Radiation shielding material

Abstract

Provided is a radiating shielding material which contains tungsten (W), boron (B), and carbon (C), wherein: the total number of moles of elements other than tungsten, boron, and carbon is not more than 10 at% of the total number of moles of tungsten, boron, and carbon; and when the contents of tungsten, boron, and carbon with respect to the total of tungsten, boron, and carbon are represented as WA, BA, and CA, the contents WA, BA, and CA are within the range surrounded by points A1, A2, A3, and A4 in fig. 1. The radiation shielding material includes a phase containing a boride of tungsten, further includes a phase containing a carbide of tungsten and / or a phase containing carbon, and is sintered.

Description

Radiation shielding materials

[0001] This disclosure relates to radiation shielding materials. This application claims priority under Japanese Patent Application No. 2024-167029, filed on 26 September 2024. All contents contained in said Japanese Patent Application are incorporated herein by reference.

[0002] Conventionally, radiation shielding materials have been disclosed, for example, in Japanese Patent Publication No. 2017-524928.

[0003] Special table 2017-524928 publication

[0004] This disclosure relates to a radiation shielding material comprising tungsten (W), boron (B), and carbon (C), wherein the total number of moles of elements other than tungsten, boron, and carbon is 10 atomic percent or less of the total number of moles of tungsten, boron, and carbon, and the content of tungsten, boron, and carbon relative to the total number of tungsten, boron, and carbon is WA, BA, and CA, respectively, and the content WA, BA, and CA are within the range enclosed by points A1, A2, A3, and A4 in Figure 1, and comprises a phase containing tungsten boride, and further comprises at least one of a phase containing tungsten carbide and a phase containing carbon, and is sintered.

[0005] Figure 1 is a ternary phase diagram showing the composition ratio of W, B, and C, WA-BA-CA, and represents points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%), A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A4 (WA: 20 atomic%, BA: 40 atomic%, CA: 40 atomic%). Figure 2 is a ternary phase diagram showing the composition ratio WA-BA-CA of W, B, and C, and shows points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A5 (WA: 23 atomic%, BA: 50 atomic%, CA: 27 atomic%). Figure 3 is a ternary phase diagram showing the composition ratio WA-BA-CA of W, B, and C, and shows points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A6 (WA: 40 atomic%, BA: 60 atomic%, CA: 0 atomic%), and A7 (WA: 25 atomic%, BA: 60 atomic%, CA: 15 atomic%).

[0006] The required performance characteristics included fracture toughness, heat resistance, neutron attenuation, and gamma ray attenuation. The embodiments of this disclosure will be listed and described first.

[0007] This disclosure relates to a radiation shielding material comprising tungsten (W), boron (B), and carbon (C), and which is sintered.

[0008] WA, BA, and CA are defined as follows, with the total number of moles being the sum of the number of moles of tungsten, boron, and carbon contained in the radiation shielding material (hereinafter also simply referred to as "the material").

[0009] WA = number of moles of tungsten contained in the material / total number of moles BA = number of moles of boron contained in the material / total number of moles CA = number of moles of carbon contained in the material / total number of moles Figure 1 is a ternary phase diagram showing the composition ratio of W, B, and C, WA-BA-CA, and shows points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%), A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A4 (WA: 20 atomic%, BA: 40 atomic%, CA: 40 atomic%).

[0010] Preferably, the total number of moles of elements other than tungsten, boron, and carbon is 10 atomic percent or less of the number of moles of tungsten, boron, and carbon. For example, if the total number of moles of tungsten, boron, and carbon is 10, then the number of moles of elements other than tungsten, boron, and carbon is 1 or less. If the content of tungsten, boron, and carbon relative to the total number of tungsten, boron, and carbon is denoted as WA, BA, and CA, then WA, BA, and CA are content rates within the range enclosed by points A1, A2, A3, and A4 in Figure 1, and the material comprises a phase containing tungsten boride, and further comprises at least one of a phase containing tungsten carbide and a phase containing carbon, and is sintered.

