A radiation protection technology that uses the effect of radiation and material to reduce the radiation level of a certain area, so as to reduce people's exposure to radiation and the radiation damage of materials
Radiation shielding refers to a radiation protection technology that uses the effect of radiation and materials to reduce the radiation level of a certain area, so as to reduce people's exposure to radiation and the radiation damage of materials. It is also a material protection measure.[1]
In nuclear power plants, the main objects of radiation shielding are gamma rays (gamma photons) and neutrons.When gamma photons pass through the shield, they transfer energy to the shield through photoelectric effect, Compton scattering, electron pair formation and other processes and are weakened or absorbed.Photoelectric effect is that photons transfer all energy to orbital electrons, making electrons leave their shells and release from atoms, which plays a major role in the absorption of low-energy gamma photons (gamma photons with energy less than a few hundred keV).Compton scattering is the combination of photons andFree electron collision, transfer part of the energy to the electron, and change its direction and energy at the same time, which plays a major role in reducing the energy of medium energy gamma photons (energy between several hundred keV and several MeV).The formation of electron pairs is the interaction between γ photons and the electric field of the nucleus. γ photons completely annihilate, and their energy is converted into the mass and kinetic energy of a pair of positive and negative electrons as well as the kinetic energy of the recoil nucleus, which plays a major role in the absorption of high-energy γ photons (energy greater than several MeV).
Fast neutrons enter the shield, mostly throughElastic scattering and inelastic scatteringIt transmits its energy to the shielding material, turns into thermal neutron or epithermal neutron, and then is absorbed by the material through radiation capture and other processes.Elastic scattering refers to the elastic collision between neutrons and the atomic nucleus of shielding material, which transfers some (in rare cases, all) energy to the recoil nucleus, and changes its own energy and direction of motion.The smaller the mass of the recoil nucleus, the more energy is transmitted to it on average in a collision.For fast neutrons with energy of 2 MeV colliding with hydrogen nucleus, they can be slowed down to thermal neutrons after an average of 18 collisions;The collision of 2 MeV fast neutrons with lead nuclei takes about 2000 times to slow down to thermal neutrons.Inelastic scatteringThe difference from elastic scattering is that the recoil nucleus is in an excited state and returns to the steady state by emitting gamma rays in addition to obtaining kinetic energy.The probability of inelastic scattering increases with the increase of neutron energy and atomic number of shielding material.One inelastic scattering can transfer a considerable amount of energy to the recoil nucleus, so inelastic scattering is the main process of deceleration of fast neutrons (energy greater than 1 MeV).Radiation capture [(n, γ) reaction] is the last process in which neutrons are absorbed by shielding materials.Most nuclides are easy to react with thermal neutrons (n, γ), and a few nuclides are also easy to react with super thermal neutrons by resonance absorption.
Shielding design
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To conduct shielding design, first determine the type and activity of radiation source, determine the radiation level of observation point and the shape of shielding body, and then select appropriate shielding materials and calculation formula to calculate the thickness of shielding body.
The radiation source of nuclear power plant is mainly the reactor, followed by the primary coolant, spent fuel elements and radioactive waste.The reactor will produce gamma rays and neutrons during operation.Gamma ray mainly refers to the prompt gamma ray emitted during nuclear fission and the gamma ray emitted during fission product decay, in addition to thermal neutron capture gamma ray and fast neutronInelastic scatteringγ - ray generated, γ - ray of nuclear reaction products, γ - ray of activation products, annihilation radiation, bremsstrahlung, etc.Neutrons are mainly fission neutrons, in addition to delayed neutrons, neutrons of activated products and photoexcited neutrons.A reactor with a power generation capacity of 1000 MW has a γ - ray emissivity close to 3.5 × 10MeV/s and a neutron emissivity of about 2.5 × 10n/s during operation.After shutdown, there is basically no neutron, but the γ ray of fission products and activation products can still reach 1021MeV/s.The main radiation of primary coolant is γ ray of fission products and activation products, and its radioactive concentration can reach 4 × 107Bq/L.The typical spent fuel pool can store about 13/3 of the core load of spent fuel, but because it has decayed for many days, its maximum activity is about 5% of the core.[2]
Shielding standard
Determine the radiation level of observation points Determine that the radiation level of each observation point is related to the purpose of shielding.For the health of workers, different radiation levels should be determined according to the frequency and time of workers approaching the radiation source, and shielding should be conducted in different zones (seeRadiation zoning of nuclear power plant)。In order to prevent the radiation damage of equipment, prevent the activation of materials and maintain the stability of shielding, the radiation level shall be determined according to the characteristics of materials and use requirements.China's nuclear industry standards stipulate that: ① the irradiation of reactor body components should be lower than the specified limit; ②The internal surface neutron fluence rate of ordinary silicate concrete shield shall be less than or equal to 5 × 10n/(cm · s), and the gamma ray energy (quantity) fluence rate shall be less than or equal to 4 × 10MeV/(cm · s); ③The thermal neutron fluence rate shall be less than or equal to 1 × 10n/(cm · s) when the reactor is in operation at the place where the staff work after shutdown.
