JPH0497973A - Neutron absorber - Google Patents

Abstract

PURPOSE:To obtain this absorber contg. fine boron particles uniformly dispersed in carbon and having superior neutron absorbing performance by using a composite material based on carbon and boron and obtd. in specified conditions as the absorber. CONSTITUTION:Boron oxide and/or a hydrated compd. thereof, e.g. B2O3 is impregnated into a carbon material such as a fine-grained isotropic graphite material and they are sintered at >=1,500 deg.C under pressure applied with inert gas such as N2 to obtain a composite material based on carbon and boron. This composite material is used as the title absorber.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to carbon-boron (hereinafter sometimes referred to as C-B) composite materials, particularly boron (hereinafter sometimes referred to as B).
The present invention relates to a neutron absorbing material made of a composite material in which ultrafine particles are uniformly dispersed in a carbon (hereinafter sometimes referred to as C) component.

[Conventional technology]

(, -B composite materials have been widely studied and used as neutron absorbers in the nuclear industry.

Currently, the manufacturing method of this C-B composite material, which is known for boat fishing, is to mix separately manufactured B and C (boron carbide) with a carbon material or a raw material that can be carbonized, and then A well-known method is to calcinate the two to form a solid solution.

For example, Japanese Patent Application Publication No. 62108767, Japanese Patent Application No. 62-29
No. 7202 is an example.

[Problem to be solved by the invention]

The C-B composite material obtained by the above conventional manufacturing method is
Coarse B and C are crushed and mixed with carbon, but since the crushing is mechanical crushing, there is a limit to the pulverization, and the target C-B composite material is the B and C parts. ,B,
There were parts where C and carbon were made into a solid solution, and parts where only carbon existed, and when closely observed, it was difficult to say that the whole was completely uniform.

In addition, in the conventional method described above, powders are mixed, molded, and sintered, so it is inconvenient to cut and mold the material after sintering, and it is expensive due to the odor of cutting powder. There was a drawback that the raw material efficiency of aC was lowered.

Furthermore, the biggest problem with the conventional method is that it contains a large amount of inorganic impurities other than boron (typically 5000 ppm). These impurities are introduced through contact with steel machinery during the grinding, mixing, molding, firing, and other processes of the blended raw materials, and are extremely difficult to avoid using conventional methods. Depending on the use of B-C composite materials, the coexistence of small amounts of inorganic impurities may not be a major problem, but as materials for the nuclear industry, inorganic impurities mixed in may cause isotope conversion, splitting, etc. due to radiation irradiation etc. Reactions occur simultaneously, and when used as a nuclear fusion material, high Z elements (high valence elements) emitted from these impurities cause a decisive adverse effect of significantly lowering the replasma temperature. Therefore, carbon materials for the nuclear power industry require particularly highly purified materials, such as ultra-high-purity materials with inorganic impurities of 5 pptm or less, preferably 2 ppm or less, substantially close to Oppw+ (by atomic absorption spectrometry or emission line spectroscopy). It is customary to use pure carbon material. As a manufacturing method for such ultra-high purity carbon material, for example, Japanese Patent Application No. 61-2241
As shown in No. 31, inorganic elements are removed by highly volatile halogenation treatment, but in composite materials made from 84C-C, due to the presence of boron in the carbon material, After that, the impurities cannot be removed using this method.

The purpose of the present invention is to solve the above-mentioned various drawbacks of the conventional method using B, C-C powder as raw materials, develop an even better BC composite material, and utilize this as a neutron absorbing material.

[Means for Solving the Problems] The present inventors have proceeded with research in order to solve the drawbacks of the above-mentioned conventional methods and develop better materials. First, B and C, which are the raw materials of the conventional method, are used as boron sources. Instead of using boron oxide or its hydrated compound, a new method was adopted in which the carbon material was impregnated with boron oxide or its hydrated compound in the form of melt or solution. By this method, the boron component penetrates into the surface of fine carbon grains or into the fine pores in the carbon material at a molecular level, and is dispersed much more finely and throughout the material than when using 84C powder. be able to.

Furthermore, since boron is used in the form of an oxide, a reaction method suitable for fixing boron in carbon and turning it into a solid solution by the reaction between the oxide and carbon, that is, applying the boron compound to the carbon material in liquid form. Following impregnation, we developed a manufacturing method in which the impregnated boron compound is not volatilized from the carbon material, and the boron and carbon undergo a solid solution reaction, in which firing is performed at high temperature and high pressure. TA C-B
They further discovered that composite materials are extremely excellent neutron absorbers and achieved their desired goal.