[0011] In radiation shielding materials constructed in this manner, performance in terms of fracture toughness, heat resistance, neutron attenuation, and gamma ray attenuation is enhanced.

[0012] Figure 2 is a ternary phase diagram showing the composition ratios of W, B, and C, WA-BA-CA, and represents points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A5 (WA: 23 atomic%, BA: 50 atomic%, CA: 27 atomic%).

[0013] Preferably, WA, BA, and CA are content levels within the range enclosed by points A2, A3, and A5 in Figure 2.

[0014] Figure 3 is a ternary phase diagram showing the composition ratios of W, B, and C, WA-BA-CA, and represents points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A6 (WA: 40 atomic%, BA: 60 atomic%, CA: 0 atomic%), and A7 (WA: 25 atomic%, BA: 60 atomic%, CA: 15 atomic%).

[0015] Preferably, WA, BA, and CA are content levels within the range enclosed by points A2, A6, and A7 in Figure 3.

[0016] Preferably, the radiation shielding material is WB phase and / or W ２ B ５ It consists of a phase and a carbon-containing phase.

[0017] Preferably, the radiation shielding material contains at least one element selected from Cr, Si, Ti, Hf, Ta, and V in an amount greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon.

[0018] Preferably, the radiation shielding material contains at least Cr in an amount greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon.

[0019] The radiation shielding material disclosed herein exhibits minimal strength reduction even when exposed to high temperatures and can be used in harsh environments, thus providing stable shielding even in environments where thermal load and radiation irradiation occur simultaneously.

[0020] Neutron radiation is broadly classified into high-energy fast neutrons and low-energy thermal neutrons based on their kinetic energy. Fast neutrons have high penetrating power and are difficult to shield. Shielding fast neutrons involves slowing them down to thermal neutrons using inelastic scattering with heavy elements, and then using the absorption reaction of thermal neutrons for shielding. Boron is known as an element with a large thermal neutron absorption cross-section. Secondary gamma rays are generated during inelastic scattering and the absorption reaction of thermal neutrons. Conventionally, shielding fast neutrons has involved a combination of iron, which handles fast neutron slowing and secondary gamma ray shielding, and concrete and boron-containing materials, which handle thermal neutron absorption. However, this approach had problems such as insufficient shielding performance unless the shielding layer was thick enough, and issues with heat resistance. There are also research examples using tungsten boride as a neutron shielding material. Tungsten boride contains heavy elements W and B, which absorbs thermal neutrons, in a specific ratio, allowing it to effectively shield against neutrons. However, it has been difficult to sinter and is brittle. While there are examples of composites with metals such as Fe and Cr as a bonding layer, these have drawbacks in terms of heat resistance and shielding properties. This disclosure has found that by adding a certain amount of carbon, the mechanical properties of the neutron shielding material can be improved without compromising its shielding properties.

[0021] The radiation shielding material of this disclosure contains at least three elements that constitute tungsten-based ceramics: tungsten (W), boron (B), and carbon (C).

[0022] The composition of the radiation shielding material is preferably within the polygon formed by connecting points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%), A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A4 (WA: 20 atomic%, BA: 40 atomic%, CA: 40 atomic%) on the WA-BA-CA ternary phase diagram, which represents the composition ratio of W, B, and C. Within this range, tungsten is responsible for slowing down fast neutrons and shielding against secondary gamma rays. Boron is responsible for absorbing thermal neutrons. Carbon is preferable because it further slows down thermal neutrons and promotes absorption by boron. Furthermore, the inclusion of carbon, when dispersed in the form of free carbon, allows for improved fracture toughness by providing resistance to crack propagation. Alternatively, if tungsten carbides with high mechanical properties are formed, the fracture toughness value can be improved, as can the hardness.

