Study on risk management during decommissioning of accident-damaged nuclear power plant

In normal nuclear power plants, three fundamental safety functions, i.e., stopping the reactor, cooling the reactor and confining radioactive materials, must be maintained to ensure nuclear safety. Similarly, during the decommissioning of accident-damaged nuclear power plants, they must be maintained. However, in severely accident-damaged nuclear power plants, such as Fukushima Daiichi Units 1 to 3, the stopping function is lost and the confinement function is degraded. In such cases, it can be considered that the cooling function during the decommissioning period before starting fuel debris retrieval becomes more important for nuclear safety than in the other periods. This is because it plays a very important role in maintaining the physical state of the fuel and fuel debris to prevent recriticality and thus suppresses the mobility of radioactive materials. Similarly, the confinement function becomes more important during the fuel debris retrieval. Because of the high radiation level, it is very difficult to access the inside of the primary containment vessel and reactor building to investigate and/or repair equipment or components. Thus, it may be necessary to propose a decontamination and decommissioning project with remaining unknowns and to perform extensive research and development to complete the project. Such a project must be carried out strategically with the efficient and effective use of risk information. In this paper, the results of considering the basic concept and methods of risk management during the decommissioning of accident-damaged nuclear power plants from the viewpoint of safety and economic risks are reported.