Method for dismantling the upper core support plate

Abstract

This invention provides a method for dismantling the upper core support plate, which enables efficient storage of radioactive waste in waste containers. [Solution] A method for dismantling an upper core support plate that constitutes an in-reactor structure of a nuclear reactor, comprising an outer peripheral cutting step of separating an annular portion including the outer peripheral edge of the upper core support plate from the central portion inside the annular portion.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses a technique for disassembling in-vessel structures in a reactor vessel in water stored in a working pool and carrying out the disassembled in-vessel structures from the working pool in a method for disassembling a nuclear power plant.

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 power plant as described in Patent Document 1 has an upper core support plate, which is a large plate having a circular contour, inside a reactor vessel. When disassembling the nuclear power plant, this upper core support plate is cut, stored in a waste container as radioactive waste, and then carried out. And, since the upper core support plate may contain a site with a high radioactivity level, a waste container for waste with a high radioactivity level is required. However, waste containers for waste with a high radioactivity level tend to have a small internal storage space. Therefore, if all the cut pieces of the upper core support plate are to be stored in a waste container for waste with a high radioactivity level, a large number of waste containers are required, resulting in problems such as an increase in the cost of the waste containers and the space for storing the waste containers.

[0005] This disclosure has been made in view of the above circumstances, and provides a method for disassembling an upper core support plate that can efficiently store radioactive waste in a waste container. [Means for solving the problem]

[0006] To address the above issues, the following configuration will be adopted. According to a first aspect of this disclosure, a method for dismantling an upper core support plate is a method for dismantling an upper core support plate constituting an in-reactor structure of a nuclear reactor, and includes an outer peripheral cutting step of separating an annular portion including the outer peripheral edge of the upper core support plate from the central portion inside the annular portion. [Effects of the Invention]

[0007] According to this disclosure, radioactive waste can be efficiently stored in waste containers. [Brief explanation of the drawing]

[0008] [Figure 1] This is a longitudinal cross-sectional view showing a pressurized water reactor, which is the reactor to be dismantled according to this embodiment. [Figure 2] This is a schematic diagram showing a pressurized water reactor, with water stored inside according to the embodiment, with the reactor vessel lid removed, positioned inside the reactor building. [Figure 3] This is a perspective view showing the upper core structure according to the embodiment. [Figure 4] This is a perspective view showing a portion of the upper core support plate according to the embodiment, with parts disassembled. [Figure 5] This is a flowchart of the method for dismantling the upper core support plate in the embodiment. [Figure 6] This figure shows the area between the central portion and the annular portion of the upper core support plate in the embodiment. [Figure 7] This is a plan view showing the cutting line of the central portion in the embodiment. [Figure 8] This figure shows the upper core support plate according to a first modified embodiment. [Figure 9] This figure shows the upper core structure according to a second modified example of the embodiment. [Modes for carrying out the invention]

[0009] Next, a method for dismantling the upper core support plate according to the embodiment of this disclosure will be described with reference to the drawings. (nuclear reactor) Figure 1 is a longitudinal cross-sectional view showing a pressurized water reactor, which is the reactor to be dismantled according to this embodiment. The reactor is a pressurized water reactor (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.

[0010] Figure 2 is a schematic diagram showing a pressurized water reactor, with water stored inside and the reactor vessel lid removed, positioned inside the reactor building according to the embodiment. 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.

[0011] As shown in Figures 1 and 2, the pressurized water reactor 1 of this embodiment comprises a reactor vessel 2, a control rod drive unit 3, an upper core structure 5, and a lower core structure 6.

[0012] The reactor vessel 2 has a reactor vessel main body 21 and a reactor vessel head 22 (upper head) so that the 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.

[0013] 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 head having a hemispherical shape. An inlet nozzle 23 (inlet header) for supplying light water (coolant) as primary cooling water (water) and an outlet nozzle 24 (outlet header) 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 header; not shown) is formed in the reactor vessel main body 21 separately from the inlet nozzle 23 and the outlet nozzle 24.

[0014] 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).

[0015] FIG. 3 is a perspective view showing an upper core structure according to the embodiment. 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 FIG. 3, the upper core structure 5 of the present embodiment has at least an upper core plate 51, an upper core support plate 52, upper core support columns 53, guide tubes 55, and a water level gauge support tube 57. Note that the upper core structure 5 is not limited to having only the above structure. For example, the upper core structure 5 may have other components (all not shown) such as a mixer and a thermocouple lead-out tube.