[Function and structure of the invention]

The manufacturing method of the C-B composite material used in the present invention will be explained.

(First step) Molten boron oxide is heated at 600 to 1400°C, preferably at 800 to 100°C, to a cut compact of a carbon material, for example, an isotropic high-density carbon material (rIG-1ll manufactured by Toyo Tanso ■) in a pressure-resistant container.
Impregnation under pressure at 200°C. At this time, it is desirable to once reduce the pressure in the pressure container to remove air contained in the pores of the carbon material before impregnation, but it is not always necessary to deaerate in advance.

In order to impregnate the carbon material with 8203, a pressure of several kg/Cdl may be used, but in order to completely penetrate the pressure to the deep part, the pressure is preferably 50 to 100 kg/Cdl.

This pressure is appropriately determined depending on the porosity, particle size, pore distribution, temperature, etc. of the carbon material.

(Second step) The carbon material impregnated with B and O is subjected to heat treatment (hereinafter sometimes referred to as HIP) under high temperature and high pressure using an inert gas as a pressure medium. By using an inert gas as a medium, the carbon material and B2O3 liquid are evenly pressured from all directions, just like water pressure, and while preventing transpiration of B2O3, it is trapped inside the carbon material, and the carbon is released due to the effect of temperature. A chemical reaction between and boron proceeds.

The pressure and temperature in the heat treatment apparatus are preferably 50 kg/cd or higher and 1500°C or higher, preferably 2000°C or higher and 1500 to 2000 kg/cd. In this case, if the temperature exceeds 2300°C, decomposition reactions of the solid solution of carbon and boron occur simultaneously, which is not preferable.

Although the first and second steps are essential, depending on the manufacturing conditions, a small amount of B, O, may remain in the carbon material.
Depending on the location and purpose of this composite material, this may be disliked. Therefore, in order to eliminate this, a third step can be added next.

Third step 10T of composite material that has undergone HIP treatment in the second step.

Under reduced pressure of less than 5 Torr, preferably less than 5 Torr, 10
By treatment at a high temperature of 00°C or higher, preferably 1500°C or higher, and under reduced pressure, the amount of BzO3 in the composite material is reduced to 0.1%.
It can be reduced to: In the B-C composite material thus obtained, boron is dispersed finely and uniformly throughout the composite material compared to a composite material using B and C powders as in the conventional method.

The carbon materials used in the present invention include isotropic carbon materials as exemplified above, general carbon materials, anisotropic carbon materials (such as pyrocarbon and pyrographite), and carbon-carbon composite materials ( It can be applied regardless of the type of carbon material, such as C/C material (hereinafter sometimes referred to as C/C material). The present invention provides carbon powder and B,
Since the carbon material is formed by cutting a separately manufactured carbon material without using C powder, the biggest feature is that it can be boronized with its shape, structure, and skeleton intact.

For example, if ultra-high purity isotropic high-density graphite material is used as a base material and boronated, the composite material obtained by using a boron compound with good purity will contain impurities of elements other than carbon and boron. A very small purity of 5 ppm or less, which is almost the same as the purity of the base material, can be obtained. This is the grinding of raw materials,
This is believed to be due to the fact that there is no contamination during mechanical processing steps such as mixing and compression molding in the case of the present invention.

A simple example showing the characteristics of the method of the present invention is the case of boronation of carbon/carbon composite materials. B like the conventional method
When using C powder, it requires special equipment and technology to grind it to a particle size of 1 μm or less even if it is very finely ground.The particles are mixed with a resin component, applied to carbon fiber, and prepreg. One possible method is to create a boronated C/C material product by forming, heat-hardening, carbonizing, and then cutting, but the biggest drawback of this method is that the carbon material cannot be completely graphitized. . This is because carbon graphitization requires 2500 ~
Despite the need for high-temperature firing at 3000°C, B
This is because the C component begins to decompose at around 2300°C. In addition, it is almost impossible to push the B and C fine powder into the fine pores of the C/C material, which has been graphitized by high-temperature firing at 3000°C. cannot be uniformly dispersed. Although the same can be said for general carbon material blocks, it is particularly difficult to prepare C/C materials for boronization while maintaining the strength of the carbon fibers.

