Activation suppression structure and method for constructing activation suppression structure

Abstract

To provide a radioactivation suppression structure and radioactivation suppression structure construction method that can suppress radioactivation, using panel materials made of timber or timber composite lumbering without keeping a water content of the timber and the like over a long period.SOLUTION: A radioactivation suppression structure of the present invention is the structure that suppresses radioactivation of a wall body closing a room where neutron generates, and comprises: a plate-like shield body; and a plate-like (or a thin film-like) rear surface plate. The shield body of the shield body and the rear surface plate is formed of a wooden plate of a wood-made (or a composite wood-made), and, on the other hand, the rear surface plate contains a boron compound. In a front surface of a wall body, the rear surface plate is arranged, and the shield body is arranged in a front surface of the rear surface plate. The shield body contains the number of carbon atoms more than 0.0261×1024 pieces in a unit pillar (becoming a quadrangular pillar to be set by a unit surface and a plate thickness).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a technology for suppressing activation of walls in rooms such as radiological medical facilities that generate neutrons, such as those used in boron neutron capture therapy (BNCT), research facilities, testing facilities, and industrial facilities. More specifically, the present invention relates to an activation suppression structure equipped with a panel-shaped shielding body formed from wooden boards, and a method for constructing this activation suppression structure. [Background technology]

[0002] Boron neutron capture therapy is a treatment in which cancer cells are taken up by boron compounds, and the cancer cells are destroyed by the nuclear reaction between the boron and neutrons. 10 B) has the property of reacting strongly with low-energy neutrons, including thermal neutrons, and as a result of a nuclear fission reaction between boron and neutrons in cancer cells, particle rays (alpha rays) are generated, which destroy the cancer cells.

[0003] The range of the particle beams generated by nuclear fission reactions is about the diameter of a cancer cell (approximately 10-14 μm), and does not affect normal cells other than cancer cells. Because conventional treatments using X-rays and gamma rays cause almost the same physical damage to normal cells as they do to cancer cells, boron neutron capture therapy is also known as "cancer cell selective treatment," and is currently considered to be the most ideal treatment, particularly for treating malignant brain tumors and malignant melanoma.

[0004] In boron neutron capture therapy, neutron beams are irradiated onto patients using an irradiator or accelerator, and naturally, this irradiation is carried out in a room sealed with concrete walls to prevent neutron beams from leaking outside. Of course, not all of the irradiated neutron beams are absorbed by the patient; some are also absorbed by the walls. Neutrons have no electric charge, so they can reach atomic nuclei in materials relatively easily. Moreover, the low-energy neutrons that are preferably used in boron neutron capture therapy exhibit a significant absorption phenomenon. Furthermore, as a result of absorbing neutrons, some of the materials that make up the walls may undergo the so-called activation phenomenon, in which stable isotopes are converted into radioactive isotopes.

[0005] Concrete that has been activated by short-lived nuclides is known to emit large amounts of radiation. As a result, anyone inside a room enclosed by concrete walls is exposed to unnecessary radiation. Furthermore, when neutrons are irradiated over many years, the concrete walls become increasingly activated, producing large amounts of long-lived nuclides. As a result, these activated concrete walls must be disposed of as radioactive waste, which incurs much higher disposal costs than conventional waste. The main substance responsible for the radiation exposure is Na-24 (sodium-24, half-life 14.96 hours), which emits gamma rays with high energy. Meanwhile, the main substances responsible for radioactive waste are Eu-152 (europium-152, half-life 13.52 years) and Co-60 (cobalt-60, half-life 5.27 years). Na-24 is produced by the nuclear reaction between neutrons in the thermal energy range (below 0.5 eV) and Na-23, which is found in large amounts in concrete, while Eu-152 and Co-60 are produced by the nuclear reaction between neutrons in the thermal energy range and Eu-151 and Co-59, which are found in trace amounts in concrete.