[0016] The upper core plate 51 is disposed at a distance below the upper core support plate 52 in the vertical direction Dv. The upper core plate 51 has a disk shape with a number of through holes.

[0017] The upper core support plate 52 is disposed 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 the reactor vessel body 21.

[0018] As shown in FIG. 3, the upper core support plate 52 exemplified in the present embodiment is formed in a disk shape having an outer diameter larger than that of the upper core plate 51. A plurality of through holes 54 are formed in the upper core support plate 52 at the same positions as a part of the through holes formed in the upper core plate 51 when viewed in the vertical direction Dv. Guide tubes 55 and water level gauge support tubes 57 are inserted through the through holes 54 of the upper core support plate 52. Among the plurality of through holes 54, a detachable lid 52b1 (see FIG. 4) is attached to and closes the through holes 54 through which the guide tubes 55 and the water level gauge support tubes 57 are not inserted. Also, as shown in FIG. 2, the upper surface of the upper core support plate 52 of the present embodiment is disposed at the same height as the first floor surface 111 in the vertical direction Dv when the upper core structure 5 is disposed in the reactor vessel 2.

[0019] As shown in FIG. 3, the plurality of upper core support columns 53 connect the upper core support plate 52 and the upper core plate 51. The plurality of upper core support columns 53 extend in the vertical direction Dv and extend linearly. 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. The plurality of upper core support columns 53 are arranged so as not to overlap with the guide tubes 55 and the water level gauge support tubes 57 when viewed in the vertical direction Dv.

[0020] The guide tube 55 is fixed to the upper core support plate 52 by being inserted through the through-hole 54 of 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 so that the control rod cluster can be inserted into it. 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 54 of the upper core support plate 52 and the through-hole 54 of the upper core plate 51. The lower end of the guide tube 55 is connected to the upper core plate 51.

[0021] 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.

[0022] (Upper core support plate) Figure 4 is an exploded perspective view of a portion of the upper core support plate according to the present disclosure. As shown in Figure 4, the upper core support plate 52 comprises an upper core support plate body 52a and an accessory 52b. The upper core support plate body 52a is plate-shaped with a plurality of through holes 54. The outline of the upper core support plate body 52a, when viewed from the plate thickness direction Dt, is circular. The plurality of through holes 54 formed in the upper core support plate body 52a are arranged in a grid pattern when viewed from the plate thickness direction Dt.

[0023] Multiple through-holes 54 formed in the upper core support plate body 52a are formed in the thickness direction Dt of the upper core support plate body 52a to allow guide tubes 55, water level gauge support pipes 57, etc., to pass through. In this embodiment, the multiple through-holes 54 through which the guide tubes 55 pass and the multiple through-holes 54 through which the water level gauge support pipes 57 pass are all circular and have the same inner diameter. The thickness of the upper core support plate body 52a can be exemplified as about 100 mm. Note that the inner diameters of the multiple through-holes 54 are not limited to being the same.

[0024] The multiple through-holes 54 form a grid pattern, arranged in a first direction D1 perpendicular to the plate thickness direction Dt of the upper core support plate body 52a, and in a second direction D2 perpendicular to both the plate thickness direction Dt and the first direction D1. More specifically, the first row L11 to the nth row L1n (n=7 in this embodiment), consisting of multiple through-holes 54 arranged linearly in the first direction D1, are arranged at intervals in the second direction D2. Of the multiple through-holes 54 constituting these first row L11 to the nth row L1n, the first through-holes 54 in the first direction D1 are located at the same position in the first direction D1, and the second through-holes 54 in the first direction D1 are located at the same position in the first direction D1. Similarly, the third to the nth through-holes 54 in the first direction D1 are located at the same position in the first direction D1. In other words, the first through-hole 54 in the first direction D1 forms a linear row L21 in the second direction D2, and the second through-hole 54 in the first direction D1 forms a linear row L22 in the second direction D2. Similarly, up to the nth through-hole 54, a linear row L2n (n=7 in this embodiment) is formed in the second direction D2. Multiple through-holes 54 are arranged in a grid pattern in this manner. Furthermore, in this embodiment, the arrangement is a grid pattern of n rows of rectangular shapes in plan view, with the through-holes 54 at the four corners of the rectangular shape in plan view omitted, compared to a grid pattern of n rows of rectangular shapes with the same number of through-holes 54 in both the first direction D1 and the second direction D2. Note that Figure 4 illustrates the case where there are up to 7 rows in both the first direction D1 and the second direction D2, but the number of rows is not limited to this.