[0023] Outside this range, for example, if WA exceeds 50 atomic percent and is excessive, the deceleration of fast neutrons and the shielding effect of gamma rays will increase, but the content of boron and carbon, which are responsible for the deceleration and absorption of thermal neutrons, will decrease, thus reducing the overall shielding performance. The same applies if BA is less than 0.1 atomic percent. If WA is less than 15% and BA is excessive at 72 atomic percent or more, the amount of tungsten, which is responsible for the deceleration of fast neutrons, will decrease, thus reducing the shielding performance. If there is an excess of carbon, a deceleration effect on thermal neutrons can be expected, but because the amounts of W and B are insufficient, the deceleration effect on fast neutrons and the absorption effect on thermal neutrons will also decrease, thus reducing the shielding performance. If there is no carbon at all, there is no dispersed free carbon or tungsten carbide, so there is a risk that mechanical properties such as fracture toughness will decrease. On the other hand, the addition of carbon has the effect of reducing residual oxygen that impairs mechanical properties, so mechanical properties can be improved.

[0024] The composition of the radiation shielding material is preferably inside the polygon formed by connecting points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A3 (WA: 50 atomic%, BA: 50 atomic%, CA: 0 atomic%), and A5 (WA: 23 atomic%, BA: 50 atomic%, CA: 27 atomic%) on the ternary phase diagram of WA-BA-CA, which is the composition ratio of W, B, and C. Within this range, tungsten responsible for slowing down fast neutrons is sufficiently present, and boron responsible for absorbing thermal neutrons is also sufficiently present, thus enhancing the shielding performance. By setting the amount of carbon to exceed 0 atomic% and be less than 30 atomic%, while sufficiently obtaining the thermal neutron slowing effect, the amount of boron responsible for absorbing thermal neutrons can be ensured, and a necessary and sufficient amount of free carbon and tungsten carbide can be dispersed, thereby ensuring fracture toughness.

[0025] Most preferably, the composition of the radiation shielding material is inside the polygon formed by connecting points A2 (WA: 28 atomic%, BA: 72 atomic%, CA: 0 atomic%), A6 (WA: 40 atomic%, BA: 60 atomic%, CA: 0 atomic%), and A7 (WA: 25 atomic%, BA: 60 atomic%, CA: 15 atomic%) on the ternary phase diagram of WA-BA-CA, which is the composition ratio of W, B, and C. In this composition range, the fast neutron slowing effect and the thermal neutron slowing and absorption effects are most efficiently achieved.

[0026] The constituent phases are composed of WB and / or W ２ B ５ phase and the balance being a phase containing at least C. By having tungsten and boron in the form of WB and W ２ B ５ present, the tungsten responsible for the fast neutron slowing effect and the boron responsible for the thermal neutron absorption effect can be uniformly present in the material. Moreover, since these substances are hard and have a high melting point, they can enhance the strength and heat resistance of the material, which is preferable. When present in the form of elemental tungsten and elemental boron, there are problems such as boron being easily hygroscopic and oxidizing, and concerns about a decrease in material strength, so it is not preferable. The phase containing the remaining carbon may exist alone in the form of free carbon or may exist as tungsten carbide by bonding with some tungsten. Alternatively, it may form a carbide with other additive elements.

[0027] WB and W ２ B ５ To obtain the constituent phases in this form, for example, a substance containing W and a substance containing B can be exposed to high temperatures in an inert gas to synthesize tungsten borides through a solid-phase reaction. After crushing the mixture to obtain powder, a high-density sintered body can be obtained by sintering using plasma discharge sintering (SPS). Alternatively, hot pressing or HIP can also be used. It is preferable that the ceramic material be solidified on its own, and it is preferable to avoid solidifying these powders via organic or inorganic binders, as this reduces heat resistance and strength.

[0028] For tungsten-based ceramics used for radiation shielding, it is preferable to include at least one element selected from Cr, Si, Ti, Hf, Ta, and V in an amount exceeding 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon. These elements readily bond with carbon and can form hard, high-melting-point carbides. By adding them, the formation of free carbon, which reduces hardness, can be avoided, thereby improving mechanical properties. In particular, Cr, Si, Ti, and Ta preferentially oxidize to form an oxide film, preventing further oxidation and potentially improving oxidation resistance. Hf also has the advantage of having a high thermal neutron absorption effect, so its addition does not significantly impair the neutron shielding effect. It is preferable to avoid adding more than 10 atomic percent, as this reduces the tungsten and boron content and thus the shielding performance. Furthermore, if a large amount is added, the additive that did not bond with carbon may exist as a metallic phase between the tungsten boride and carbide phases of the matrix, which would significantly reduce heat resistance. These elements can be added individually or selected in combination.

