Radiation shielding materials

Abstract

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 of WA, BA, and CA falls within the range enclosed by points A1, A2, A3, and A4 in Figure 1, and 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.

Description

[Technical Field]

[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. [Background technology]

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

[0003] [Patent Document 1] Special Publication No. 2017-524928 [Overview of the Initiative]

[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 of WA, BA, and CA falls 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. [Brief explanation of the drawing]

[0005] [Figure 1]Figure 1 is a ternary phase diagram showing the composition ratios of W, B, and C, namely WA-BA-CA, and represents points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%), A2 (WA: 29 atomic%, BA: 71 atomic%, CA: 0 atomic%), A3 (WA: 45 atomic%, BA: 55 atomic%, CA: 0 atomic%), and A4 (WA: 20 atomic%, BA: 40 atomic%, CA: 40 atomic%). [Figure 2] Figure 2 is a ternary phase diagram showing the composition ratios of W, B, and C, namely WA-BA-CA, and represents points A2 (WA: 29 atomic%, BA: 71 atomic%, CA: 0 atomic%), A3 (WA: 45 atomic%, BA: 55 atomic%, CA: 0 atomic%), and A5 (WA: 23 atomic%, BA: 50 atomic%, CA: 27 atomic%). [Figure 3] Figure 3 is a ternary phase diagram showing the composition ratios of W, B, and C, namely WA-BA-CA, and represents points A2 (WA: 29 atomic%, BA: 71 atomic%, CA: 0 atomic%), A3 (WA: 45 atomic%, BA: 55 atomic%, CA: 0 atomic%), and A7 (WA: 25 atomic%, BA: 60 atomic%, CA: 15 atomic%). [Modes for carrying out the invention]

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

[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 in the material / Total number of moles BA = Number of moles of boron in the material / Total number of moles CA = Moles of carbon in the material / Total number of moles Figure 1 is a ternary phase diagram showing the composition ratios of W, B, and C, WA-BA-CA, with points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%), A2 (WA: 2 9 Atomic %, BA:7 1 Atomic %, CA: 0 atomic %, A3(WA: 45 Atomic %, BA: 55 This indicates 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 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 WA-BA-CA of W, B, and C, with point A2(WA:2 9 Atomic %, BA:7 1 Atomic %, CA: 0 atomic %, A3(WA: 45 Atomic, BA: 55 This indicates atomic percentage, CA: 0 atomic%, and A5 (WA: 23 atomic%, BA: 50 atomic%, CA: 27 atomic%).

[0013] Preferably, WA, BA, and CA are the contents WA, BA, and CA within the range surrounded by points A2, A3, and A5 in FIG. 2.

[0014] FIG. 3 is a ternary phase diagram showing WA-BA-CA, which is the composition ratio of W, B, and C, and shows point A2 (WA: 2 9 atomic %, BA: 7 1 atomic %, CA: 0 atomic %), A3 (WA: 45 atomic %, BA: 55 atomic %, CA: 0 atomic %), and A7 (WA: 25 atomic %, BA: 60 atomic %, CA: 15 atomic %).

[0015] Preferably, WA, BA, and CA are the contents WA, BA, and CA within the range surrounded by points A2, A 3 and A7 in FIG. 3.

[0016] Preferably, the radiation shielding material comprises a WB phase and / or a W2B5 phase and a carbon-containing phase.

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

[0018] Preferably, the radiation shielding material contains at least Cr in an amount exceeding more than 0 atomic % and less than 10 atomic % with respect to the total number of moles of tungsten, boron, and carbon.

[0019] The radiation shielding material of the present disclosure has a small decrease in strength even when exposed to high temperatures and can be used in harsh environments, so it can exhibit a stable shielding effect even in an environment where it is simultaneously subjected to a heat load and radiation irradiation.