[0025] (Accessories) The accessory 52b protrudes from the upper core support plate body 52a in the thickness direction Dt. The upper core support plate 52 in this embodiment includes a cover 52b1 and a reinforcing beam 52b2 as accessories 52b. Both the cover 52b1 and the reinforcing beam 52b2 are detachably fixed to the upper core support plate body 52a by fastening members (not shown) such as bolts.

[0026] (lid) The lid 52b1 closes off the unused through-holes 54 among the multiple through-holes 54 formed in the upper core support plate body 52a. The lid 52b1 is sized to be able to close off each of the through-holes 54. The lid 52b1 of this embodiment has a rectangular shape when viewed from the plate thickness direction Dt. The lid 52b1 of this embodiment also has a plate-shaped lid body and circular protrusions that protrude from the lid body and fit into the through-holes 54. The four rectangular corners of the lid 52b1 of this embodiment are fixed to the upper core support plate body 52a by fastening members (not shown) such as bolts. The fastening members (not shown) that fix the lid 52b1 of this embodiment also have anti-rotation and anti-loosening parts (not shown) by welding or the like.

[0027] In this embodiment, the lid 52b1 is attached to the third to fifth three through-holes 54 located in the center of the second row L12 and the sixth row (n-1 row) L16, and to the second and sixth through-holes 54 in the third row L13 to the fifth row (n-2 row) L15, respectively, among the multiple through-holes 54 forming the first row L11 to the seventh row (n-th row) L17. Note that the number of rows of multiple through-holes 54 formed in the upper core support plate body 52a, the number of through-holes 54 in each row, and the positions of the through-holes 54 to which the lid 52b1 is attached are examples and are not limited to the above configuration. "n" is a natural number of 2 or more, and is not limited to 7.

[0028] (Reinforcement beam) The reinforcing beam 52b2 is a member that reinforces the upper core support plate body 52a. In this embodiment, the reinforcing beam 52b2 has a grid-like structure when viewed from the plate thickness direction Dt. Furthermore, the reinforcing beam 52b2 extends perpendicularly from one surface of the upper core support plate body 52a in the plate thickness direction Dt. The reinforcing beam 52b2 has one central frame portion 56 and four outer frame portions 58. In this embodiment, the reinforcing beam 52b2 is fixed to the upper core support plate body 52a by a plurality of fastening members (not shown), such as bolts. In addition, the fastening members (not shown) that fix the reinforcing beam 52b2 in this embodiment have anti-rotation and anti-loosening parts (not shown) by welding or the like, similar to the fastening members that fix the cover 52b1 described above.

[0029] (Method for dismantling the upper core support plate) Figure 5 is a flowchart of the method for dismantling the upper core support plate in an embodiment of the present disclosure. Figure 6 is a diagram showing the area between the central portion and the annular portion of the upper core support plate in an embodiment. As shown in Figure 5, the method S10 for dismantling the upper core support plate in this embodiment includes the steps of: obtaining the distribution of the radioactivity level of the upper core support plate in S11; determining the range to be annular in S12; determining a waste container according to the radioactivity level of the annular in S13; separating the annular in S14; dividing the annular in S15; and storing the divided annular in a waste container in S16.

[0030] In step S11, which involves obtaining the distribution of radioactivity levels of the upper core support plate, information on the distribution of radioactivity levels of the upper core support plate 52 is obtained. The radioactivity level of the upper core support plate 52 may be the radioactivity level measured by a measuring instrument, or it may be the radioactivity level obtained by simulation or calculation.

[0031] Here, the radioactive nuclides that contribute to determining the radioactivity level of the upper core support plate 52 are mainly generated by the activation reaction between neutrons emitted from the fuel assemblies 71 in the lower core structure 6 and the upper core support plate 52. This activation reaction is particularly pronounced with low-energy neutrons (also called thermal neutrons). Neutrons emitted from the fuel assemblies 71 collide with the coolant (light water) and metal structures such as guide tubes 55 present in the reactor vessel 2, scattering and diffusing. This neutron scattering effect is greater with coolant (light water) than with metal structures. The energy of neutrons passing through the region where coolant (light water) is present tends to decrease particularly easily, so a large number of low-energy neutrons (thermal neutrons) are present in the region where coolant (light water) is present.