[0029] As a method for adding these elements, they can be easily added by adding and mixing powders containing these elements to the synthesized tungsten boride powder. Alternatively, after forming a coating of these metals on the surface of the fabricated tungsten-based ceramics for radiation shielding by means such as vapor deposition, sputtering, or plating, heat treatment in an inert gas can be performed to allow them to diffuse and penetrate into the material interior. In this case, functionality can be added only near the surface. For example, when Cr, Ti, Si, etc. are preferentially present on the surface, it is possible to improve the oxidation resistance of the material.

[0030] It is more preferable that the tungsten-based ceramics for radiation shielding contain at least Cr in an amount exceeding 0 atomic % and less than 10 atomic % with respect to the total number of moles of tungsten, boron, and carbon. Cr easily binds with carbon to form ２３ C ６ and Cr ３ C ２ such hard carbides as to increase the material strength, and it is also possible to uniformly disperse them in the material because they substitute and solid-solve with tungsten. By solid-solving in the matrix phases of WB and W ２ B ５ it is possible to enhance the oxidation resistance.

[0031] For determining the composition ratios of W, B, and C in the sintered tungsten-based ceramics and investigating the abundance of additive elements, an analytical method using an electron beam microanalyzer in accordance with ISO 22489:2016 can be used. Also, for identifying the constituent phases, the X-ray diffraction method is usually used.

[0032] (1) Sample Nos. 1 - 24 Production of Tungsten-Based Ceramics for Radiation Shielding (1 - 1) Raw Material Preparation Step

[0033]

[0034] As the W source, WO powder with a purity of 99% and a particle size of 30 μm ３ was used, as the B source, a 20% aqueous solution of H ３ BO ３ was used, and as the C source, powder of pyrolytic carbon with a particle size of 0.5 μm was used.

[0035] These raw materials are weighed, for example, in sample number 1, so that the amounts of W, B, and C are 40 atomic%, 40 atomic%, and 20 atomic%, respectively, and in addition, WO ３ , H ３ BO ３ After adding more pyrolysis carbon for reduction, the mixture was placed in a cemented carbide container, along with cemented carbide balls with a diameter of Φ6 mm, and wet mixing was performed for 30 minutes.

[0036] When adding a third element, as in samples 15 to 22, the source of the third element was a powder of the third element itself, or a boride or carbide powder of the third element.

[0037] After mixing, the powder was heated and dried in air at 200°C. The dried powder was placed in a carbon crucible and subjected to a pressure of 5 × 10⁻¹⁰. －３ A mixture of tungsten boride and carbon was obtained by heating synthesis at 1800°C for 1 hour in a Pa vacuum. The obtained mixture was coarsely ground using a jaw crusher and finely ground using a jet mill.

[0038] The raw materials used as W, B, and C sources can be selected as appropriate. While these can be obtained by reduction or thermal decomposition reactions of oxides and nitrides, industrially, supplying them as individual powders is preferable in terms of simplifying the process.

[0039] The synthesis temperature for tungsten boride is preferably 1400°C or higher. At temperatures below this, the solid-phase reaction proceeds slowly, and unreacted tungsten or boron compounds may remain.

[0040] The synthesis temperature for the raw materials of tungsten-based ceramics for radiation shielding is preferably below 1950°C. At temperatures higher than this, although the solid-phase reaction proceeds sufficiently, a liquid phase is formed by the eutectic reaction between tungsten and boron, causing the material to solidify densely, which may make subsequent grinding difficult and is therefore undesirable.