In this respect, according to the method of the present invention, it is possible to boronize the C/C material very easily. That is, as described above, the boron component is forcibly injected into the pores of the carbon material at a molecular level in the form of a melt or solution, and can be uniformly dispersed deep into the pores of the carbon material. In addition, there is no change in the structure of the C/C material due to the forced injection of boron components and the subsequent firing work, and since the carbon material has been graphitized at 3000°C in advance, it is difficult to proceed with the boronization reaction. Even when fired at 2000°C, the resulting boronated molded product has sufficient physical properties as a C/C material.

In principle, boron compounds that can be melted by heating or made into a solution with a solvent can be used as the boron component impregnated into the material. This is undesirable because it causes contamination of the material and limits its usage. Therefore, it is desirable to use a compound that decomposes and volatilizes by thermal decomposition or reaction with carbon during firing, leaving only boron. From this point of view, boron-containing organic compounds and boron halides have been tried, but from the viewpoint of economy and ease of handling, boron oxide (B t Os ) and its hydrate are used in the present invention. A compound such as H, BO, orthoboric acid can be exemplified as the most suitable one. For example, as a chemical reaction between B20° and carbon, 2B,03+7C-+BJC+6CO is known as the calcination reaction of B a C, but as in the present invention, molecular-level particles are present in a very large amount of carbon. It is not clear whether the (carbon-boron) composite material produced by dispersing 8203 proceeds according to the above reaction formula. As a result of performing various analyzes on the composite material obtained by the method shown in Example 1, a boron component of 4% by weight was determined by chemical analysis (free #BgOsO, 02%), and from the results of neutron irradiation. Despite the fact that neutron absorption by the boron component is clearly observed, there are only a few peaks indicating the presence of B and C when observed using an X-ray diffraction device. There are also few peaks indicating other specific crystal systems (and there are many broad parts, so it is assumed that the peaks are in the form of an amorphous substance or a solid solution. Therefore, the final product shows specific compounds B and C. Rather than a clear form, (B, x
It is presumed that the solid solution is in the form of Cy+C), but the present invention is not limited to such a solid solution form.

As the boron component, in addition to boron oxide (Bt◯), hydrated compounds thereof can also be used. Examples of hydrated compounds include boric acid (H3BO3, B (OH)
3).

These boric acids, compared to boron oxide (B z O3),
It has a relatively low melting point (185°C), and at higher temperatures it decomposes while releasing moisture, forming (BzCh・"H3BO
3) Becomes a solid solution and remains liquid.

Therefore, when boric acid is used as a raw material, the boric acid is kept at a temperature that maintains an appropriate viscosity in the container, that is, 300 to 500°C, and the carbon material is immersed in the melted boric acid. Forcibly press into the pores of the material. The second step (HIP treatment step) performed subsequent to the above first step (impregnation) is the above-mentioned B and O.

It can be implemented in the same way as in the case of .

Next, a mode of carrying out the method of the present invention using these boron compounds and carbon materials as raw materials will be described.

The boron compound is heated and melted and impregnated under pressure in a liquid state or in a solution state dissolved in an appropriate solvent. For example, B20. Melting point is 450'C1 boiling point is 1500 at normal pressure
°C, and becomes liquid in this temperature range, but the impregnation operation is carried out at 600 to 1400 °C, preferably 800 to 1200 °C.
A temperature range of

First, in the first step, B z 03 and a carbon material are placed in a pressure-resistant container, and B2O3 is pressurized into the pore spaces of the carbon formed body by vacuum, heating, and pressure methods. At this time, before press-fitting B, O, the inside of the container is once depressurized to remove the air present in the pores of the carbon material (and B20. is completely press-fitted.
Although it is easy, since the press-in pressure is high, this depressurization operation is not essential.

The press-in pressure may be several kg/ai, but is preferably 50 kg/ai.
~100 kg/cd.