[0032] In the upper core structure 5, many tubular in-reactor structures such as guide tubes 55 are present in the central part close to the central axis O1 of the reactor vessel 2, which has a circular horizontal cross-sectional shape, making it difficult for neutron energy to decrease. On the other hand, in the upper core structure 5, near the inner circumference of the reactor vessel 2, there are almost no metal structures and the area is filled with coolant (light water), so neutron energy decreases and there are many thermal neutrons, which are lower energy neutrons. Therefore, there is a difference in the activation reaction between the upper core structure 5 near the inner circumference of the reactor vessel 2 (in other words, the region including the outer edge 52e of the upper core support plate 52) and the central part close to the central axis O1. As a result, the radioactivity level of the annular portion 52p, which includes the outer edge 52e of the upper core support plate 52, where the activation reaction is significant, tends to be higher than the radioactivity level of the central portion 52c, which is inside the annular portion 52p. In step S11, which involves obtaining the distribution of radioactivity levels in the upper core support plate, information on the distribution of radioactivity levels in the radial direction Dr of the upper core support plate 52 is obtained as described above. In the following explanation, an example will be given in which the annular portion 52p has an extremely low radioactivity level of L3, and the central portion 52c has a CL level that does not need to be treated as radioactive material.

[0033] In step S12, which determines the range to be the annular portion, the range of the upper core support plate 52 that will be the annular portion 52p is determined. In this embodiment, as shown in Figure 6, based on the distribution of radioactivity levels obtained in step S11, the range of the upper core support plate 52 that is at the L3 level and is to be stored in a waste container (not shown) is determined, starting from the outer peripheral edge 52e and moving inward. In this embodiment, since the contour of the upper core support plate 52 as viewed from the plate thickness direction Dt is substantially a perfect circle, the annular portion 52p is an annular range with a certain width extending radially inward from the outer peripheral edge 52e in the Dr direction. In this embodiment, the annular portion 52p is the part located radially in the Dr direction centered on the central axis O1 that is outside the circular dashed-dotted boundary line 60 centered on the central axis O1 shown in Figure 6.

[0034] In step S13, which determines a waste container according to the radioactivity level of the annular portion, the type of waste container to store the annular portion 52p is determined based on the distribution of radioactivity levels obtained in step S11. In step S13 of this embodiment, a waste container capable of storing L3 level radioactive waste is determined. The radioactive container determined in step S13 has a predetermined storage space. In this embodiment, the storage space of the radioactive container determined in step S13 is smaller than the overall size of the annular portion 52p.

[0035] In step S14, the annular portion 52p is separated from the central portion 52c. Specifically, in step S14, the upper core support plate 52 is cut into a circular shape, for example, using a tool such as a handsaw or a plasma cutter, to separate the annular portion 52p from the central portion 52c. Note that a robotic arm or the like may be used to cut the upper core support plate 52.

[0036] In step S15, which divides the annular portion, the annular portion 52p is divided to a size that can be stored in a waste container according to the radioactivity level. For example, the spacing in the circumferential direction Dc for dividing the annular portion 52p is determined, and the annular portion 52p is cut to form multiple divided pieces 85. In this embodiment, as shown in Figure 6, multiple dividing lines 65 are set that extend radially in Dr at intervals in the circumferential direction Dc centered on the central axis O1, and the annular portion 52p is cut at the positions of these dividing lines 65. As a result, multiple divided pieces 85 are formed as shown in Figure 7. Although the case of dividing the annular portion 52p in the circumferential direction Dc has been described, further division in the radial direction Dr may be added to the division in the circumferential direction Dc. This makes the size of the divided pieces 85 smaller, so that the size of the divided pieces 85 can be more reliably made to a size that can be stored in a waste container according to the radioactivity level. In addition, the position and direction of cutting the annular portion 52p when forming the divided pieces 85 can be set as appropriate, as long as it is possible to form multiple divided pieces 85 divided in the circumferential direction Dc. Furthermore, although this embodiment describes the case where process S15 is performed after process S14 as an example, process S14 may also be performed after process S15.

[0037] In step S16, which involves storing the divided annular portion in a waste container, the divided pieces 85 of the annular portion 52p formed in step S15 are stored in L3 level waste containers and transported out of the reactor building pool 100.

[0038] In this embodiment, since process S15 is performed after process S14, the central portion 52c is left undivided. In this embodiment, this remaining central portion 52c is cut in a grid pattern. Cutting the central portion 52c in a grid pattern makes it easier to transport. To cut the central portion 52c in a grid pattern, a cutting device having a cutting blade that moves in a straight line can be used. In this embodiment, at least a lid 52b1 and a reinforcing beam 52b2 are attached to the central portion 52c as accessories 52b. The central portion 52c may be cut after removing the lid 52b1 and the reinforcing beam 52b2 from the central portion 52c, respectively. The timing of cutting the central portion 52c in a grid pattern may be between process S14 and process S15.

