What is a Radiation Shield?

Radiation shielding refers to a kind of radiation protection technology that uses radiation and substances to reduce the radiation level in a certain area, thereby reducing people's exposure and radiation damage to materials, and is also a material protection measure. ^[1]

In nuclear power plants, the main objects of radiation shielding are gamma rays (gamma photons) and neutrons. photons pass through the shield and pass through the photoelectric effect, Compton scattering, and electron pair formation to transfer energy to the shield and are weakened or absorbed. The photoelectric effect is that the photon transmits the entire energy to the orbiting electrons, causing the electrons to escape from the shell and release them from the atom. This plays a major role in the absorption of low-energy gamma photons (gamma photons with energy less than several hundred keV). Compton scattering is the collision of a photon with a free electron, passing part of the energy to the electron, and changing its direction and energy at the same time. It plays a major role in reducing the energy of the medium-energy gamma photon (energy between several hundred keV and several MeV). The formation of an electron pair is the action of the electric field between the photon and the nucleus. The photon is completely annihilated. Its energy is converted into the mass and kinetic energy of a pair of positrons and negative electrons, and the kinetic energy of the recoil nucleus. main effect.

Fast neutrons enter the shield. In most cases, their energy is transferred to the shielding substance through elastic scattering and inelastic scattering, and they become thermal neutrons or superthermal neutrons, which are then absorbed by the material through processes such as radiation capture. Elastic scattering is the elastic collision between the neutron and the nucleus of the shielding material, which transfers a part (in rare cases, all) of the energy to the recoil nucleus, while changing its own energy and direction of motion. The smaller the mass of the recoil core, the more energy is averagely transmitted to it in a collision. For a collision of 2 MeV fast neutrons and hydrogen nuclei, 18 collisions on average can slow down to thermal neutrons; while a collision of 2 MeV fast neutrons and lead nuclei takes about 2000 times to slow down to thermal neutrons. The difference between inelastic scattering and elastic scattering is that in addition to obtaining kinetic energy, the recoil nucleus itself is in an excited state and returns to a steady state by emitting gamma rays. The probability of inelastic scattering increases with the neutron energy and the atomic number of the shielding material. A single 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, ) reaction] is the last process in which neutrons are absorbed by the shielding substance. Most nuclides are prone to (n, ) reactions with thermal neutrons, and a few nuclides are also prone to resonance absorption reactions with superthermal neutrons.

The design of the shield must first determine the type and activity of the radiation source, determine the radiation level of the observation point and the shape of the shield, and then select the appropriate shield material and calculation formula to calculate the thickness of the shield.

The main sources of radiation in nuclear power plants are reactors, followed by primary coolants, spent fuel components and radioactive waste. The reactor will generate gamma rays and neutrons during operation. -rays are mainly instantaneous -rays emitted during nuclear fission and -rays emitted during decay of fission products. In addition, there are -rays generated by thermal neutron capture -rays and fast neutron inelastic scattering, and -rays of nuclear reaction products. -rays of activation products, annihilation radiation and bremsstrahlung radiation. Neutrons are mainly fission neutrons, in addition to delayed neutrons, activated neutrons, and photoexcited neutrons. A reactor with a generating capacity of 1000 MW has a gamma-ray emission rate close to 3.5 × 10 MeV / s and a neutron emission rate of about 2.5 × 10 n / s during operation. After the shutdown, there are basically no neutrons, but the gamma rays of fission products and activation products can still reach 1021 MeV / s. The primary radiation of the primary coolant is the gamma rays of fission products and activation products, and its radioactive concentration can reach 4 × 107Bq / L. A typical spent fuel pool can store spent fuel at about 13/3 of the core load, but its maximum activity is about 5% of the core because it has been decaying for many days. ^[2]

The radiation related to the shielding of the radiation source includes: (a) a useful harness of the radiation source. (B) Leaked radiation passing through the radiation source assembly housing, which is an unused wiring harness. (c) Scattered radiation, that is, scattered radiation from objects, patients, device components, and building walls that are directly exposed to useful beams and leaking radiation. (D) Sky-scattered radiation, that is, radiation that passes through the top of the shielded room (mainly useful wiring harnesses and leaking radiation) interacts with air above the top of the shielded room and is scattered into the surrounding environmental area of the shielded room. (e) Side-scattered radiation, that is, the radiation emitted by the radiation source into the top of the shielded room and the roof shielded room a certain distance outside the building where the person resides is heavier than the roof. (f) When the radiant energy is high (such as proton therapy), the useful beam and leaking radiation directly irradiate the neutron and related radiation caused by the nuclear reaction on the material, which is the associated secondary radiation.