Next, HIP processing is performed as a second step. Surprisingly, even if the carbon material impregnated with a boron compound in the first step is heated at 2000"C under normal pressure, the carbon material is hardly boronized. The boron component evaporates due to high temperature heating, and the carbon material is separated from the carbon material. This seems to be because there is no solid solution formation in the reaction.Heating in the second step needs to be performed under high pressure.In the treatment carried out at high temperature and high pressure, for example, an inert gas such as Ar is used as a medium. 100 kg/cd or more, temperature of 1500°C or more, preferably 100 to 2000 kg/ci, 2
The test is carried out at a temperature of 000°C or higher. By this HIP treatment, the boron compound can be diffused into the carbon material as a solid solution and fixed chemically.

The above is the second step, which is an essential operation along with the first step.
It is sufficient for the purpose and use as a normal [carbon-boron] solid solution, and if necessary, it can be provided on the market after being subjected to finishing treatments such as cutting and forming processing.

However, in order to use the material as a neutron absorber for nuclear reactors as in the present invention, it is preferable that the amount of residual B2O3 is as small as possible. If such a material in which 203 remains is used in a nuclear reactor under high temperature conditions, the evaporated Bt
This is because O3 precipitates and solidifies in relatively low-temperature areas, causing problems that inhibit operation, corroding metal parts, and solidifying during storage.

Therefore, remaining B20 must be used for nuclear reactors.
．． It is preferable to remove as much as possible, and in this case, the following third step can be added as necessary.

(Third step) The solid solution obtained in the second step is placed in a pressure-resistant container and heated under reduced pressure, preferably 10 Torr or less, particularly preferably 5 T.
A step of heat treatment at 1,500° C. or higher under strongly reduced pressure of orr or less to evaporate and remove Bt○3 is additionally added.

By performing such treatment, the residual amount of B2O3 can be reduced to 0.01% by weight.

The material obtained by the method of the present invention has extremely excellent neutron absorption performance and is effectively used as a neutron absorbing material, and is practically used as a nuclear reactor material, a plasma facing wall for nuclear fusion reactors, a material for aerospace, etc. Demonstrates its power in the field of

〔Effect of the invention〕

In the composite material of the present invention, boron is uniformly diffused in carbon in the form of fine particles. For this reason, it is extremely excellent as a neutron absorbing material, and is widely used in applications such as absorbing neutrons, and has extremely large industrial effects.

〔Example〕

The present invention will be described in detail below with reference to reference examples and examples of production of C-B composite materials.

Reference Example 1 <First step> B2O melted at 1200°C using an autoclave in a fine-grained isotropic graphite material (rIG-11 manufactured by Toyo Tanso Co., Ltd.)
The graphite material was immersed in 3 and heated at 150 kg/c- with N2 gas.
The graphite material was pressurized for 1 hour at a pressure of 100 mL to impregnate B2O3 into the pores of the graphite material.

〈Second step〉 After the impregnation is completed, a HIP treatment device is used and the temperature is 2000℃.
A pressure of 12000 kg/ci was maintained for 1 hour to diffuse boron into the graphite material and form a solid solution.

During the HIP treatment, the product to be treated was placed in a cylindrical graphite sheath and covered with a lid.

<Third Step> Thereafter, vacuum treatment was performed at I Torr and 2000° C. for 1 hour using a vacuum container. The boron concentration of the obtained composite material was measured by the mannilatol method and was 4.0% by weight (as elemental boron). Among them, B20° is 0.02% by weight, and almost all unreacted B z Ox transpires.
It had been removed.

Reference Example 2 The carbon-boron composite material obtained in Reference Example 1 was repeatedly subjected to the same treatment as in Reference Example 1. The boron concentration of the boron composite material thus obtained was 7% by weight. Of these, 8.03 was 0.03% by weight.

As is clear from the above, it was found that by repeating the treatment shown in Reference Example 1, it was possible to increase the boron content in the composite material.

Reference Example 3 A phenol resin solution (resol type phenol resin diluted 2 to 3 times with methanol) was applied to a plain weave cloth made of PAN-based high-end high-strength carbon fiber 000 filament, fiber diameter 7 μm, tensile strength 300 kg/mm''). A,) was applied by impregnation and air-dried for 24 hours to obtain a prepreg sheet.

This prepreg sheet is laminated in a dryer and heat treated.
00° (xo, 5 hours), and then packed into a mold and held in a hydraulic press at 140'c and 50 kg/d for 1 hour to obtain a 2D bottom molded product consisting of two laminates.

The obtained compact was packed in coke powder and treated in a non-oxidizing atmosphere at a heating rate of 10'C/hour to too'C, and then heated to 200℃ under a reduced pressure of 5 Torr using a vacuum furnace.
High temperature treatment was carried out at a rate of 100°C/hour to 0°C.