[0039] For example, to remove the lid 52b1 and reinforcing beam 52b2 from the upper core support plate body 52a, first, the locking and unlocking mechanisms of all fastening members that secure the lid 52b1 and reinforcing beam 52b2 must be released. When releasing the locking and unlocking mechanisms that are welded, one example is to remove the welded parts of the locking and unlocking mechanisms using tools such as a disc grinder or rotary cutter. Releasing the locking and unlocking mechanisms allows the fastening members to be rotated in the direction of release. Therefore, the fastening members are removed by rotating them using a tool such as a wrench. If the fastening members are stuck and cannot be rotated due to seizing or other reasons, the head of the fastening member may be removed to release the fastening.

[0040] Figure 7 is a plan view showing the cutting line of the central portion in the embodiment. As shown in Figure 7, in order to cut the central portion 52c, which has been separated from the annular portion, into a grid pattern, a straight cutting line 80 is set on the central portion 52c. Here, the cutting line 80 is a straight line that indicates the position through which the cutting blade passes when cutting the central portion 52c. In this embodiment, two types of cutting lines 80 are set: a first cutting line 81 and a second cutting line 82. Multiple first cutting lines 81 are set extending in the first direction D1 and spaced apart in the second direction D2. Multiple second cutting lines 82 are set extending in the second direction D2 and spaced apart in the first direction D1. These cutting lines 80 pass through multiple through holes 54. Note that the cutting lines 80 may not pass through the through holes 54.

[0041] In this embodiment, two first cutting lines 81 and two second cutting lines 82 perpendicular to the two first cutting lines 81 are defined. The two first cutting lines 81 divide the area on the central portion 52c into three parts in the second direction D2, and the two second cutting lines 82 divide the area on the central portion 52c into three parts in the first direction D1. By defining these two first cutting lines 81 and two second cutting lines 82, the upper core support plate body 52a of this embodiment is divided into nine areas.

[0042] Then, the central portion 52c is cut along the first cutting line 81 and the second cutting line 82 using a cutting blade. Since the radioactivity level of the cut pieces of the central portion 52c is at the CL level, they can be removed from the reactor building pool 100 without being stored in a waste container. Alternatively, the cut pieces of the central portion 52c may be stored in a waste container with a radioactivity level lower than that of the annular portion 52p and removed from the reactor building pool 100. Furthermore, although the case of cutting the central portion 52c in a grid pattern has been explained as an example, the cutting shape of the central portion 52c is not limited to a grid pattern.

[0043] (Effects and Benefits) The method for dismantling the upper core support plate in the above embodiment includes a step S14 of separating the annular portion from the central portion 52c which is inside the annular portion 52p, including the outer peripheral edge 52e of the upper core support plate 52. This allows the annular portion 52p, which includes the outer edge 52e of the upper core support plate 52 where the activation reaction is more pronounced compared to the central portion 52c, to be cut from the central portion 52c. As a result, only the annular portion 52p, including the outer edge 52e with a high radioactivity level, can be efficiently stored in a high-radioactivity waste container. On the other hand, since the central portion 52c does not contain any high-radioactivity parts, handling by workers can be made easier.

[0044] Furthermore, the method for dismantling the upper core support plate in the above embodiment further includes step S11 of obtaining the distribution of radioactivity levels of the upper core support plate 52. Then, in step S14 of separating the annular portion, based on the distribution of radioactivity levels of the upper core support plate 52, the portion that has reached a radioactivity level requiring storage in a waste container is separated as the annular portion 52p. This allows for more precise separation of the portion with radioactivity levels requiring storage in a waste container as the annular section 52p. As a result, the annular section 52p can be made to its minimum size, enabling more efficient storage in the waste container.

[0045] Furthermore, in step S15 of dividing the annular portion of the embodiment described above, the annular portion 52p is further divided in the circumferential direction Dc to form a plurality of divided pieces 85. This allows the annular portion 52p to be divided into smaller pieces 85, making it possible to efficiently store the annular portion 52p in a waste container with a high level of radioactivity.

[0046] Furthermore, in the method for dismantling the upper core support plate of the above embodiment, the central portion 52c is cut in a grid pattern along a plurality of first cutting lines 81 extending in a first direction D1 perpendicular to the thickness direction Dt of the upper core support plate 52, and a plurality of second cutting lines 82 extending in a second direction D2 perpendicular to the thickness direction Dt and the first direction D1. This makes it easy to standardize the size of the cut pieces obtained by cutting the central portion 52c. Therefore, the burden on workers involved in cutting the central portion 52c and transporting the cut pieces can be reduced.

