Study on evaluation of safety function reliability and risk management during the decommissioning of an accident-damaged nuclear power plant

In severely accident-damaged nuclear power plants, fuel rods may melt through the reactor vessel wall and drop to the floor inside the primary containment vessel (PCV). The PCV and the reactor building may also be damaged by a severe accident. Even in such a case, the three fundamental safety functions, i.e., stopping the reactor (preventing recriticality), cooling the reactor, and confining radioactive materials, must be maintained so that the uncontrolled release of radioactive materials to the outside can be minimized during decommissioning. However, the success of this strongly depends on the availability of the cooling function of the reactor cooling system, which can maintain the physical form of the fuels and fuel debris by water cooling and therefore prevent their recriticality and the uncontrolled release of radioactive materials to the outside. In other words, the cooling function before completing the retrieval of fuels and fuel debris is very important for safety. Thus, we focused on the reactor cooling system of an accident-damaged boiling water reactor (BWR) during decommissioning, evaluated its reliability, and then considered how the evaluation results should be used in the risk management during the decommissioning of an accident-damaged BWR. (author)