[0041] (1-2) Sintering process of tungsten-based ceramics for radiation shielding 30 g of the obtained powder was taken out and placed in a Φ20 mm carbon mold and sintered using an electrically heated sintering apparatus (LABOX315 model manufactured by Sinterland Co., Ltd.) under the following conditions: heating rate of 50 °C / min, sintering temperature of 1900 °C, holding time of 30 minutes, and pressurized pressure of 50 MPa. As a result of sintering, a cylindrical sintered body with a diameter of 20 mm and a height of 12 mm was obtained.

[0042] (1-3) Compositional analysis The obtained sintered body was removed, both sides were planar ground, and 1 mm was removed from each side to obtain a flat surface. Then, one side was mirror polished using a diamond suspension.

[0043] The constituent phases of the obtained sintered body were identified using powder X-ray diffraction. A Rigaku Smart Lab was used, and CuKα radiation was irradiated, with measurements taken in the range of 20° to 100°. The results of the measurements, as shown in Table 1, are WB, W ２ B ５ It was found that it is composed of C, etc.

[0044] To investigate the composition ratios of elements such as W, B, C, Cr, Si, Ti, Hf, Ta, and V, spot analysis was performed using an electron beam microanalyzer in accordance with ISO 22489:2016. The characteristic X-ray spectrum was spectrally analyzed when an electron beam with an acceleration voltage of 15 kV, a beam current of 50 nA, and a spot diameter of 10 μm was irradiated, and the composition ratio of each element was calculated using the ZAF method, and the composition ratios of W, B, and C were calculated in atomic percent. As a result, for example, in sample number 1, the amount of W (WA) was 40.1 atomic percent, the amount of B (BA) was 39.9 atomic percent, and the amount of C (CA) was 20.0 atomic percent, confirming that it was in line with the target composition.

[0045] The sintering of tungsten-based ceramics for radiation shielding is preferably carried out at 1600°C or higher. If the temperature is lower than this, sintering may not proceed sufficiently, and a dense sintered body may not be obtained. Failure to achieve densification may lead to a decrease in mechanical properties and a decrease in shielding performance. Typically, a sintered body with a relative density of 97% or higher, more preferably 99% or higher, is preferred.

[0046] (1-4) Evaluation of the mechanical properties of tungsten-based ceramics for radiation shielding The density of the obtained sintered body was measured by the Archimedes method based on JIS R1634, and the relative density was determined by dividing the measured bulk density by the theoretical density calculated based on the composition of the sample. As a result of the measurement, for example, sample number 1 had a relative density of 99.5%, indicating that a dense sintered body was obtained.

[0047]

[0048] A Vickers hardness test was performed on the mirror-polished surface of the obtained sintered body, in accordance with JIS R1610. An AVK test machine manufactured by Akashi Corporation was used, with a test force of 30 kgf and a holding time of 15 seconds. Seven measurements were taken, and the average of the five measurements (excluding the maximum and minimum values) was used as the hardness. The results showed, for example, that sample number 1 exhibited a high hardness of 1728 HV.

[0049] The fracture toughness of the obtained sintered body was evaluated by the IF method on the mirror-polished sample surface, according to JIS R1607, with a test force of 30 kgf and a holding time of 15 s. The average value of five measurement points was calculated. For the calculation, the elastic modulus of the sintered body was measured in advance by the ultrasonic pulse method according to JIS R1602 and used in the calculation of the fracture toughness. As a result of the series of measurements, the value was 4.8 MPa·m. １／２ The value was shown.

[0050] The fracture toughness value of the tungsten-based ceramics used for radiation shielding is 4.0 MPa·m. １／２ It is preferable that the value is greater than or equal to this. If it is less than this value, cracks will propagate easily and the object will become prone to fracture.

