51 to the upper core support plate 52, the method comprising the steps of removing the guide tube 55 from a through hole 521 formed in the upper core support plate 52, and attaching shielding covers 200, 200A so as to close the through hole 521.

[0057] With this configuration, the through-hole 521 in the upper core support plate 52 is closed by shielding covers 200 and 200A after the guide tube 55 is removed. The installation of shielding covers 200 and 200A suppresses the diffusion of radiation through the through-hole 521. Therefore, it is possible to secure working time in the process after the worker has pulled out the guide tube 55, and work efficiency can be improved. Thus, workers can work while minimizing the effects of radiation.

[0058] (2) The method for dismantling the upper core structure 5 according to the second embodiment is the method for dismantling the upper core structure 5 according to (1), further comprising the step of having workers enter the area above the upper core support plate 52 and the shielding covers 200 and 200A after the shielding covers 200 and 200A have been installed.

[0059] This configuration allows workers to have sufficient time to work on the upper core support plate 52, thereby improving work efficiency. As a result, workers can perform their tasks while minimizing the effects of radiation.

[0060] (3) A method for dismantling the upper core structure 5 according to the third embodiment is the method for dismantling the upper core structure 5 according to (1) or (2), wherein the shielding cover 200, 200A has an insertion portion 202 formed to be insertable into the through hole 521, and a cover portion 204 positioned above the insertion portion 202 and formed to have a diameter larger than the diameter of the through hole 521.

[0061] With this configuration, the shielding covers 200 and 200A can be positioned with the insertion portion 202 inserted into the through-hole 521. The cover portion 204 is formed to close the through-hole 521 and be supported by the upper core support plate 52. Therefore, the shielding covers 200 and 200A can close the through-hole 521 and suppress the diffusion of radiation through the through-hole 521.

[0062] (4) The method for dismantling the upper core structure 5 according to the fourth embodiment is the method for dismantling the upper core structure 5 according to (3), wherein the insertion portion 202 is formed to have a thickness in the vertical direction Dv greater than that of the cover portion 204.

[0063] With this configuration, the portion of the shielding covers 200 and 200A that protrudes above the upper core support plate 52 can be reduced, while suppressing the diffusion of radiation through the through-holes 521. Therefore, workers can work while minimizing the effects of radiation, as they are less likely to be obstructed when moving above the upper core support plate 52.

[0064] (5) The method for dismantling the upper core structure 5 according to the fifth embodiment is the method for dismantling the upper core structure 5 according to (3) or (4), wherein the shielding covers 200, 200A are formed such that their thickness in the vertical direction Dv is equal to the thickness of the upper core support plate 52.

[0065] With this configuration, the shielding covers 200 and 200A can suppress the diffusion of radiation to the same extent as the upper core support plate 52. Therefore, workers can perform their duties while minimizing the effects of radiation. [Explanation of Symbols]

[0066] 1 Pressurized water reactor 2 Reactor vessel 21. Reactor vessel body 22 Reactor vessel lid 23 Inlet nozzle 24 Outlet nozzles 3. Control rod drive mechanism 5. Upper core structure 51 Upper core plate 52 Upper core support plate 521 Through hole 53 Upper core support column 55 Guide tube 551 Upper tube 553 Upper tube body 554 Upper flange 555 Lower tube 557 Lower tube body 558 Lower flange 58 Fixing bolts 59 Connecting bolts 6. Lower core structure 100 Reactor building indoor pool 110 First Cavity 111 First floor 120 Second Cavity 121 Second floor 200 shielding lid 202 Insertion section 204 Lid 206 Handle section 200A shielding lid 208 Mounting part Dv Vertical direction Dh horizontal direction

Claims

1. A method for dismantling an upper core structure comprising an upper core plate, an upper core support plate positioned vertically above the upper core plate, and a guide tube having a lower tube and an upper tube, extending vertically from the upper core plate to the upper core support plate, wherein The steps include removing the guide tube from the through hole formed in the upper core support plate, A step of attaching a shielding cover so as to close the through hole, A method for dismantling the upper core structure, including the core structure.

2. The process further includes, after the shielding cover has been installed, having a worker enter the area above the upper core support plate and the shielding cover. A method for dismantling an upper reactor core structure according to claim 1.

3. The aforementioned shielding cover is, An insertion portion formed to be insertable into the aforementioned through hole, It has a lid portion positioned above the insertion portion and having a diameter larger than the diameter of the through hole, A method for dismantling an upper reactor core structure according to claim 1 or 2.

4. The insertion portion is formed with a thickness in the vertical direction that is greater than that of the lid portion. The method for dismantling the upper core structure according to claim 3.

5. The shielding cover is formed such that its vertical thickness is equal to the thickness of the upper core support plate. The method for dismantling the upper core structure according to claim 3.

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