Safety Function Reliability Evaluation and Risk Management during Decommissioning of Accident-Damaged Nuclear Power Plant ～Based on Results of Confinement Function Reliability Evaluation

During the decommissioning of accident-damaged nuclear power plants, three fundamental safety functions, i.e., stopping the reactor, cooling the reactor, and confining radioactive materials, have to be maintained as well as normally operating plants. However, for 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. Even in such cases, it is very important to suppress the additional release of radioactive materials to outside during the decommissioning. 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 repair equipment or components. Because of this, a lot of time and effort may be needed to complete the project, and it may be necessary to take a strategic, efficient, and effective step by taking a risk-informed approach. In this paper, we focused on a negative pressure control system for a primary containment vessel and a reactor building during fuel debris retrieval, evaluated its reliability, and, on the basis of the evaluation results, considered appropriate safety management for nuclear decommissioning.