JPS6249305B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a neutron shielding material comprising a polyethylene resin and a relatively large amount of an inorganic boron compound.

In recent years, with the development of the nuclear power industry, radiation shielding in nuclear facilities has become important. The purpose of this radiation shielding is to keep the amount of radiation received by the human body below the permissible level by using some kind of shielding material, and to prevent the structural and equipment materials of nuclear facilities from being damaged by radiation, heating, and activation. The goal is to prevent radiation exposure and keep the radiation background to measurement equipment low. In particular, regarding reactor shielding, gamma rays and neutrons coexist, and secondary radiation must also be taken into consideration. Since the attenuation characteristics of γ-rays and neutrons differ depending on the substance, combinations of various substances are being considered as shielding materials. Currently, concrete is the most commonly used shielding material as it is effective against both gamma rays and neutrons. Lead is commonly used as a gamma ray shielding material, but it is said to be weak in strength. Water is also widely used as a neutron shielding material. Water is circulated and used to cool the reactor body of the neutron breeder reactor. Water is widely used to increase the water content in shielding concrete, thereby enhancing the shielding effect of concrete. Hydrocarbon compounds that are similar to water and have relatively few impurities and a relatively large number of hydrogen atoms (for example, paraffins, polyethylene resins) are used in the form of blocks or thick plates.

On the other hand, from the standpoint of radiation absorption, elements with large atomic numbers are effective at attenuating gamma rays, and elements with large inelastic scattering cross sections for neutrons are suitable for attenuating fast neutrons into slow neutrons. There is. When fast neutrons become thermal neutrons or slow neutrons through scattering, they are easily absorbed by atomic nuclei, and elements with large absorption cross sections increase the absorption effect. For these reasons, even if neutrons are absorbed, secondary gamma rays must not be emitted, or even if they are emitted, the energy must be extremely low.

As mentioned above, it is important for neutron shielding materials to contain as many elements as possible that have effective shielding performance, but shielding design is an issue of optimal design from the viewpoints of weight, volume, cost, etc. exists. In particular, lightweight, low-volume neutron shielding materials are needed in place of concrete, lead, water, etc., as shielding materials for vessels with limited weight and volume, such as nuclear-powered ships.

Radiation shielding has also become an issue in fields other than nuclear power, especially in the electronics industry, which has made remarkable progress recently. Especially integrated circuits (IC circuits)
The package material,
The development of processes to prevent alpha rays is becoming important.

Thus, in the fields of the nuclear power industry and the electronics industry, there is an urgent need to develop neutron shielding materials that are lightweight and have a low volume.

Based on the above, the present inventors searched for various neutron shielding materials that do not have the above problems, and found that by filling 100 parts by weight of polyethylene resin with 350 to 750 parts by weight of an inorganic boron compound. , neutron strength is reduced by 1/2 in the form of a sheet with a thickness of 3 to 5 mm.
It has been discovered that a neutron shielding material capable of attenuating neutrons to 100% or less can be obtained, and the present invention has been achieved.

It has been known that compounds such as inorganic boron compounds and inorganic lithium compounds have a neutron shielding effect. Once the composition, degree of dispersion, thickness, etc. are determined from the absorption cross section of those elements, the shielding ability can be calculated. Regarding shielding materials in which a resin material such as polyethylene resin is filled with boron nitride, US Pat. No. 3,261,800 and British Patent No.
Already known as number 1169855. According to these specifications, in the former, the blending ratio of boron nitride is 0.05 to 70 parts by weight per 100 parts by weight of the resinous material, and in the latter, the blending ratio is 25 to 50 parts by volume per 100 parts by weight of the resinous material. Department. As these specifications make clear, when filling a commonly used resinous material with boron nitride, etc., the limit is 70 parts by weight per 100 parts by weight of the resinous material. If more than this is filled in order to increase the shielding effect, it will be difficult to fill the resinous material with inorganic powder such as boron nitride, and even if it is filled, the mechanical strength of the resulting composition will be significantly reduced.

Hereinafter, the method for manufacturing the neutron shielding material of the present invention will be explained in detail.

The polyethylene resin used in the present invention is polyethylene with a density of 0.950 g/cc or more. That is, it is desirable that the degree of branching is relatively low (the degree of branching is 5 or less per 1000 carbon atoms). In addition, its molecular weight is over 30,000, especially
50000 or more is desirable. In order to give this polyethylene resin flexibility and further improve its moldability, low-density polyethylene (so-called high-pressure polyethylene), ethylene-vinyl acetate copolymer, ethylene and α-olefin (carbon number is at most 8 Although a polymeric material such as a copolymer with a polymer (2) may be used, it is preferable to keep the proportion of these resins in the total polymer to 50 parts by weight or less.

Furthermore, among the inorganic boron compounds used in the present invention, inorganic boron compounds that do not contain metal are desirable. Representative examples of desirable inorganic boron compounds include boron carbide, boron nitride, boric anhydride, orthoboric acid, metaboric acid, and tetraboric acid. Among these inorganic boron compounds, boron nitride is preferred. These inorganic boron compounds are preferably in powder form with a weight average diameter of 0.5 to 500 microns, particularly 3.0 to 300 microns.
Micron ones are preferred. Boron nitride, which is a preferred inorganic boron compound, preferably has a true density of 1.88 to 2.27 g/cm ³ , a weight average diameter of 0.7 to 6.0 microns, and a surface area of 14 to 35 m ² /g, depending on the degree of crystal development. In particular, those having a true specific gravity of 2.0 g/cm ³ or more and a weight average diameter of 3.0 microns or more are preferred.