[0006] Thus, in facilities where neutrons that cause activation are generated (hereinafter referred to as "neutron generating facilities"), activation of the walls that enclose the interior has been a major problem. Therefore, various technologies have been proposed to suppress activation of the walls of neutron generating facilities. For example, Patent Document 1 proposes a technology to suppress activation of walls, etc. by using a radiation shield made of wood or a material derived from wood. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-228327 Summary of the Invention [Problem to be solved by the invention]

[0008] Conventionally, polyethylene, paraffin, or water have been the main materials used for neutron shielding. However, polyethylene and paraffin are very expensive, and liquid water is difficult to handle. In this regard, the technology disclosed in Patent Document 1 focuses on the moisture content of wood or wood-derived materials, and is a technology characterized by shielding radiation using the moisture contained in the wood or other materials that form the radiation shield. In other words, the technology makes handling easier by using solid wood or other materials without using expensive materials. Patent Document 1 states that the higher the average moisture content of the wood or other materials, the greater the expected effect, with an average moisture content of 30% or more being preferable, 50% or more being more preferable, and 70% or more being most preferable.

[0009] The technology of Patent Document 1 requires that wood or the like have a high moisture content, but since some of the moisture contained in wood evaporates, it is not easy to maintain a high moisture content over a long period of time. Even if a drying prevention measure is applied to prevent moisture from evaporating, it is difficult to maintain a high moisture content over a long period of time, and it is necessary to periodically check that a high moisture content is being maintained, so it is not practical to use this technology.

[0010] On the other hand, panels (boards) made of wood or wood composite lumber are easy to procure, easy to process, relatively inexpensive, and have a beautiful appearance. In addition, they also have the property of suppressing radioactivity, making them a desirable material for radiation shielding. However, as mentioned above, the moisture content of wood and other materials fluctuates significantly, making it difficult to control it to maintain the required amount.

[0011] The object of the present invention is to solve the problems associated with conventional technology, namely, to provide an activation suppression structure and a method for constructing the same that can suppress activation using panel materials (board materials) made of wood or wood composite lumber without maintaining a high moisture content in wood or the like for a long period of time, i.e., while allowing for fluctuations in the amount of moisture contained in wood or the like. [Means for solving the problem]

[0012] Through extensive research, the inventors of the present application have discovered that the carbon contained in wood has the ability to shield neutrons. The present invention was developed based on an unprecedented concept, focusing on the use of a shielding body made of carbon-containing wood and the fact that neutron shielding performance depends on the number of carbon atoms in the thickness direction of the plate.

[0013] The activation suppression structure of the present invention is a structure for suppressing activation of a wall that encloses a room where neutrons are generated, and is equipped with a plate-shaped shielding body and a plate-shaped (or thin film) back plate. The shielding body is formed from a wooden board made of wood (or composite wood), and one of the back plates contains a boron compound. The back plate is disposed on the front surface of the wall, and the shielding body is disposed on the front surface of the back plate. The shielding body has a density of 0.0261 x 10 within a unit column (a square column formed by a unit surface of 1 cm x 1 cm and a plate thickness). 24 It contains 1 or more carbon atoms.

[0014] The radiation suppression structure of the present invention may also be configured such that the shielding is made of two or more wooden boards (each made of a different material) stacked in the thickness direction. 24 In this case, the wooden board with the larger number of carbon atoms contained in the unit column can be placed closer to the wall.

[0015] The method for constructing an activation suppression structure of the present invention is a method for constructing an activation suppression structure of the present invention, and includes a shielding planning step and a structure installation step. In the shielding planning step, the specifications of a plate-shaped shielding formed from a wooden plate made of wood (or composite wood) are planned. In the structure installation step, a plate-shaped (or thin-film) back plate containing a boron compound is placed on the front surface of a wall, and the back plate and the shielding are installed so that the shielding is placed on the front surface of the back plate. In the shielding planning step, the "required number of carbon atoms" to be contained in each unit column of the shielding is planned, and in the shielding installation step, a shielding containing at least the required number of carbon atoms is installed.

[0016] The method for constructing an activation suppression structure of the present invention requires that the number of carbon atoms contained in a unit pillar be 0.0261 × 10 24 It can also be planned as an individual. [Effects of the Invention]