[0047] Furthermore, the method for dismantling the upper core support plate of the above embodiment includes a step S13 of determining a waste container according to the radioactivity level of the annular portion, a step S15 of dividing the annular portion 52p so that it can be stored in a waste container according to the radioactivity level, and a step S16 of storing the divided annular portion 52p in a waste container. The space required for a waste container to contain radioactive waste tends to decrease as the radioactivity level of the waste increases. Therefore, by performing steps S13 and S15 above, the size of the annular portion 52p can be adjusted to match the dimensions of the waste container corresponding to the radioactivity level of the annular portion 52p. Consequently, the annular portion 52p separated from the central portion 52c can be easily stored in a waste container appropriate to its radioactivity level.

[0048] (First modified example of the embodiment) Figure 8 shows the upper core support plate according to the first modified embodiment. In the above embodiment, an example was given in which the annular portion 52p and the central portion 52c are cut along a circular boundary line 60. However, the boundary line between the annular portion 52p and the central portion 52c is not limited to a circle. For example, as shown in the first modified example of the embodiment in Figure 8, the cut may be made along a polygonal boundary line 60B made up of straight lines. In Figure 8, an example is given in which the cut is square, but it may also be a polygon with five or more sides.

[0049] (Second modified example of the embodiment) Figure 9 shows the upper core structure according to a second modified example of the embodiment. In the above embodiment, an example was given in which the internal reactor structure (guide tube 55, etc.) extending vertically through the upper core support plate 52 is removed, and then the annular portion 52p is separated from the central portion 52c. However, the embodiment is not limited to this.

[0050] For example, as shown in the second modified embodiment in Figure 9, in the upper core structure 5, with an in-core structure extending vertically, such as a guide tube 55, penetrating the upper core support plate 52, the upper core structure 5 may be cut horizontally in the direction Dh along the upper core support plate 52, and then the annular portion 52p of the upper core support plate 52 may be separated from the central portion 52c. In Figure 9, an example of the cutting position is shown by the dashed-dot cutting line. Although only the tubular guide tube 55 has been given as an example of an in-core structure extending vertically, the upper core support column 53, the water level gauge support pipe 57, and the flow column (not shown) can also be cut as in-core structures extending vertically.

[0051] In this case as well, similar to the embodiment described above, the annular portion 52p can be separated from the central portion 52c. Furthermore, since the in-core structure extending vertically with a higher radioactivity level below the upper core support plate 52 of the upper core structure 5 is cut and removed first, the remaining portion can be handled together with the central portion 52c as a portion with a lower radioactivity level. Therefore, the portion of the upper core structure 5 with a lower radioactivity level can be quickly and easily separated from the portion with a higher radioactivity level, thereby reducing the burden on workers.

[0052] (Third modified example of the embodiment) Furthermore, in the above embodiment, the range to be designated as the annular portion 52p was determined based on the distribution of radioactivity levels of the upper core support plate 52, but the determination of the range to be designated as the annular portion 52p is not limited to this method. For example, the entire area radially outside Dr of the grid-like arrangement of through holes 54 formed in the upper core support plate 52 may be designated as the annular portion 52p. Also, the boundary between the annular portion 52p and the central portion 52c may be set radially outside Dr by a predetermined distance from the through holes 54. Here, the predetermined distance is preferably a distance that allows for easy cutting even when internal reactor structures extending vertically through the through holes 54, and examples include a distance equivalent to the diameter of the through holes 54, more preferably equivalent to the radius of the through holes 54.

[0053] This allows the entire range of high radiation levels (L3 level in this embodiment) to be included in the annular portion 52p without obtaining the radiation level distribution of the upper core support plate 52. Therefore, the operation of separating the annular portion 52p from the central portion 52c can be carried out quickly.

[0054] (Fourth modification of the embodiment) Furthermore, in the above embodiment, step S14 for separating the annular portion illustrates a case where the annular portion 52p is separated from the central portion 52c and then divided into multiple pieces. However, the configuration is not limited to this. For example, the upper core support plate 52 may be cut into a grid pattern or the like to form multiple cut pieces, and then the annular portion 52p may be separated from the central portion 52c. In this way, the annular portion 52p can be separated from the central portion 52c while it is still in the state of the cut pieces, thus eliminating the need for a large workspace and improving work efficiency compared to the case where a large annular portion 52p is separated from the central portion 52c and then divided.

[0055] (Other embodiments) This disclosure is not limited to the configuration of the embodiments described above, and design modifications are possible without departing from the spirit thereof.