[0051] (2) Sample numbers 101-118 (2-1) Sample number 101

[0052]

[0053]

[0054] For sample number 1, a 99% pure W powder with a particle size of 3 μm was used as the W source, and a 99.9% pure boron powder was used as the B source. W and B were weighed to be 70 atomic percent and 30 atomic percent, respectively, and placed in a tungsten crucible, 5 × 10⁻⁶－２ The tungsten boride was obtained by heating and synthesizing in a Pa vacuum at 1800°C for 1 hour, followed by coarse grinding and fine grinding, and then sintered using SPS in the same manner as in Example 1. The resulting sintered body was W ２ B ５ It is a single-phase material with a relative density of 99.7%, a Vickers hardness of 2132 HV, and a fracture toughness of 3.0 MPa·m. １／２ That was the case.

[0055] (2-2) Sample No. 102 For Sample No. 102, a 99% purity W powder with a particle size of 3 μm was used as the W source, and a pyrolysis carbon powder with a particle size of 0.5 μm was used as the C source. The W and C were weighed to be 50 atomic%, and the mixture was placed in a tungsten crucible and measured in 5 × 10⁻⁶ units. －２ The tungsten carbide was obtained by heating and synthesizing in a Pa vacuum at 1800°C for 1 hour, followed by coarse and fine grinding, and then sintered using SPS in the same manner as in Example 1. The resulting sintered body was a single layer of WC with a relative density of 100%, a Vickers hardness of 1988 HV, and a fracture toughness of 3.6 MPa·m. １／２ In samples 101 and 102, since they consist only of a tungsten boride phase, there is no phase to stop crack propagation, resulting in low fracture toughness values.

[0056] (2-3) Sample No. 103 Sample No. 103 is tungsten boride (W) prepared using the method of Sample No. 101. ２ B ５ A composite material consisting of a tungsten boride layer and an FeCr alloy layer was fabricated by adding 10 wt% FeCr alloy powder (Fe-8%Cr) to the powder of ) and sintering at 1600°C in a vacuum atmosphere. ２ B ５ The amount of FeCr alloy added relative to the base material was 46 atomic percent. The relative density was 99.2%, Vickers hardness was 1640 HV, and fracture toughness was 9.8 MPa·m. １／２ That was the case.

[0057] (2-4) Sample No. 104 For Sample No. 104, 10 wt% of FeCr alloy (Fe-8%Cr) powder was added to tungsten carbide (WC) powder prepared using the method of Sample No. 102, and sintering was performed at 1600°C under a vacuum atmosphere to produce a composite material consisting of tungsten carbide and FeCr alloy layers. The amount of FeCr alloy added relative to WC, expressed in atomic percent, was 28 atomic percent. Relative density 99.1%, Vickers hardness 1721 HV, fracture toughness value 10.1 MPa·m １／２ That was the case.

[0058] (3) Evaluation of the heat resistance of tungsten-based ceramics for radiation shielding In order to evaluate the heat resistance of the obtained sintered body, the sintered body was cut into 5 mm × 5 mm × 5 mm cubes and then placed in a vacuum furnace, 5 × 10 －４ Under a Pa vacuum atmosphere, samples were heated at 1700°C for 1 hour, and changes in appearance and weight loss before and after heating were evaluated. Samples 1, 101, and 102, which did not contain a metal phase in their constituent phases, showed no changes in appearance or weight loss. In contrast, samples 103 and 104, which contained an FeCr alloy phase, showed evidence of melting and volatilization of the FeCr alloy phase on the surface, and their weight decreased. Materials that are susceptible to such melting and volatilization should be avoided because the bonding phase decreases in areas exposed to high temperatures in a vacuum, gradually reducing mechanical properties and shielding performance, or because volatilized metal elements may accumulate and contaminate other parts of the device.

[0059] These evaluations indicate that samples 1 through 22 possess a good balance of mechanical properties such as hardness and fracture toughness, as well as excellent heat resistance, suggesting their potential to function as shielding materials even in harsh environments.

[0060] Note that in Tables 1 and 3, the values ​​of WA, BA, and CA are measured values ​​rounded to two decimal places, so the sum of WA, BA, and CA may not equal 100.

