The safety of nuclear power plants is of paramount importance to the nuclear energy sector. A vital element of nuclear plants is the sturdy shielding doors that are installed in critical areas. A study published in Frontiers in Energy Research has presented research into new temperature-resistant materials for these critical power plant infrastructure elements.
Study of a High Temperature–Resistant Shielding Material for the Shielding Doors of Nuclear Power Plants. Image Credit: Parilov/Shutterstock.com
The Importance of Nuclear Shielding
Early on in the atomic age, the potential of radiation to cause fatalities were discovered. This discovery led to the urgent need to create effective shielding materials and infrastructure for nuclear power plants to ensure the safety of workers and the environment, as well as protect vital equipment outside of the shielded area.
Samples of the lead–boron polyethylene shielding composite. Image Credit: Xiao-ling, L et al., Frontiers in Energy Research
Elements of nuclear power plants that require shielding include reactors, pressure valves, and main circuit systems. Conventional materials used for shielding in doors include boron steel, epoxy resin, and lead-boron polyethylene. Because shielding doors are close to radioactive areas, they must be designed to withstand elevated levels of temperature, humidity, and radiation.
Because of this, shielding materials must not only display superior mechanical properties, performance, and resistance to aging from irradiation and hydrothermal pressures over their recommended 20-year service life. They must also be able to withstand extreme temperatures of up to 190oC when a loss of coolant accident (LOCA) occurs.
Shielding doors and materials are subjected to intense 48-hour LOCA simulations before being approved for installation in nuclear power plants. After this time period, shielding doors must remain intact and display no significant deformation. They must also be easy to repair and replace, and all performance indicators must be within acceptable ranges.
Combining Materials in Shielding Doors
The optimal design of a shielding door for nuclear power plants combines different materials to protect against neutrons and Gamma radiation. Materials like lead absorb and scatter Gamma-rays through effects like the Compton effect and photoelectric effect, and slow down fast neutrons through inelastic scattering. High-carbon content polyethylene can further moderate intermediate neutrons through elastic scattering, which are then absorbed by 10B of boron carbide.
Environmental condition test parameters of the simulated design basis accident. Image Credit: Xiao-ling, L et al., Frontiers in Energy Research
However, polyethylene-based shielding materials exhibit lower melting temperatures and heat deformation temperatures, meaning that they do not resist the elevated temperatures caused by a loss of coolant accident. This leads to mechanical deformation of the materials such as softening and splashing, which affects shielding effectiveness and increases the risk of radiation leaks.
Even when they are fitted with protective elements such as lead or steel plates, temperatures can still reach more than the safe level on the surface of polyethylene-based materials.
Improving the Materials Used in Radiation Shielding
To investigate how to improve the thermal resistance and mechanical properties of radiation shielding doors, the study published in Frontiers in Energy Research presented a composite lead-born polyethylene shielding material. This material would perform better during the intense heat of a loss of coolant accident scenario and prevent radiation leaks, improving safety levels for nuclear power plants.
Raw material modification and optimization of composition design further improved the properties and performance of the composite shielding material presented in the research. The door design underwent stringent, comprehensive performance tests and sample trials. The shielding design was evaluated at the Hualong One reactor chamber adit under both normal and accident conditions.
The composition ratio of the shielding material was designed according to the Monte Carlo method and the genetic algorithm. An ultrahigh-molecular weight polyethylene was used with block-and-graft copolymerizations throughout the mixing process. This improved the material’s resistance to elevated temperatures and achieved the maximum shielding effect.
MCNP calculation simplified model for the shielding door of the reactor pit chamber. Image Credit: Xiao-ling, L et al., Frontiers in Energy Research
The modification molecule chosen was maleic anhydride. This modification molecule optimized the uniformity and mechanical properties of elements such as lead and boron carbide that were mixed with polyethylene. Extensive environmental tests were conducted on factors such as the neutron shielding properties of the composite material, its mechanical performance, resistance to hydrothermal and irradiation-induced aging, and performance in the event of a LOCA.
The results of the intensive tests conducted on the material displayed all-round superior performance. Shielding integrity was maintained even in the harsh atmospheric temperature of a loss of coolant accident. It was further demonstrate3d that adding a 60-mm thick layer of this novel material to the reactor adit shielding door reduced gamma radiation dose levels by five times and neutron levels by ten times. This was within zoning requirements for the shielding door.
The research demonstrated a composite shielding material that displayed superior performance even under catastrophic accident conditions. Widespread use of the material would drastically improve the safety of personnel and protect sensitive equipment from damage by exposure to irradiation.
Xiao-ling, L et al. (2021) Study of a High Temperature–Resistant Shielding Material for the Shielding Doors of Nuclear Power Plants [online] Front. Energy Res. | frontiersin.org. Available at: https://www.frontiersin.org/articles/10.3389/fenrg.2021.751654/full#h1