[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 bonding layers, these have drawbacks in terms of heat resistance and shielding properties. This disclosure has found that adding a certain amount of carbon improves the mechanical properties of neutron shielding materials without compromising 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 radiation shielding material is represented by points A1 (WA: 50 atomic%, BA: 0 atomic%, CA: 50 atomic%) and A2 (WA: 2 atomic%) on the WA-BA-CA ternary phase diagram, where W, B, and C are in a specific composition ratio. 9 Atomic %, BA:7 1 Atomic %, CA: 0 atomic %, A3(WA: 45 Atomic %, BA: 55It is preferable that the material is located within the polygon formed by connecting A4 (WA: 20 atomic%, BA: 40 atomic%, CA: 40 atomic%) with straight lines. 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, if carbon is dispersed in the form of free carbon, the dispersed carbon particles can resist crack propagation, thereby improving the fracture toughness. Alternatively, if tungsten carbides with high mechanical properties are formed, the fracture toughness can be similarly improved, as can the hardness.

[0023] Outside this range, for example, if WA exceeds 50 atomic percent (excess), 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 (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 carbon is in excess, 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 no carbon is present 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, adding carbon has the effect of reducing residual oxygen that impairs mechanical properties, thus improving mechanical properties.

[0024] The composition of radiation shielding material is the WA-BA-CA ternary phase diagram, where W, B, and C are in the same ratio. Point A2(WA:2 9 Atomic %, BA:7 1 Atomic %, CA: 0 atomic %, A3(WA: 45 Atomic, BA: 55It is more preferable that the carbon content is within the polygon formed by connecting A5 (WA: 23 atoms, BA: 50 atoms, CA: 27 atoms) with straight lines. Within this range, there is sufficient tungsten to slow down fast neutrons and sufficient boron to absorb thermal neutrons, thus improving shielding performance. By setting the amount of carbon to more than 0 atoms but less than 30 atoms, it is possible to obtain sufficient thermal neutron slowing effect while ensuring a sufficient amount of boron to absorb thermal neutrons, and to disperse a necessary and sufficient amount of free carbon and tungsten carbide, thereby ensuring fracture toughness.

[0025] The composition of radiation shielding material is the ratio of W, B, and C, as shown at point A2 (WA:2) on the WA-BA-CA ternary phase diagram. 9 Atomic %, BA:7 1 Atomic %, CA: 0 atomic %, A3(WA: 45 Atomic, BA: 55 The most preferable composition range is one that lies within a polygon formed by connecting A7 (WA: 25 atoms, BA: 60 atoms, CA: 15 atoms) in a straight line. Within this composition range, the deceleration effect of fast neutrons and the deceleration and absorption effect of thermal neutrons are performed most efficiently.

[0026] The constituent phase consists of a WB and / or W2B5 phase and a phase whose remainder is at least C. The presence of tungsten and boron in the form of WB and W2B5 as the main phases is preferable because it allows for the uniform distribution of tungsten, which is responsible for the fast neutron moderation effect, and boron, which is responsible for the thermal neutron absorption effect, within the material. Furthermore, these materials are hard and have high melting points, thus enhancing the material's strength and heat resistance. The presence of elemental tungsten and elemental boron is undesirable due to the boron's easy absorption of moisture and oxidation, as well as concerns about reduced material strength. The remaining carbon-containing phase may exist alone in the form of free carbon, or it may exist as a tungsten carbide by bonding some tungsten. Alternatively, it may form carbides with other additive elements.

[0027] To obtain the constituent phases in the form of WB and W2B5, 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 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] These elements can be easily added by adding and mixing powders containing these elements with the synthesized tungsten boride powder. Alternatively, a coating of these metals can be formed on the surface of the fabricated tungsten-based ceramic for radiation shielding using methods such as vapor deposition, sputtering, or plating, and then heat-treated in an inert gas to allow the elements to diffuse into the material. In this case, functionality can be added only to the vicinity of the surface; for example, if Cr, Ti, and Si are preferentially present on the surface, the oxidation resistance of the material can be improved.

[0030] For radiation shielding tungsten-based ceramics, it is more preferable that at least Cr be present in greater than 0 atomic percent and less than 10 atomic percent relative to the total number of moles of tungsten, boron, and carbon. Cr readily combines with carbon to form Cr 23 It can form hard carbides such as C6 and Cr3C2, thereby increasing material strength, and can be uniformly dispersed in the material by substituting and solid-solving with tungsten. By solid-solving it in the matrix phases WB and W2B5, oxidation resistance can be improved.