[0061] (4) Evaluation of the shielding performance of tungsten-based ceramics for radiation shielding The shielding performance against neutrons and gamma rays was analyzed using the Monte Carlo calculation code PHITS (PHITS-3.34, Particle d Heavy Ion Transport code System, The Nuclear Energy Agency). The composition of the neutron shield was estimated from the mixing ratio of the raw materials, and the density was the bulk density of the sintered body. In the analysis, a model simulating a neutron shield material with a thickness of 30 cm was created, and the neutron and gamma ray fluxes at the incident surface and the neutron and gamma ray fluxes at the exit surface were determined when the neutron shield material was irradiated with 2.5 MeV fast neutrons. The shielding performance was evaluated by defining the attenuation rate as the value obtained by dividing the attenuation of neutron and gamma ray fluxes before and after passing through the neutron shielding material by the flux at the incident surface.

[0062] It is preferable that both the neutron and gamma ray attenuation rates be 99.990% or higher. If they are lower, the shielding performance will decrease, and there is a risk of adverse effects from leaked neutrons and gamma rays. In addition, the required thickness for shielding will increase, which may lead to constraints on installation space. More preferably, the neutron and gamma ray attenuation rates are 99.992% or higher. Most preferably, the neutron and gamma ray attenuation rates are 99.995% or higher, which minimizes the leakage of neutrons and gamma rays.

[0063] Sample No. 1 showed a neutron attenuation rate of 99.991% and a gamma-ray attenuation rate of 99.994%, indicating effective shielding against both neutrons and gamma rays. A comprehensive evaluation was conducted, combining the results of previous mechanical property and heat resistance evaluations, and the sample exhibited favorable properties in all categories, demonstrating that it can be effectively used as a shielding material even in harsh environments.

[0064] Sample No. 101 showed a neutron attenuation rate of 99.998% and a gamma-ray attenuation rate of 99.994%, indicating effective shielding against both neutrons and gamma rays. However, its fracture toughness evaluation showed low values, suggesting that it may easily break when subjected to impact.

[0065] Sample No. 102 showed a neutron attenuation rate of 99.986% and a gamma-ray attenuation rate of 99.995%, indicating a small shielding effect for neutrons.

[0066] Sample No. 103 showed a neutron attenuation rate of 99.992% and a gamma-ray attenuation rate of 99.994%, indicating effective shielding against both neutrons and gamma rays. However, the heat resistance evaluation revealed melting and volatilization of the bonding layer. This suggests that when used in components subjected to high temperatures under vacuum, the bonding layer of the shielding material may gradually decrease, potentially leading to a decline in shielding performance and mechanical properties.

[0067] Sample No. 104 showed a neutron attenuation rate of 99.981% and a gamma ray attenuation rate of 99.991%, indicating a low shielding effect for neutrons. Furthermore, the heat resistance evaluation revealed melting and volatilization of the bonding layer, resulting in an overall low performance as a shielding material.

[0068] In the table, items that meet the desirable range in the characteristic evaluation are rated A, those that meet the more desirable range are rated AA, those that meet the most desirable range are rated AAA, and those that fall within the undesirable range are rated B.

[0069] In the overall evaluation, emphasis is placed on the shielding performance. Products that are within the most favorable range for both neutron and gamma-ray attenuation rates, and also show favorable results in other evaluation items, are designated as "Most Favorable" (AAA). Products that are within a more favorable range for both neutron and gamma-ray attenuation rates, and also show favorable results in other evaluation items, are designated as "More Favorable" (AA). Products that are within a favorable range for both neutron and gamma-ray attenuation rates, and also show favorable results in other evaluation items, are designated as "Favorable" (A). Products that show unfavorable results in any one evaluation item are designated as "Unfavorable" (B).

[0070] Samples 2 through 16 are examples of shielding materials prepared by varying the composition ratios of W, B, and C. All of them showed good results in mechanical properties, heat resistance, and shielding performance evaluations.

[0071] Samples 17 through 24 are examples of shielding materials prepared by adding less than 10 atomic percent of Cr, Ti, Hf, Si, Ta, and V relative to the total number of moles of W, B, and C. These samples also showed good results in evaluation of mechanical properties, heat resistance, and shielding performance.