A crack-free 2DC/C composite material was obtained.

Orthoboric acid (H3B
A solution obtained by adding 1% by weight of water to 1% by weight of O, ) was added and immersed for impregnation.

The water was evaporated in a dryer maintained at 120°C. After that, the aqueous solution impregnation treatment was performed one more time.

It was confirmed that the aqueous solution had a relatively low viscosity and was easily impregnated deep into the voids and pores in the C/C composite material.

The above was the first step (impregnation treatment), and the second step was carried out under the same conditions as shown in Reference Example 1 to obtain a carbon-boron composite material using the C/C composite material as a base material. . The boron concentration in the obtained product was 3.7% by weight (calculated as boron element).

Reference example 4 Mesophase spherulite carbon (Kawasaki Steel ■
rKMFCJ) is pulverized to an average particle size of 5 μm or less, hot-press molded, and fired again at 2500 to 3000°C, and the obtained high-purity ultrafine isotropic graphite material (hereinafter abbreviated as 50880) is used. Then, a boronation reaction was carried out in the same manner as in Reference Example 1.

This carbon base material is a carbon material with dense and high strength properties.
Although it is a material with a small pore volume, when boronation was performed using the method shown in Reference Example 1, the boron concentration in the obtained C-B composite material was 2.6% (weight), and Remaining B20 after processing the process. The amount was determined to be less than 0.01%.

Table 1 shows the analytical values of elements other than boron before and after the boronization treatment, that is, for the 130-880 raw material and after the boronization reaction according to this reference example.

Table 1 (1) Table 1 (2) Table 1 (3) General carbon materials usually contain impurities of around 400111)111, which can be treated by high-temperature halogenation treatment (for example, JP-A-63-79759). ), the total ash content can be reduced to 10 ppm or less, and depending on the purpose, the total ash content can be reduced to 1 to 2 ppm or less. In this example, 130-880 is a material obtained by removing impurities in advance by halogenation of r5o-88. The analysis method is a combination of atomic absorption spectrometry and emission line spectroscopy. Also, (-) indicates no detection.

As is clear from the analysis results of the amount of impurities before and after the boriding treatment shown in Table 1, it can be seen that elements other than boron have not increased.

Example 1.2 and Comparative Example 1 A neutron irradiation test was conducted using the samples prepared in Reference Examples 1 and 3 and the samples prepared by the conventional method.

<Test Samples> Sample-A (Comparative Example 1): Commercially available powders B and C were ground, and those having a particle size of 3 to 7 μm were selected and prepared. Separately, coal coke powder (average particle size 1
5 μm or less) 50 parts by weight, artificial graphite powder (average particle size 40a
m or less) and 40 parts by weight of pitch are mixed together, kneaded under heating (230° C. for 12 hours), and then molded and pulverized. 7.7 parts by weight of the above-mentioned 84C grains were added to 100 of the pulverized product, and the mixture was heated and kneaded together with a small amount of a binder. This kneaded material was pressure-molded and fired at 2000°C to obtain a raw material. As a result of chemical analysis, the boron content was 4.2% by weight (in terms of pure boron).

Sample-B (Example 1): Raw material obtained by the method described in Reference Example 1 above.

Sample-C (Example 2): Raw material obtained by the method described in Reference Example 3 above.

The three types of raw materials obtained by the above method were 2m thick.
It was cut into a thin plate shape of n and subjected to a neutron irradiation test.

Neutron irradiation test bag W: Manufactured by Sumiju Kenken Co., Ltd. Neutron radiography beam irradiation amount: 34.4 μA ・4653 sec (160,0 m
Cb) Neutron irradiation method: A sample was placed on a dry plate and neutron irradiation was performed. The areas where neutrons were absorbed are exposed in white, and the areas where neutrons were not absorbed are exposed in black.

Test results: The test results are shown in Figures 1-2.

In the case of sample-A, the boron component exists as particles B and C as shown in FIG. 2, and the portions where neutrons have been absorbed remain unexposed as white spots.

Areas without boron, ie areas irradiated with neutrons, are exposed black. This photo has been enlarged 10 times to show the spots clearly.

In the case of Sample-B, the boron component is very finely and uniformly dispersed. No white spots are visible even under magnification.