[0017] The activation suppression structure and the method for constructing the activation suppression structure of the present invention have the following effects. (1) The shielding body (wooden board) used in the present invention is easy to procure, easy to process, and relatively inexpensive, and the radiation suppression structure formed by this shielding body has an aesthetically pleasing appearance. (2) The back plate absorbs thermal neutrons moderated by the shielding, which can more effectively suppress the activation of the walls than conventional technology. As a result, unnecessary exposure of people in the neutron generation chamber can be avoided, and the discharge of radioactive waste can be minimized. (3) No drying prevention means is required to maintain a high moisture content, and wood of various moisture contents, such as air-dried wood and bone-dried wood, can be used. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a plan view showing the state in which the activation suppression structure of the present invention is installed in a neutron generation chamber. [Figure 2] A cross-sectional view of the radiation suppression structure and concrete wall cut vertically. [Figure 3] (a) is a front view showing the surface of the shielding body as seen from inside the room, and (b) is a perspective view explaining the "unit column." [Figure 4] (a) is a graph showing the amount of activation (Eu-152) at each depth in concrete when seven types of wooden boards with different carbon atom content are placed, and (b) is a graph showing the amount of activation (Co-60) at each depth in concrete when seven types of wooden boards are placed. [Figure 5] (a) is a graph showing the amount of activation (Eu-152) at each concrete depth when wooden boards of different thicknesses are placed, and (b) is a graph showing the amount of activation (Co-60) at each concrete depth when wooden boards of different thicknesses are placed. [Figure 6] This is a graph showing the experimental results of irradiating neutrons onto air-dry and bone-dry shielding placed on concrete, and showing the amount of activation of the concrete measured. [Figure 7]A cross-sectional view showing a shield made of two different types of wooden boards. [Figure 8] (a) is a flow diagram showing the main steps in constructing an activation suppression structure in which the shielding body and back plate are integrated, and (b) is a flow diagram showing the main steps in constructing an activation suppression structure in which the shielding body and back plate are separate. DETAILED DESCRIPTION OF THE INVENTION

[0019] 1.Overview An example of an embodiment of the activation suppression structure and activation suppression structure construction method of the present invention will be described with reference to the drawings. FIG. 1 is a plan view from above of the activation suppression structure 100 of the present invention installed in a space where neutrons are generated (hereinafter referred to as a "neutron generation chamber"). The neutron generation chamber shown in this figure is closed (sealed) by a concrete wall (hereinafter simply referred to as a "concrete wall CW"), and an accelerator ND that generates neutrons is installed inside the chamber. Note that while FIG. 1 shows a neutron generation chamber installed with an accelerator ND, the present invention can be effectively implemented in any neutron generation chamber that generates neutrons, not limited to an accelerator ND.

[0020] 2.Activation suppression structure Next, an example of the activation suppression structure 100 of the present invention will be described in detail with reference to the drawings. Note that the activation suppression structure construction method of the present invention is a method for constructing the activation suppression structure 100 of the present invention, and therefore the activation suppression structure 100 of the present invention will be described first, and then the activation suppression structure construction method of the present invention will be described.

[0021] As shown in FIG. 1, the activation suppression structure 100 of the present invention is installed on the indoor side (front side) of the concrete wall CW, i.e., so as to cover the surface of the concrete wall CW. As shown in detail in FIG. 2, the activation suppression structure 100 is a structure including a shielding body 110 and a back panel 120. The back panel 120 is disposed on the indoor side (front side) of the concrete wall CW, and the shielding body 110 is disposed on the indoor side (front side) of the back panel 120. FIG. 2 is a cross-sectional view taken along the line AA in the plan view of FIG. 1, in which the activation suppression structure 100 and the concrete wall CW are cut along a vertical plane. While the activation suppression structure 100 is installed on the indoor side (front side) of the concrete wall CW that forms the wall surface in FIGS. 1 and 2, the activation suppression structure 100 can also be installed on the front side of the concrete wall CW that forms the ceiling or floor, not just the wall surface.

[0022] The back plate 120 is a plate-like or thin-film member made of a boron-containing resin, and may be, for example, a member molded from a resin containing B4C. Of course, the resin material is not limited to B4C resin as long as it contains boron; other members may also be used as the back plate 120, such as a member made by mixing boric anhydride into a resin or a member made by mixing powdered colemanite into a resin.

[0023] The shielding body 110 is formed from a "wooden board." Here, a "wooden board" refers to a panel-like (board-like) member made of wood (for example, hardboard such as purple heart or ipe) or composite wood such as laminated timber or CLT (Cross Laminated Timber).