[0072] Samples 105 to 113 are examples of shielding materials prepared with compositions where the W, B, and C composition ranges were not within the preferred range. The analysis results showed a significant decrease in shielding performance.

[0073] Samples 114, 115, and 117 are examples prepared by adding more than 10 atomic percent of Cr or Si relative to the total number of moles of W, B, and C. In these cases as well, the analysis results showed a significant decrease in shielding performance.

[0074] Sample No. 115 is an example of a shielding material with the same composition as Sample No. 1, but with a lower relative density. Even with a low relative density, the presence of numerous voids within the material that do not contribute to shielding resulted in a significant decrease in the shielding performance of the shielding material.

[0075] Sample number 118 is a rolled material of pure tungsten (W). The analysis results showed that the neutron attenuation rate was low, and pure tungsten (W) could not effectively shield neutrons. (Note 1) A radiation shielding material containing tungsten (W), boron (B), and carbon (C), and sintered. (Note 2) The radiation shielding material described in Note 1, in which the total number of moles of elements other than tungsten, boron, and carbon is 10 atomic percent or less of the total number of moles of tungsten, boron, and carbon, and the content of tungsten, boron, and carbon relative to the total of tungsten, boron, and carbon is WA, BA, and CA, respectively, and the content WA, BA, and CA are within the range enclosed by points A1, A2, A3, and A4 in Figure 1, and it is sintered as a phase containing tungsten boride, tungsten carbide, and carbon. (Note 3) A radiation shielding material according to Note 1 or 2, wherein the content of WA, BA, and CA is within the range enclosed by points A2, A3, and A5 in Figure 2. (Note 4) A radiation shielding material according to any one of Notes 1 to 3, wherein the content of WA, BA, and CA is within the range enclosed by points A2, A6, and A7 in Figure 3. (Note 5) A radiation shielding material according to any one of Notes 1 to 4, comprising a WB phase and / or a W2B5 phase and a carbon-containing phase. (Note 6) A radiation shielding material according to any one of Notes 1 to 5, comprising at least one element selected from Cr, Si, Ti, Hf, Ta, and V in an amount exceeding 0 atomic% and less than 10 atomic% relative to the total number of moles of tungsten, boron, and carbon. (Note 7) A radiation shielding material according to any one of Notes 1 to 6, comprising at least Cr in an amount greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon.

[0076] It should be understood that at least one configuration or feature described in each embodiment and example can be combined with other embodiments and examples, or modified in various ways.

[0077] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than the embodiments and examples described above, and all modifications within the scope of the claims are intended to be included in the meaning of equivalents and within the scope.

Claims

1. A radiation shielding material comprising tungsten (W), boron (B), and carbon (C), wherein the total number of moles of elements other than tungsten, boron, and carbon is 10 atomic percent or less of the total number of moles of tungsten, boron, and carbon, and the content of tungsten, boron, and carbon relative to the total number of tungsten, boron, and carbon is WA, BA, and CA, respectively, and the content WA, BA, and CA are within the range enclosed by points A1, A2, A3, and A4 in Figure 1, comprising a phase containing tungsten boride, and further comprising at least one of a phase containing tungsten carbide and a phase containing carbon, and being sintered.

2. The radiation shielding material according to claim 1, wherein the content of WA, BA, and CA is within the range enclosed by points A2, A3, and A5 in Figure 2.

3. The radiation shielding material according to claim 1 or 2, wherein the content of WA, BA, and CA is within the range enclosed by points A2, A6, and A7 in Figure 3.

4. WB phase and / or W ２ B ５ A radiation shielding material according to claim 1 or 2, comprising a phase and a carbon-containing phase.

5. The radiation shielding material according to claim 1 or 2, comprising at least one element selected from Cr, Si, Ti, Hf, Ta, and V in an amount greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon.

6. The radiation shielding material according to claim 1 or 2, comprising at least Cr in an amount greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon.

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