Selection of shielding materials
Shields of nuclear power plants should have the following characteristics: ① high density, which can effectively absorb primary and secondary gamma rays, and slow down fast neutrons through inelastic scattering; ②It contains enough hydrogen to effectively slow down neutrons below the inelastic scattering threshold into thermal neutrons; ③Sufficient mechanical strengthMechanical stability, thermal stability and chemical stability; ④Low price, easy to process and build.Only a mixture of several materials can meet these requirements.The shielding materials commonly used in nuclear power plants are steel, water and concrete (including concrete containing heavy metal aggregates containing crystal water or boron), and lead or boron containing plastics are also used locally.
Shielding calculation
The point nuclear technique and accumulation factor are usually used to calculate the attenuation of gamma rays in the shielding body.For the shield containing hydrogen materials, a diffusion method is often used to calculate the attenuation of fast neutrons and the distribution of thermal neutrons in the shield.The test shows that when there is enough hydrogen (about 6 g/cm hydrogen, or about 50 cmHtwoO), the decay of fission neutrons in it approximately follows the exponential law.It can be considered that the neutron which reduces energy and changes direction after scattering is "separated" from the fast neutron beam, so this weakened cross section is called the separation cross section.The separated neutrons will enter the diffusion process. The distribution of intermediate energy neutrons and thermal neutrons in the shield can be calculated according to the diffusion theory, which is essential for the calculation of the secondary gamma ray distribution in the shield.It must be noted that when the hydrogen content in the shield is insufficient, the intermediate energy neutron dose often becomes the main contributor to the neutron dose.
With the development of computer technology, people can use numerical methods to solve the problem of neutron or gamma photon transport in the shield.There are two ways, one is macro, that is, to solve Boltzmann equation. The methods used are moment method, spherical harmonic function method, discrete ordinate method, etc.The other is microscopic, that isMonte Carlo methodIt describes the process of individual particles passing through the shield until they are absorbed or penetrated. As long as the number of samples is large enough, it can accurately describe the transport process of neutrons or gamma photons in the shield.These calculation methods have been well used in engineering design.
In classified shielded nuclear power plant
Radiation shielding
The shielding of the reactor building generally adopts a two-stage shielding scheme, that is, the shielding of the reactor body (primary shielding) and the shielding of the primary coolant system (secondary shielding).Because the pipes and equipment of the primary coolant system are arranged around the reactor, they are also strong radiation sources and inaccessible when the reactor is operating.Therefore, the shielding of the reactor body is mainly to prevent the activation of these equipment and secondary coolant and ensure that the radiation level from the reactor outside the shielding is lower than the radiation level from these equipment itself after shutdown.The shielding of the primary coolant system is to protect the health of workers and reduce the γ radiation level of the environment around the reactor building.The overview of the primary and secondary shielding is shown in figure.
Heap body
The shielding (primary shielding) of the reactor body is composed of multiple steel and water shielding in the pressure vessel and the surrounding circular concrete wall with a thickness of about 2m.Several layers of steel and water shields are respectively composed of the core bulkhead, core barrel, thermal shield, pressure vessel and the water layer in the middle.In addition to the purpose of safety protection, these shields also have some engineering considerations. For example, thermal shields can be used to protect the mechanical properties of pressure vessels and will not deteriorate due to excessive neutron irradiation;Reduce the heat in the concrete and prevent the activation of equipment outside the primary shielding.
Reactor coolant system
Reactor coolant systemThe shielding (secondary shielding) of includes the polar crane bearing wall around the reactor coolant system and the cement operation floor above it.The concrete structure of the containment is also considered as secondary shielding.Reactor coolantThe main radiation source inside is N.The purpose of the secondary shielding is to reduce the radiation to a safe level, so that the workers can enter the containment for a short time to carry out necessary inspection and maintenance when the reactor stack is operating at full power.The secondary shielding also has such a role, that is, when the reactor stack is operating at full power, people can work normally outside the containment, and protect workers and surrounding residents from excessive exposure when the reactor core melts and a large amount of radioactive substances enter the containment.
Shielding factor
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Radiation related to radiation source shielding includes: (a) useful harness of radiation source.(b) Leakage radiation passing through the radiation source assembly shell is a non useful harness.(c) Scattered radiation refers to the scattered radiation of objects, patients, device parts and building walls directly exposed by useful harness and leakage radiation.(d) Sky scattering radiation, that is, the radiation (mainly useful harness and leakage radiation) passing through the top of the shielding room acts on the air above the top of the shielding room and scatters to the surrounding area of the shielding room.(e) Lateral scattering radiation, that is, the radiation emitted by the radiation source into the top of the shielding room is heavier than the floor on the roof of the building where people stay at a certain distance from the roof shielding room.(f) When the radiation energy is high (such asProton therapy）The neutron and related radiation generated by the nuclear reaction when the useful harness and leakage radiation directly irradiate the material, which is the associated secondary radiation.