[0024] As mentioned above, the inventors of the present application have discovered that the carbon contained in wood has the ability to shield neutrons. They have also found that for effective neutron shielding, the "required number of carbon atoms" in the thickness direction of the shielding body 110 (i.e., the wooden board) is important, and that the thickness of the shielding body 110 (hereinafter referred to as "board thickness") also has an effect. The "required number of carbon atoms" will now be explained with reference to FIG. 3. FIG. 3(a) is a front view showing the surface of the shielding body 110 as seen from the indoor side, and FIG. 3(b) is a perspective view illustrating a "unit pillar." To determine the required number of carbon atoms, a "unit pillar" must first be established. Specifically, as shown in FIG. 3(a), a 1 cm × 1 cm rectangle (hereinafter referred to as the "unit surface") is established on the surface of the shielding body 110. As shown in FIG. 3(b), a rectangular pillar defined by this unit surface and the board thickness (FIG. 2) is the "unit pillar." In other words, this unit pillar is a rectangular pillar with the unit surface as its base and the board thickness as its height. The number of carbon atoms contained in this unit column is the "number of contained carbon atoms," and the minimum number of contained carbon atoms required to shield neutrons is the "required number of contained carbon atoms."

[0025] To suppress activation of the concrete wall CW, it is necessary to restrict the movement of neutrons in the thickness direction. Therefore, it is desirable that the number of carbon atoms contained is always greater than or equal to the required number no matter where neutrons pass through the surface of the shielding 110. If the number of carbon atoms contained is significantly high in some areas of the surface of the shielding 110 and significantly low in other areas, these areas will become weak points and cause the activation of the concrete wall CW to progress. Therefore, it is desirable that the shielding 110 used in the activation suppression structure 100 has a number of carbon atoms contained on each unit surface (i.e., each unit column) that is greater than or equal to the required number of carbon atoms no matter where the unit surface is set on its surface.

[0026] The analytical results and experimental results carried out by the inventors of the present application are described below. The inventors have carried out two types of analyses, namely, an analysis to determine the necessary number of carbon atoms contained (hereinafter referred to as "first analysis") and an analysis to determine the appropriate board thickness (hereinafter referred to as "second analysis"), as well as an experiment to confirm that the shielding body 110 (i.e., the wooden board) can shield neutrons regardless of its moisture content (hereinafter referred to as "confirmation experiment").

[0027] (First analysis) Several types of wooden boards containing different numbers of carbon atoms were prepared (assumed), and a simulation analysis was performed to confirm the changes in Eu-152 and Co-60 produced in the concrete in each case. The simulation analysis codes used were the PHITS code and the DCHAIN code, developed by JAEA and others. The neutron energy conditions used in the simulation analysis were epithermal neutrons (40 keV), which are used in BNCT facilities, which are expected to expand in the future and are considered to be the most effective for this invention, and are also used in research and development facilities for neutron-based engines and fuel cells.

[0028] The amount of activation in concrete varies depending on the neutron spectrum at the concrete surface, the neutron irradiation time and interval, and the elemental composition and content of the concrete. The first analysis calculates the amount of activation after 30 years (at the end of the facility's operation period) when concrete is irradiated with epithermal neutrons (40 keV) for 1,588 hours per year, based on actual facility usage. Furthermore, Co-60 and Eu-152, which are problematic in concrete activation, are present in trace amounts in concrete, and their content and distribution within concrete are not constant and are complex. Therefore, the values for the amount of Co-60 and Eu-152 contained in concrete are used here as specified in the standard content (NUREG / CR-3474) published by the United States Nuclear Regulatory Commission.

[0029] Specifically, we analyzed the amount of activation at each depth (distance from the surface) of concrete by placing a B4C board (thickness t = 1 cm) on the surface of the concrete, and then placing an 8cm-thick wooden board in front of the B4C board and irradiating neutrons from the surface of the wooden board. Figure 4 shows the analysis results for the amount of activation at each depth of concrete when seven types of wooden boards T11 to T17, each containing different amounts of carbon atoms, were placed. (a) shows the amount of activation for Eu-152, and (b) shows the amount of activation for Co-60. For comparison, the case without a panel (T10) is also shown.

[0030] The number of carbon atoms contained in the wooden board T11 set in the analysis is 0.0261 × 10, which is the same as that of purple heart wood. 24 The number of carbon atoms contained in the wooden board T12 is twice that of the wooden board T11, that is, 2 × 0.0261 × 10 24 Similarly, the number of carbon atoms contained in the wooden board T13 is five times that of the wooden board T11 (5 × 0.0261 × 10 24 The number of carbon atoms contained in the wooden board T14 is half that of the wooden board T11 (1 / 2 × 0.0261 × 10 24 The number of carbon atoms contained in the wooden board T15 is 1 / 5 times that of the wooden board T11 (1 / 5 × 0.0261 × 10 24 The number of carbon atoms contained in the wooden board T16 is 1 / 10 times that of the wooden board T11 (1 / 10 × 0.0261 × 10 24 The number of carbon atoms contained in the wooden board T17 is 1 / 100 times that of the wooden board T11 (1 / 100 × 0.0261 × 10 24 (pieces).