[0056] In the above embodiment, the case in which a first cutting line 81 and a second cutting line 82 are set to be mutually orthogonal has been described, but it is not limited to the case in which the first cutting line 81 and the second cutting line 82 are orthogonal. Also, although two types of cutting lines, the first cutting line 81 and the second cutting line 82, have been set, other straight cutting lines may also be set in addition to the first cutting line 81 and the second cutting line 82.

[0057] Furthermore, in the above embodiment, the case in which the annular portion 52p has an extremely low radioactivity level of L3 and the central portion 52c has a radioactivity level of CL, which does not require handling as a radioactive material, was explained as an example. However, the radioactivity levels of the annular portion 52p and the central portion 52c are not limited to the above radioactivity levels, as long as the radioactivity level of the central portion 52c is lower than that of the annular portion 52p. For example, the annular portion 52p may have a radioactivity level higher than L3, or the central portion 52c may have a radioactivity level higher than CL.

[0058] <Note> The method for dismantling the upper core support plate described in the embodiment can be understood, for example, as follows.

[0059] (1) According to the first embodiment, the method S10 for dismantling the upper core support plate is a method S10 for dismantling the upper core support plate which constitutes an in-reactor structure of a nuclear reactor, and includes the step of separating the annular portion 52p from the central portion 52c which is inside the annular portion 52p including the outer peripheral edge 52e of the upper core support plate 52.

[0060] This allows only the annular portion 52p, including the outer edge 52e with a high radioactivity level, to be efficiently stored in a high-radioactivity waste container. On the other hand, since the central portion 52c does not contain any high-radiioactivity areas, handling by workers can be made easier. Therefore, radioactive waste can be efficiently stored in waste containers.

[0061] (2) According to the second embodiment, the method S10 for dismantling the upper core support plate is the method S10 for dismantling the upper core support plate of (1), wherein in the step S14 for separating the annular portion 52p, the portion of the upper core support plate 52 on the outer side of the through hole 54 that penetrates the tubular in-core structures 55, 57 is separated as the annular portion 52p. Examples of tubular in-reactor structures include guide tubes and water level gauge support tubes.

[0062] This allows the entire area of ​​high radioactivity to be included in the annular portion 52p without obtaining the radiation level distribution of the upper core support plate 52. Therefore, the operation of separating the annular portion 52p from the central portion 52c can be carried out quickly.

[0063] (3) According to the third embodiment, the method S10 for dismantling the upper core support plate is the method S10 for dismantling the upper core support plate according to (1) or (2), further comprising the step S11 of obtaining the distribution of radioactivity levels of the upper core support plate 52, wherein in the step S14 of separating the annular portion, based on the distribution of radioactivity levels of the upper core support plate 52, the portion that has a radioactivity level requiring storage in a waste container is separated as the annular portion 52p.

[0064] This allows for more precise separation of the portion with radioactivity levels requiring storage in a waste container as the annular portion 52p. Consequently, the annular portion 52p can be made smaller, enabling more efficient storage of radioactive waste in waste containers.

[0065] (4) According to the fourth embodiment, the method S10 for dismantling the upper core support plate is any one of the methods S10 for dismantling the upper core support plate from (1) to (3), wherein in the step S14 for separating the annular portion 52p, the upper core support plate 52 is cut to form a plurality of cut pieces, and then the annular portion 52p is separated from the central portion 52c.

[0066] This allows the annular portion 52p to be separated from the central portion 52c while it is still in the cut state, eliminating the need for a large space and improving work efficiency.

[0067] (5) According to the fifth embodiment, the method S10 for dismantling the upper core support plate is any one of the methods S10 for dismantling the upper core support plate from (1) to (3), and further includes a step S15 for dividing the annular portion 52p, which is divided in the circumferential direction Dc to form a plurality of divided pieces, after the annular portion 52p has been cut from the central portion 52c by a step S14 for separating the annular portion 52p.

[0068] This allows the annular portion 52p to be divided into smaller pieces, making it possible to efficiently store the annular portion 52p in a waste container with a high level of radioactivity.

[0069] (6) According to the sixth embodiment, the method S10 for dismantling the upper core support plate is the method S10 for dismantling the upper core support plate according to (1) or (2), further comprising: a step S13 for determining a waste container according to the radioactivity level of the annular portion; a step S15 for dividing the annular portion so that it is sized to fit into the waste container according to the radioactivity level; and a step S16 for storing the divided annular portion 52p into the waste container.

