The alternate mitigation strategies on the extreme event of the LOCA and the SBO with the TRACE Chinshan BWR4 model

The LOCA combined SBO accident is analyzed using the TRACE code which has been designed by USNRC to perform best-estimate analyses of loss-of-coolant accidents (LOCAs), operational transients, and other accident scenarios in reactor systems. From the accident at the Japanese Fukusima NPP, an extreme event beyond the design basis is realized to be possible. The current mitigation strategies for the emergency core cooling systems (ECCSs) can be easily voided in the event of an extended station blackout (SBO), where all the onsite and offsite electrical power is failed. Although the electrical power of the critical control systems can be recovered by portable electrical generators, the electrical pumps are difficult to recover by any portable device. The only possible driving force of the pumps in SBO is the steam generated by residual heat. The current strategies in an extended SBO are mostly focused on low pressure injection, but the reactor water level will decrease sharply while the reactor pressure is reduced and that results in a higher PCT. In this paper, the alternate mitigation strategies adopting the turbine driven pumps, the high pressure injection systems, are analyzed to maintain an “enough” water level before the reactor pressure is reduced. The Chinshan NPP, the same BWR/4 reactor as the Fukusima NPP, is the first nuclear power plant in Taiwan, operating at thermal power at 1828 MWt. Three break sizes, 100%, 10% and 1%, on the recirculation suction line which is the most serious LOCA in BWR/4 reactor are analyzed with three sensitivity studies: (1) the scram time, (2) the increase of RCIC injection flow rate, and (3) the earlier HPCI injection. Through the paper, the alternate mitigation strategy using the turbine driven pumps and residual steam is evaluated for the emergency operational procedures (EOPs) and the severe accident mitigation guidelines (SAMGs).

► Increasing RCIC flow and injecting HPCI flow earlier are effective to reduce the PCT.
► To utilize the best of residual steam with RCIC and HPCI is necessary in SBO.
► The current designed RCIC flow is not sufficient even in a very small break LOCA.
► The reactor water level is not recovered until the success HPCI injecting in a LOCA.
► To inject the HPCI flow earlier is more effective than to increase the RCIC flow.