Therefore, the entire surface of the photograph obtained is exposed as a uniform neutral color between black and white, and no white spots are observed as shown in FIG.

As shown in Reference Example 1, the boron component is 4
The fact that absorption points do not appear as white spots, regardless of the amount of boron present, indicates that boron is very finely dispersed.

In the case of sample - (0), boron is impregnated into a (carbon-carbon) composite material, and although there are no photographic analysis results, the boron is uniformly distributed throughout the sample in an ultra-finely dispersed state.

The above conventional product (sample-A) and the product of the present invention (sample B and C)
From the comparison, there is a noticeable difference in the dispersion state of the boron component between the two, and in the case of the method of the present invention, boron is dispersed uniformly throughout and in a finer manner than in powdered materials B and C. It is clear that there are

When the same test as above was conducted for Reference Examples 2 and 4, almost the same results were obtained.

Furthermore, the C-B composite material used in the present invention has extremely excellent oxidation resistance. This oxidation resistance is one of the characteristics that is desirable to have when used as a neutron absorbing material. This is preferable for the material. Here, an example in which the oxidation resistance was measured will be shown.

Example 3 and Comparative Example 2.3 The oxidation resistance of the (boron-carbon) composite material manufactured in Reference Example was investigated.

Sample-D (Comparative Example 2): A sample (boron content: 4.2%) prepared by the conventional method used in Comparative Example 1 (using powders B and C).

Sample-E (Example 3): Sample prepared by the method of the present invention used in Reference Example 4 (
boron content 4.0%).

Sample-F (Comparative Example 3): Carbon raw material (boron content 0.0%) used in preparing the sample used in Reference Example 2.

The above three samples (D-F) (32 x 20 x 12
．． The sample was cut into pieces (5 circles) and left in an air bath heater kept at 700°C, and the weight remaining was measured at appropriate intervals to measure the oxidation loss rate. The measurement results are shown in Figure 3.

As is clear from FIG. 3, Sample-E prepared by the method of the present invention is a raw material (IC-
When compared with Sample F, which was used as Example 11) (see Reference Example 1), it is clear that the oxidation resistance is significantly improved by impregnating with the boron component. Surprisingly, samples added with B and C powders using the conventional method -
When the boron content is kept at the same level, compared to
It was found that the oxidation resistance was extremely high.

The reason for this is that in the conventional method, the boron component as B 4 C powder, which has an oxidation-inhibiting effect, is locally unevenly distributed in the form of granules, and microscopically there are many areas where there is no boron component, and oxidation starts from around these areas. On the other hand, in the case of the method of the present invention, the oxidation reaction is suppressed as a whole because it is finely dispersed uniformly throughout.

When producing the C-B composite material of the present invention, the characteristics of the boronation reaction of carbon materials by this method are that uniform and ultrafine dispersion is possible, and carbon materials of any type and shape can be used. Another feature is that boronization can be carried out on materials without substantially impairing the properties and physical properties of the raw material.

Table 1 shows the physical properties of the carbon materials used in the present invention before and after the unrefining reaction, that is, the raw materials used in the experiments of Reference Examples 1 and 4 (Comparative Examples 4 and 5). ) and the physical properties of
Table 2 compares the physical properties of the products subjected to the boronation reaction (Reference Examples 1 and 4) As is clear from Table 2 above, the structure and skeleton of the raw carbon material are etc. do not change, indicating that the physical properties do not change either.

[Brief explanation of the drawing]

Figures 1 and 2 are all photographs exposed by neutron irradiation. FIG. 3 is a graph showing the oxidation consumption rate of various carbon materials. (That's all) O Former 1st η (E) γ December 25, 1990

Claims (4)

[Claims] (1) A composite material whose main components are carbon and boron, obtained by impregnating a carbon material with boron oxide or (and) its hydrated compound and firing it under the pressure of an inert gas at a temperature of 1500°C or higher. A neutron absorbing material using as a neutron absorbing material. (2) Claim in which the carbon material is a high-density isotropic graphite material (
The neutron absorbing material described in 1). (3) Carbon-carbon material reinforced with carbon fibers
The neutron absorbing material according to claim 1, which is a carbon composite material. (4) The remaining amount of liberated B_2O_3 is 0.1% by weight
The neutron absorbing material according to claim (1), which is the following composite material.