[0070] This allows the annular portion 52p to be divided into sections corresponding to the dimensions of the waste container, based on the radioactivity level of that portion.

[0071] (7) According to the seventh embodiment, the method S10 for dismantling the upper core support plate is any one of the methods S10 for dismantling the upper core support plate from (1) to (6), wherein the central portion 52c is cut in a grid pattern along a plurality of straight first cutting lines 81 that extend in a first direction D1 perpendicular to the thickness direction Dt of the upper core support plate 52 and pass through a plurality of through holes 54, and a plurality of straight second cutting lines 82 that extend in a second direction D2 perpendicular to the thickness direction Dt and the first direction D1 and pass through a plurality of through holes 54.

[0072] This makes it easy to standardize the size of the cut pieces obtained by cutting the central portion 52c. Therefore, the burden on workers involved in cutting the central portion 52c and transporting the cut pieces can be reduced.

[0073] (8) According to the eighth aspect, the method S10 for dismantling the upper core support plate is any one of the methods S10 for dismantling the upper core support plate from (1) to (7), wherein in the step S14 for separating the annular portion, with the tubular in-core structures 55, 57 penetrating the upper core support plate 52, the in-core structures 55, 57 are cut horizontally Dh along the upper core support plate 52, and then the annular portion 52p of the upper core support plate 52 is separated from the central portion 52c.

[0074] This allows the annular portion 52p to be separated from the central portion 52c while it is still in the cut piece state. Compared to separating the large annular portion 52p from the central portion 52c before dividing it, this eliminates the need for a large workspace and improves work efficiency. [Explanation of Symbols]

[0075] 1… Pressurized water reactor 2…Reactor vessel 3…Control rod drive mechanism 5…Upper core structure 6…Lower core structure 21…Reactor vessel 22...Reactor vessel lid 23... Inlet nozzle 24…Outlet nozzle 51…Upper core plate 52…Upper core support plate 52a... Upper core support plate body 52b...Accessories 52b1…Lid 52b2…Reinforcement beam 52c…Central part 52e...Outer edge 52p... Circular section 53… Upper core support column 54…Through hole 55... Guide tube 56…Central frame section 57…Water level gauge support pipe 58...Outer frame section 60,60B…boundary line 71…Fuel assembly 80…cutting line 81…First cutting line 82…Second cutting line 85…divided piece 91...Cutting blade 100... Pool inside the reactor building 110... First Cavity 111...First floor 120...Second cavity 121…Second floor surface

Claims

1. A method for dismantling the upper core support plate that constitutes the internal structure of a nuclear reactor, A method for dismantling an upper core support plate, comprising the step of separating the annular portion from the central portion inside the annular portion including the outer edge of the upper core support plate.

2. In the step of separating the annular portion, the portion of the upper core support plate that is on the outer side of the through-hole that penetrates the tubular internal structure is separated as the annular portion. A method for dismantling the upper core support plate according to claim 1.

3. The process further includes obtaining the distribution of radioactivity levels of the upper core support plate, In the process of separating the annular portion, Based on the distribution of radioactivity levels in the upper core support plate, the portion with radioactivity levels requiring storage in a waste container is separated as the annular portion. A method for dismantling the upper core support plate according to claim 1.

4. In the step of separating the annular portion, the upper core support plate is cut to form a plurality of cut pieces, and then the annular portion is separated from the central portion. A method for dismantling the upper core support plate according to claim 1.

5. The step of separating the annular portion includes, after cutting the annular portion from the central portion by the step of separating the annular portion, a step of dividing the annular portion in the circumferential direction to form a plurality of divided pieces. A method for dismantling the upper core support plate according to claim 1.

6. A step of determining a waste container according to the radioactivity level of the ring portion, A step of dividing the annular portion so that it is sized to fit into the waste container corresponding to the radioactivity level, The step further includes storing the divided annular portion in the waste container. A method for dismantling the upper core support plate according to claim 1.

7. The central portion is cut in a grid pattern along a plurality of linear first cutting lines that extend in a first direction perpendicular to the thickness direction of the upper core support plate and pass through a plurality of through holes, and a plurality of linear second cutting lines that extend in a second direction perpendicular to the thickness direction and the first direction and pass through a plurality of through holes. A method for dismantling the upper core support plate according to claim 1.

8. In the process of separating the annular portion, With the tubular in-core structure penetrating the upper core support plate, the in-core structure is cut horizontally along the upper core support plate, and then the annular portion of the upper core support plate is separated from the central portion. A method for dismantling the upper core support plate according to claim 1.

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