[0031] The standard value (i.e., clearance level) for treating concrete as radioactive waste is 0.1 Bq / g for both Eu-152 and Co-60. Therefore, as can be seen from Figure 4, in order to keep both Eu-152 and Co-60 below the clearance level, the number of carbon atoms contained must be 0.0261 x 10 24In other words, according to this analysis, the number of carbon atoms required to be contained in the shield 110 employed in the activation suppression structure 100 of the present invention is 0.0261 × 10 for both Eu-152 and Co-60. 24 Becomes an individual.

[0032] (Second analysis) Several types of wooden boards with different thicknesses were prepared (assumed), and a simulation analysis was performed to confirm the changes in Eu-152 and Co-60 generated in the concrete in each case. The analysis conditions, such as the simulation analysis code used, the neutron energy used in the simulation analysis (epithermal neutrons 40 keV), the annual irradiation time (1588 hours), and the time point of the analysis results (30 years later), were the same as those explained in the first analysis.

[0033] Specifically, a B4C plate (thickness t = 1 cm) was placed on the surface of the concrete, and a carbon-containing material with a carbon content of 0.0261 × 10 24 We analyzed the amount of activation for each depth of concrete (distance from the surface) by assuming that neutrons were irradiated from the surface of the wooden boards after placing wooden boards of different thicknesses. Figure 5 shows the analysis results showing the amount of activation for each depth of concrete when wooden boards of different thicknesses were placed, with (a) showing Eu-152 as the activation amount and (b) showing Co-60 as the activation amount.

[0034] As mentioned above, the clearance level for treating concrete as radioactive waste is 0.1 Bq / g for both Eu-152 and Co-60. Therefore, as can be seen from Figure 5, in order to keep both Eu-152 and Co-60 below the clearance level throughout the entire concrete (i.e., even if the concrete is 5 cm deep), it is advisable to place shielding 110 with a thickness of 8 cm or more. For convenience, the minimum thickness of shielding 110 that keeps both Eu-152 and Co-60 below the clearance level will be referred to as the "minimum thickness" here.

[0035] (Confirmation experiment) Air-dried and bone-dry shields 110 (wooden boards) were prepared, and experiments were conducted to confirm that each shield 110 could shield neutrons. Specifically, a B4C board (thickness t = 1 cm) was placed on the surface of concrete, and an 8 cm thick purple heart (containing 0.0261 x 10 carbon atoms) was placed in front of the B4C board. 24 After placing neutrons (number of pieces) on the surface of the Purple Heart (i.e., wooden board), neutrons containing a large amount of components in the thermal to epithermal energy range (cadmium ratio 5) were irradiated, and the amount of activation of the concrete was measured. The experimental conditions are similar to those of BNCT facilities and research and development facilities using neutrons for engines and fuel cells. However, of the activated substances generated in the concrete, Eu-152 and Co-60 have a small generation probability (cross section) and can only be generated after neutron irradiation over a period of years. Therefore, in this confirmation experiment, Na-24 and Mn-56 (manganese-56), which are generated by neutrons in the same thermal energy range but in a short irradiation time, were measured.

[0036] The results of the confirmation experiment are shown in Figure 6. As shown in this figure, in Case D, where a B4C board and air-dried Purpleheart (wooden board) were placed on the concrete surface, and Case E, where a B4C board and bone-dried Purpleheart (wooden board) were placed on the concrete surface, the generation of Na-24 and Mn-56 was more suppressed than in Case A, which was concrete only, Case B, where air-dried Purpleheart was placed on the concrete surface, Case C, where bone-dried Purpleheart was placed on the concrete surface, Case F, where a B4C board was placed on the concrete surface, and Case G, which was limestone concrete only.Furthermore, when comparing air-dried Purpleheart (Case D) and bone-dried Purpleheart (Case E), no significant difference was observed between the two. Excluding the cases (D and E) in which a B4C plate and air-dried Purpleheart were placed on the concrete surface, the generation of Na-24 and Mn-56 was most suppressed in the cases (F) in which a B4C plate was placed on the concrete surface, followed by the cases (B and C) in which Purpleheart was placed on the concrete surface. Again, the generation of Na-24 and Mn-56 was not significantly suppressed in the cases (A and G) in which only concrete was used. However, it was found that the generation of Na-24 and Mn-56 was more suppressed when limestone concrete, a low-activation concrete (Case G), was used than when ordinary concrete (Case A) was used. In other words, the activation suppression structure 100 of the present invention can shield neutrons regardless of the moisture content of the shielding body 110 (i.e., the wooden board). In other words, it can suppress the activation of concrete while allowing for variations in the moisture content of the wooden board.

