Analysis of Fukushima Unit 2 Accident by Considering the Operating Condition of RCIC System and Torus Room Flooding

摘要

The severe accident of Fukushima Daiichi occurred on March 11, 2011, which originated from an earthquake and tsunami. Core degradations were expected in Units 1, 2 and 3 from the results of Phase Ⅰ of the OECD/NEA BSAF Project. Among the three plants, it was estimated that the core degradation of unit 2 is less serious. The operation of the reactor core isolation cooling (RCIC) system in unit 2 during the initial stage of the accident was expected to reduce the core damage because it removes the decay heat in the reactor pressure vessel. During normal operation, the steam from the main steam line drives the RCIC turbine and the mixture of steam and liquid water flow into the wetwell. The liquid water in the wetwell or condensate storage tank was supplied to the downcomer by using the energy from RCIC turbine. However, the efficiency of the RCIC pump was not clear when a mixture of steam and liquid water was supplied into the RCIC inlet. The enthalpy of the outlet of the RCIC pump can also be varied according to the efficiency of the RCIC pump. The energy in the outlet flow after passing the RCIC turbine can affect the pressure of the wetwell and drywell, and thus it can be important to analyze the accident. Furthermore, it was expected that torus room flooding occurred in the case of unit 2. The energy in the wetwell water can also be transferred to the water in the torus room through conduction. In this research, the accident analysis of Fukushima Unit 2 was conducted by considering the RCIC turbine efficiency and degree of torus room flooding.