[0031] To determine the composition ratio of W, B, and C in sintered tungsten-based ceramics and to investigate the abundance of additive elements, an analysis method using an electron beam microanalyzer in accordance with ISO 22489:2016 can be used. Furthermore, X-ray diffraction is typically used for identifying the constituent phases.

[0032] (1) Sample number 1-24 Manufacturing of tungsten-based ceramics for radiation shielding (1-1) Preparation process of raw materials

[0033] [Table 1]

[0034] As the W source, a WO3 powder with a purity of 99% and a particle size of 30 μm was used; as the B source, a 20% aqueous solution of H3BO3 was used; and as the C source, a pyrolysis carbon powder with a particle size of 0.5 μm was used.

[0035] These raw materials were weighed, for example, in sample number 1, so that the amounts of W, B, and C were 40 atomic%, 40 atomic%, and 20 atomic%, respectively. In addition, pyrolysis carbon was further added to reduce WO3 and H3BO3. The mixture was then 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 The mixture was synthesized by heating at 1800°C for 1 hour in a vacuum of Pa to obtain a mixture of tungsten boride and carbon. 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 through reduction and thermal decomposition reactions of their 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 produced 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 30g of the obtained powder was taken out and placed in a Φ20mm carbon mold. It was then 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 pressure of 50MPa. As a result of sintering, a cylindrical sintered body with a diameter of 20mm and a height of approximately 12mm was obtained.

[0042] (1-3) Analysis of composition The resulting 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. Using a Rigaku Smart Lab, CuKα radiation was irradiated, and measurements were taken in the range of 20° to 100°. The measurement results showed that the body was composed of WB, W2B5, C, etc., as shown in Table 1.

[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 a temperature of 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. Generally, 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 using 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] [Table 2]

[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. Prior to the calculation, the elastic modulus of the sintered body was measured using the ultrasonic pulse method according to JIS R1602 and used in the calculation of the fracture toughness. The result of the series of measurements was 4.8 MPa·m. 1 / 2 The value was shown.

[0050] The fracture toughness value of tungsten-based ceramics 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 number 101-118 (2-1) Sample number 101

[0052] [Table 3]

[0053] [Table 4]

[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 70 atomic percent and 30 atomic percent respectively, and placed in a tungsten crucible, 5 × 10⁻⁶ -2 The tungsten boride was synthesized by heating at 1800°C for 1 hour in a vacuum of Pa, and after rough grinding and fine grinding, it was sintered using SPS in the same manner as in Example 1. The obtained sintered body was a single-phase W2B5 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 number 102 For sample number 102, a 99% purity, 3 μm particle size W powder was used as the W source, and a 0.5 μm particle size pyrolysis carbon powder was used as the C source. The W and C were weighed to be 50 atomic percent and 50 atomic percent respectively, and placed in a tungsten crucible, 5 × 10⁻⁶. -2 Tungsten carbide was obtained by heating synthesis at 1800°C for 1 hour in a vacuum of Pa, followed by coarse and fine grinding, and then sintered using SPS in the same manner as in Example 1. The obtained 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 solely of a tungsten boride phase, there is no phase to stop crack propagation, resulting in low fracture toughness values.

[0056] (2-3) Sample number 103 As Sample No. 103, 10 wt% of FeCr alloy (Fe-8%Cr) powder was added to the tungsten boride (W2B5) powder prepared by the method of Sample No. 101, and sintering was carried out at 1600 °C in a vacuum atmosphere to produce a composite material composed of tungsten boride and an FeCr alloy layer. When the addition amount of the FeCr alloy to W2B5 was expressed in atomic%, it was 46 atomic%. The relative density was 99.2%, the Vickers hardness was 1640 HV, and the fracture toughness value was 9.8 MPa·m 1 / 2 It was.

[0057] (2-4) Sample No. 104 As Sample No. 104, 10 wt% of FeCr alloy (Fe-8%Cr) powder was added to the tungsten carbide (WC) powder prepared by the method of Sample No. 102, and sintering was carried out at 1600 °C in a vacuum atmosphere to produce a composite material composed of tungsten carbide and an FeCr alloy layer. When the addition amount of the FeCr alloy to WC was expressed in atomic%, it was 28 atomic%. The relative density was 99.1%, the Vickers hardness was 17 21 HV, and the fracture toughness value was 10.1 MPa·m 1 / 2 It was.