[0037] As described above, the shield 110 constituting the activation suppression structure 100 of the present invention has a carbon atom content of 0.0261×10 24 In particular, the number of carbon atoms contained must be 0.0261 × 10 or more. 24When using a shielding body 110 (i.e., a wooden board), it is desirable that the board thickness be equal to or greater than the minimum board thickness (e.g., 8 cm). Note that the shielding body 110 can be formed from one type of wooden board as shown in Fig. 2, or can be formed by stacking (laminarizing) two or more wooden boards made of different materials (two types of wooden boards in the figure) as shown in Fig. 7. Fig. 7 is a cross-sectional view that schematically shows a shielding body 110 made from two types of wooden boards made of different materials.

[0038] In Fig. 7, a back plate 120 is arranged on the indoor side (front side) of a concrete wall CW, a first wooden plate 111 is arranged on the indoor side (front side) of this back plate 120, and a second wooden plate 112 is arranged on the indoor side (front side) of the first wooden plate 111. The first wooden plate 111 and the second wooden plate 112 form a shielding body 110. In this case, it is preferable that the wooden plate 111 and the second wooden plate 112 are made of different materials, that is, materials having different numbers of carbon atoms. Note that a shielding body 110 made of two or more wooden plates also has a number of carbon atoms that is equal to or greater than 0.0261 x 10 24 It is desirable that the number of carbon atoms contained is 0.0261 × 10 or more. 24 When using a shield 110 consisting of two or more wooden boards (i.e., wooden boards), it is desirable that the board thickness be 8 cm or more, which is the minimum board thickness. However, in the case of a unit column set in a shield 110 consisting of two or more wooden boards, that is, a unit column set by each wooden board (in FIG. 7, the first wooden board 111 and the second wooden board 112), the number of carbon atoms contained in the unit column is set to be equal to or greater than the required number of carbon atoms contained (i.e., 0.0261×10 24 It is recommended to combine two or more wooden boards to make a total of 100 pieces.

[0039] When the shielding body 110 is formed using two or more wooden boards, it is preferable to arrange the wooden boards containing fewer carbon atoms closer to the room, and the wooden boards containing more carbon atoms closer to the concrete wall CW. Naturally, the wooden boards containing more carbon atoms have a higher neutron shielding effect. On the other hand, the wooden boards on the room side are inevitably more likely to be damaged by accidental contact, etc. Therefore, the wooden boards with a relatively low shielding effect (containing fewer carbon atoms) are arranged on the room side, and the wooden boards with a relatively high shielding effect (containing more carbon atoms) are arranged on the concrete wall CW side. In this way, even if the wooden boards on the room side are damaged, the activation suppression structure 100 of the present invention can continue to function by simply replacing the wooden boards containing fewer carbon atoms, i.e., by maintaining the wooden boards containing more carbon atoms as they are.

[0040] 3. Activation suppression structure construction method Next, the method for constructing an activation suppression structure of the present invention will be described with reference to Figure 8. Note that the method for constructing an activation suppression structure of the present invention is a method for constructing the activation suppression structure 100 described up to this point, and therefore, we will avoid any explanation that overlaps with the content explained for the activation suppression structure 100, and will only explain content that is unique to the method for constructing an activation suppression structure of the present invention. In other words, content not described here is the same as that explained in "2. Activation suppression structure."

[0041] Figure 8 is a flow chart showing the main steps of the method for constructing an activation suppression structure of the present invention, where (a) shows a case in which the shielding body 110 and the back plate 120 are integrated and installed on the front surface of the concrete wall body CW, and (b) shows a case in which the shielding body 110 and the back plate 120 are each installed individually on the front surface of the concrete wall body CW.

