What Is Nuclear Shielding?
During the operation of the nuclear reactor, the core will generate a variety of radiation and neutron (n), proton (p), deuterium (d), and fission product (FP) radiation, among which the penetration of gamma rays and neutrons Strong, can cause radiation damage to surrounding objects and people [1] .
- The shielding materials are divided into non-metallic materials and metallic materials for discussion and introduction. In addition, concrete is not only a very effective shielding material for -rays and neutrons, but also has many advantages as a structural material, so it will be introduced separately [1] .
- Non-metallic shielding material
- (1) Water
- Water is the most readily available and inexpensive material, and because it contains a large amount of hydrogen, it is an extremely effective neutron shielding material. Oxygen contained in the water captures neutrons and emits less secondary gamma rays. The main reason is that hydrogen emits captured gamma rays with an energy of 2.2 megaelectron volts. Therefore, it can be said that water is a neutron shielding material that generates relatively few secondary gamma rays. However, because the electron density of water is low, it cannot be said to be a good shielding material for gamma rays.
The advantage of water is that it does not generate micro-gap, and it is a stable and effective coolant. However, it also has some defects. For example, it must be completely waterproofed. In addition, if it is left for a long time, chaos will occur. [1]
- (2) Graphite
- Graphite has excellent properties as a neutron moderator and reflector, and because high-purity graphite has stable physical, chemical, and mechanical properties at very high temperatures, it is widely used as a reactor material and shielding material. In liquid metal-cooled fast neutron reactors, graphite is used as the primary shield of the fast neutron shield. In order to improve the seed shielding performance of graphite, thermal neutron absorbers such as boron compounds are sometimes mixed. When graphite is used as the neutron shielding body, the density is preferably 1.69 / m 3 or more.
- (3) Boron and boron-containing substances
- boron
- Boron is used as a thermal neutron absorber, which is achieved by the (n, ) reaction of boron isotope 10 B (abundance ratio 18.45-18.98%) because of its thermal neutron The absorption cross section is extremely large, about 3840 targets. The alpha rays generated in this reaction are easily absorbed in the shield, but when the incident neutron flux is large, such alpha rays are generated and generate heat. Boron can be used directly or mixed with graphite and polyethylene, or used in combination with other materials in the form of boron oxide and boron carbide [1] .
- Boron ore
- In Japan, boron-containing ore suitable as a shielding material is hardly produced. Substances containing small amounts of boron include celite and axe. In addition, Pergat is a fibrous agglomeration that coexists with ferromagnetic ore and other ores. The boron content of the raw ore is 36% (B 2 O 3 ), and the concentrate is 11-13%.
- Aluminum-clad boron carbide plate
- Boron carbide (B 4 C) is mixed with aluminum powder, and then sintered. The sintered material is sandwiched with an aluminum plate to form a sandwich plate.
- boron
- Boron concrete
- In order to improve the thermal neutron shielding performance of concrete, boron is often added to the concrete. This boron also plays a role in reducing secondary gamma rays in concrete. Boron-containing materials generally have water solubility, because it has the property of retarding the hardening of concrete, so the amount of boron added is greater than 1%, and special attention should be paid.
- (4) sand, soil, clay
- Because sand, soil, and clay are easily available and inexpensive, they are often used as shields. When using these materials, care must be taken not to cause sand and soil to sink and collapse due to rain. The density of soil and sand after ordinary tamping is about 1.4 g / cm3, and its composition is roughly the same as that of ordinary concrete. Therefore, when using sand as a shielding body, as long as its thickness is 1.7 times that of ordinary concrete, it can be expected to have the same shielding performance as concrete.
- (5) Organic shielding material
- Organic materials have various advantages as commonly used materials, because most organic materials contain a large amount of hydrogen atoms, so they are often used alone as a moderate retarder and shielding material. However, the organic material monomer itself is not suitable as a shielding material for -rays because its effective atomic number is small. In addition, because the absorption efficiency of neutrons is not necessarily large, in order to further improve the shielding efficiency, Combine materials with large atomic coefficients such as lead, and boron and lithium with large absorption cross sections for slow neutrons and thermal neutrons, together with organic materials, or mix these metal powders with organic materials, or make them overlap Of multilayer boards to use [1] .
- paraffin
- It is a widely used and familiar material. Because it is not only easy to process but also cheap, it is often used as a neutron shielding material in the laboratory. Because it has a specific gravity of 0.87 ~ 0.91 and a low melting point (40 ~ 60 ), it is easy to process. It can be used as long as it is injected into the model.
- Polyethylene and other plastics
- Polyethylene is pure hydrocarbon with a specific gravity of 0.92. In each cubic centimeter volume, water contains 6.7x1022 hydrogen atoms, and polyethylene contains about 8x1022. Therefore, it can be said that this material is a neutron shielding material that is beautiful with water . In addition, the reason why it is suitable as a shielding material is because it does not generate activation, but it has the disadvantage of being easily damaged by radiation (for general organic materials). Usually softens at 100 , some high density (~ o.96g / cm3),
- Polyethylene
In addition to polyethylene, polystyrene, methyl methacrylate and phenol-formaldehyde resins also contain a large amount of hydrogen, so they are sometimes used as neutron shielding materials. No matter which kind of materials, they are brittle and radiant. Damage problems cannot be used as shielding materials like polyethylene.