[0058] (3) Evaluation of the heat resistance of tungsten-based ceramics for radiation shielding To evaluate the heat resistance of the obtained sintered body, after cutting the sintered body into cubes of 5 mm×5 mm×5 mm, it was placed in a vacuum furnace, and heating was carried out at 1700 °C for 1 h in a vacuum atmosphere of 5×10 -4 Pa, and the appearance change and the presence or absence of weight loss before and after heating were evaluated. For Samples No. 1, 101, 102, etc. that did not contain a metal phase in the constituent phases, no appearance change or weight loss was observed, while for Samples No. 103, 104 containing an FeCr alloy phase, traces of melting and volatilization of the surface FeCr alloy phase were observed, and the weight had decreased. Such materials with concerns about melting and volatilization should be avoided because the bonding phase will decrease at sites exposed to high temperatures in a vacuum, and there is a concern that the mechanical properties and shielding performance will gradually decrease, or the volatilized metal elements will deposit on other parts of the device and cause contamination.

[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 ​​for 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 and 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 determined using the bulk density of the sintered body. In the analysis, a model simulating a 30 cm thick neutron shield was created, and the neutron and gamma ray fluxes at the incident and exit planes were determined when the neutron shield 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 the neutron and gamma ray fluxes before and after transmission through the neutron shield by the flux at the incident plane.

[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 can minimize the leakage of neutrons and gamma rays.

[0063] Sample number 1 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 assessments, and the material exhibited favorable characteristics in all categories, demonstrating its effectiveness as a shielding material even in harsh environments.

[0064] Sample number 101 Sample number 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 number 102 Sample number 102 showed a neutron attenuation rate of 99.986% and a gamma-ray attenuation rate of 99.995%, indicating a small shielding effect for neutron beams.

[0066] Sample number 103 Sample number 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 number 104 Sample number 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 their characteristic evaluation values ​​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 perform favorably 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 perform favorably in other evaluation items, are designated as "Even More Favorable" (AA). Products that are within a favorable range for both neutron and gamma-ray attenuation rates, and also perform favorably in other evaluation items, are designated as "Favorable" (A). Products that perform unfavorably 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 mechanical properties, heat resistance, and shielding performance evaluations.

[0072] Samples 105 to 113 are examples of shielding materials prepared with compositions where the W, B, and C composition ranges were outside the desirable 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 made of pure walnut. The analysis results showed that the neutron attenuation rate was low, indicating that pure walnut cannot effectively shield against neutrons. (Note 1) A radiation shielding material containing tungsten (W), boron (B), and carbon (C), which is sintered. (Note 2) The radiation shielding material described in Appendix 1, 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 of WA, BA, and CA falls within the range enclosed by points A1, A2, A3, and A4 in Figure 1, and is sintered as a phase containing tungsten boride, tungsten carbide, and carbon. (Note 3) A radiation shielding material as described in Appendix 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 as described in any one of the appendices 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 the appendices 1 to 4, comprising a WB phase and / or a W2B5 phase and a carbon-containing phase. (Note 6) A radiation shielding material as described in any one of the appendices 1 to 5, containing 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. (Note 7) A radiation shielding material as described in any one of the appendices 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, wherein the content of tungsten, boron, and carbon relative to the total number of tungsten, boron, and carbon is WA, BA, and CA, and the content WA, BA, and CA within the range enclosed by points A1, A2, A3, and A4 in Figure 1 is the WA, BA, and CA of the radiation shielding material, 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 WA, BA, and CA within the range enclosed by points A2, A3, and A5 in Figure 2 are WA, BA, and CA of the radiation shielding material.

3. The radiation shielding material according to claim 1 or 2, wherein the content WA, BA, and CA within the range enclosed by points A2, A3, and A7 in Figure 3 are WA, BA, and CA of the radiation shielding material.

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. A 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 exceeding 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.

Images