[0042] As shown in Fig. 8(a), first, the specifications of the shielding 110 constituting the activation suppression structure of the present invention are planned (Step 10). At this time, at least the required number of carbon atoms contained in the shielding 110 is planned, and it is also advisable to plan a minimum plate thickness. Then, wooden boards that satisfy the required number of carbon atoms contained and minimum plate thickness planned here are fabricated, and the shielding 110 is fabricated using one or more wooden boards. The shielding 110 and a back plate 120 are then stacked and fixed together, and the resulting structure is transported to the site (work site) (Step 21). Once the structure in which the shielding 110 and back plate 120 are integrated is delivered, the structure is installed in front of the concrete wall CW (Step 30).

[0043] 8(a) illustrates an example in which a structure in which the shielding body 110 and the back plate 120 are integrated is installed, but the shielding body 110 and the back plate 120 can also be delivered and installed as separate bodies. In this case, as shown in FIG. 8(b), once the specifications for the shielding body 110 are planned (Step 10), a wooden board that satisfies the required carbon atom content and minimum board thickness is manufactured, and the separate shielding body 110 and back plate 120 are delivered to the site (worksite) (Step 22). Then, the back plate 120 is installed in front of the concrete wall CW (Step 31), and the back shielding body 110 is installed in front of the back plate 120 (Step 32). [Industrial Applicability]

[0044] The activation suppression structure and the method for constructing an activation suppression structure of the present invention can be particularly effectively used in medical facilities that generate neutrons, such as proton therapy, heavy ion therapy, boron neutron capture therapy, and PET (positron emission tomography) facilities, as well as research facilities, testing facilities, industrial facilities, etc. The present invention solves the current problems faced by facilities that generate neutrons, namely, it promotes the spread of particle beam cancer therapy, reduces unnecessary exposure of radiation workers, and reduces the generation of radioactive waste, so the present invention is not only applicable industrially but is also expected to make a significant contribution to society. [Explanation of symbols]

[0045] 100 Activation suppression structure of the present invention 110 (Radiation Suppression Structure) Shielding 111 First Wooden Plank (of Cover) 112 Second Wooden Plank (of Cover) 120 (Radiation Suppression Structure) Back Plate CW Concrete wall ND accelerator

Claims

1. A structure that suppresses activation of a wall that closes a room where neutrons are generated, A plate-shaped shield formed by a wooden board made of wood or composite wood; a plate-shaped or thin-film-shaped back plate containing a boron compound; the rear plate is disposed on a front surface of the wall body, and the shield is disposed on a front surface of the rear plate; The shielding body is formed by stacking two or more wooden boards made of different materials in a thickness direction, The shielding body consisting of two or more wooden boards is formed into a unit column having a unit surface of 1 cm x 1 cm and a board thickness of 0.0261 x 10 ２４ contains 1 or more carbon atoms, The shielding body is arranged closer to the wall body as the wooden board has a larger number of carbon atoms contained in the unit column. A radiation suppression structure characterized by:

2. A method for constructing an activation suppression structure that suppresses activation of a wall that closes a room where neutrons are generated, comprising the steps of: a shielding planning step of planning specifications for a plate-shaped shielding body formed of a wooden board made of wood or a composite wood; a structure installation step of installing a plate-like or thin-film-like back plate containing a boron compound on the front surface of the wall body and installing the back plate and the shielding body so that the shielding body is located on the front surface of the back plate, The shielding design step includes an analysis step of determining the minimum number of carbon atoms contained in a unit pillar having a unit surface of 1 cm x 1 cm and a plate thickness, In the analysis step, a case is assumed in which neutrons are irradiated from the surface of the wooden board placed in front of the back panel, and a plurality of wooden board cases with different numbers of carbon atoms contained in the unit columns are set, and an analysis is performed to determine the amount of activation for each depth of the concrete wall in each of the wooden board cases, and the smallest number of carbon atoms among the wooden board cases that makes the amount of activation of the concrete wall below the "standard value for treating concrete as radioactive waste" is determined as the minimum number of carbon atoms; In the shielding planning step, a shielding body is planned to be installed so that the number of carbon atoms in the unit pillar is equal to or greater than the minimum number of carbon atoms, In the structure installation step, the shield planned in the shield planning step is installed. A method for constructing a radiation suppression structure.

3. In the structure installation step, two or more wooden boards made of different materials are stacked in a thickness direction and installed such that the wooden board having a larger number of carbon atoms contained in the unit column is closer to the wall body.

3. The method for constructing an activation suppression structure according to claim 2.

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