The blending ratio of the inorganic boron compound to 100 parts by weight of the polyethylene resin (including other polymeric substances, if included) is 350 to 750 parts by weight, particularly preferably 350 to 600 parts by weight. If the blending ratio of the inorganic boron compound to 100 parts by weight of the polyethylene resin is less than 350 parts by weight, the effect of the addition is not necessarily satisfactory. on the other hand,
If it exceeds 750 parts by weight, it is difficult to obtain a uniform composition and it is impossible to form it into a plate.

In producing the composition of the polyethylene resin and inorganic boron compound of the present invention, the polyethylene resin and a part of the inorganic boron compound are dry blended in advance, then melted and kneaded to a uniform state, and then the remaining inorganic boron compound is blended into a uniform state. It is preferable to perform melt-kneading while sequentially adding the components so that the following results are obtained.

When melt-kneading the inorganic boron compound into the polyethylene resin, the polyethylene resin is first put into a Brabender or kneader, and the resin is sufficiently melted and kneaded while keeping the temperature above the melting point of the polyethylene resin. After this melting and kneading has been sufficiently performed, a portion of the inorganic boron compound is added and thoroughly kneaded. Once this kneading has been sufficiently performed, the inorganic boron compound is added while kneading to form the final blended components. In this mixing, the mixing method commonly used in the field of use of polyethylene resins is used, for example, dry blending is performed in advance using a mixer such as a ribbon mixer and a tumbler, and then the resulting mixture is blended using a mixer such as an on-pull roll or an extruder. Even if melt-kneading is performed using a mixer, it is impossible to fill 100 parts by weight of polyethylene resin with 300 parts by weight or more of boron nitride.

In order to fill the inorganic boron compound into the polyethylene resin of the present invention, a part of the inorganic boron compound is filled into the polyethylene resin and kneaded almost uniformly, and then the inorganic boron compound is kneaded into this mixture almost uniformly. It is important to invest.

The mixture obtained in the above manner is molded using a molding machine such as a press molding machine, an extrusion molding machine, or an injection molding machine used in the field of synthetic resins at a temperature above the melting point of polyethylene resin (generally,
It may be formed into a desired shape at 100 to 200°C.

The neutron shielding material of the present invention obtained as described above does not generate gamma rays, which was a drawback of conventionally used cadmium plates, lead plates, etc., and is in the form of a plate with a thickness of about 3 to 5 mm. Not only does it have a neutron shielding effect, but it can also be easily thermoformed.
It is thought that it can be molded into various shapes and can be used in a wide range of fields, not only in the nuclear energy field but also in the radiation shielding field of electronic equipment.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

In addition, in the examples and comparative examples, melt
The index (hereinafter referred to as “MI”) is ASTM
According to D-1238, the load is 2.16Kg and the temperature is
Measurement was performed at 190°C.

Example 1 High ^- density polyethylene (M.
I.1.0g/10 minutes) 100 parts by weight were put into a Brabender set at 180°C and melt-kneaded. 400
Part by weight of boron nitride (manufactured by Showa Denko Co., Ltd., trade name SHOB S HPS, density 2.27 g/cm ³ , weight average diameter
A portion of the boron nitride (3.5 microns) was put into the molten high-density polyethylene and kneaded almost uniformly, and then the remaining boron nitride was gradually added while kneading. After the entire amount was added, the mixture was thoroughly kneaded for about 10 minutes, and then the mixture was taken out. the resulting mixture
A sheet (5 mm thick) was produced using a 50 ton press set at 230°C. The obtained sheet had a hard tile shape with good surface smoothness, good dispersion, and excellent surface condition.

In order to measure the neutron shielding ability of this sheet, we conducted a transmission experiment using a thermal neutron flux from a triggered nuclear reactor. Neutron intensity after passing through is 1/12
0, indicating good shielding ability.

Example 2 Melt-kneading was carried out in the same manner as in Example 1, except that high-density polyethylene with a density of 0.956 g/cm ³ (MI 0.5 g/10 minutes) was used instead of the high-density polyethylene used in Example 1. I did it. After this kneading was completed, the set temperature of the Brabender was raised by 200°C, and 600 parts by weight of the boron nitride used in Example 1 was added in the same manner as in Example 1. After the entire amount was added, the mixture was thoroughly kneaded for about 10 minutes, and then the mixture was taken out. A sheet was prepared from the obtained mixture in the same manner as in Example 1. The obtained sheet had a good dispersion state and a hard tile-like shape with a good surface condition.

When the neutron shielding ability of the obtained sheet was measured in the same manner as in Example 1, the thermal neutron intensity was 1/89.
This result shows that the shielding ability is good.

Comparative Example 1 The amount of nitrogen and boron used in Example 1 was
Melt-kneading was carried out in the same manner as in Example 1 except that the amount was changed to 250 parts by weight. Then, a sheet was created in the same manner as in Example 1. When the neutron shielding ability of the obtained sheet was measured in the same manner as in Example 1, the neutron intensity after transmission was 1/15.

Claims (1)

[Claims] 1 The density of 100 parts by weight is 0.950 g/cc or more,
A neutron shielding material comprising a polyethylene resin having a molecular weight of 30,000 or more and 350 to 750 parts by weight of an inorganic boron compound.