- Wood and its products
- Because wood also contains a large amount of hydrogen, it can also be used as a neutron shielding material. Wood is basically composed of cellulose and very little lignin. It varies from species, with a specific gravity of 0.5 to 0.8, and contains about 6% by weight of hydrogen. Easy processing, suitable strength for structural materials, and cheap price. However, the amount of moisture contained in it is easily affected by the environment, and it is easy to deform. Therefore, logs are rarely used, and the wood is usually cut into Small pieces or sawdust, then compressed into a plate for use. In this way, as a structural material, its strength is not worse than that of wood. Because of changes in strength and shape and changes in moisture absorption, it is superior to ordinary wood [1] .
- Metal shielding material
- (1) Iron
- Because of its large specific gravity and high mechanical strength, iron is widely used as a reactor structural material, heat shielding material, and pressure shell material. In addition, in addition to the shielding effect on thermal neutrons, it is difficult to say that it is a good neutron shielding material. Through inelastic scattering, iron can decelerate high-energy neutrons, but within a certain energy range, the effect of reducing neutron energy is small. Therefore, it is rare to use iron alone as a shielding body, cooperate with other shielding materials, such as water, to make an iron-water multilayer structure, or mix it in concrete. Due to the capture of thermal neutrons, iron can emit a large amount of 10MeV
- stainless steel
- (2) Stainless steel
- Stainless steel has better shielding performance against gamma rays and neutrons than iron. Especially because of the large inelastic scattering cross section, shielding fast neutrons is more effective. However, chromium, nickel, manganese, and other elements in stainless steel are activated after neutron irradiation, and personnel are restricted from approaching after the reactor is shut down. This problem is more serious than iron.
- (3) Boron-added steel
- In order to increase the shielding effect against thermal neutrons, boron is added to iron to become boron-added steel. In the past, in order to improve the hardenability of steel, the steel added with a small amount of boron was called boron steel. In order to distinguish this, the material used for shielding was called boron-added steel. If boron is added to steel,
- Boron steel
- (4) Lead
- Lead has a density of 11.3 g / cm 3. It is generally used as a gamma-ray shielding material in places with limited space. However, because lead is so soft that it cannot be used as a structure, this is its disadvantage. In addition, lead has a low melting point (327.4 ° C) and is easily attacked by alkali, so its use is limited.
- (5) other metal materials
- Sodium, aluminum, chromium, manganese, nickel, copper, zinc and other elements are rarely used as monomers. In steel and concrete, these elements are present as impurities or mixtures.
When these elements reach a considerable content in the material, of course, the shielding properties of the material are greatly affected, and even in small amounts, sometimes it cannot be ignored that it causes the activation of the material and generates secondary gamma rays. Therefore, when choosing and using shielding materials, it is necessary to know exactly what kind of element is contained.
The atomic numbers of tungsten and bismuth are large, and the shielding effect on -rays is great. In particular, tungsten has a melting point of 3400 ° C. Compared with the melting points of lead and bismuth (corresponding to 327.4 ° C, 271.0 ° C), it can be used at high temperature because it is about an order of magnitude higher. The atomic number of uranium is also large (92), and its shielding ability against -rays is the largest compared with other materials, so sometimes a low-grade nuclear material, diuranium, is used as a -ray shielding material [1] .
- Concrete
- Because concrete has the following characteristics, it is often used as a shielding material.
- (A) contain an appropriate amount of substances necessary for shielding and gamma rays;
- Concrete
- (B) If necessary, a certain element or substance can be mixed into it;
- (C) It is easy to form and process, and it can also be used to form a complex shield.
(D) have the necessary strength and durability of the structure;
- (E) Compared with other shielding materials, it is cheaper.
- When using concrete as a shielding material, in addition to the properties that are commonly used for structural concrete, the following properties are also required:
- (A) The material of the concrete is uniform, no matter where it is, the density and chemical composition meet the requirements;
- (B) the volume change is small, and the defects on the shield are not caused by the occurrence of cracks;
- (C) When the wall thickness is thick, the calorific value during assimilation is also small;
- (D) The temperature rise caused by the absorption of radiation will not cause thermal expansion and a significant reduction in moisture;
- (E) In terms of thermal performance, the thermal conductivity is large and the thermal expansion coefficient is small;
- (F) The radiation damage is small, and no harmful radioactive materials are generated [1] .
If ordinary concrete absorbs 1 mW / cm3 of radiation locally, the temperature inside the concrete will increase by about 3 ° C. If the temperature rises above 80 ° C, the moisture in the concrete will be rapidly lost, and the neutron shielding effect will decrease. Therefore, as a design condition of the shielding material concrete, the following three points must be considered:
- (A) The total incident energy should be below 4x1010 MeV / cm 3 .s;
- (B) Control the ambient temperature below 80 ° C;
- (C) Do not make the temperature gradient greater than 0.8 ° C / cm [1] .