Effectiveness of the raw-water system as a severe-accident-management measure in station blackout accidents

摘要

Chinshan is a Mark-I boiling water reactor nuclear power plant (NPP) located in north Taiwan. It incorporates several severe-accident-mitigating features, especially two raw-water tanks in the mountain. According to a probabilistic risk assessment (PRA) of Chinshan NPP, station blackout (SBO) sequences are the most dominant sequences in internal core damage frequency. No credit is taken for the raw-water system in the development of a Chinshan PRA. Therefore, two dominant sequences (T3UTERDGX and T3UTERDG) of the SBO in the Chinshan PRA are cited as reference cases to evaluate the capacity of the raw-water system in the PRA and severe accident. The T3UTERDGX sequence is initiated by loss of off-site power (T-3) followed by failure of both diesel generators (D-G), failure of gas turbine generators, and failure to recover alternating current (ac) power (E-R). That results in loss of all on- and off-site ac power. The high-pressure injection systems fail (UT) initially and timely reactor depressurization fails (X). The T3UTERDG sequence is the same as the T3UTERDGX sequence, except for failure of timely reactor depressurization (X). The MAAP4 code is used as a tool to evaluate the effectiveness of the raw-water system. Based on MAAP4 analysis, the raw-water system cannot cool down the core in the T3UTERDG sequence after introducing severe-accident-management guidelines. The raw-water system cannot flood dry-well water level above minimum debris submerge level (MDSL) in the T3UTERDGX sequence after reactor pressure vessel (RPV) breach. Sensitivity studies show that raw-water injection before the vessel water level reaches level 2 (L-2) can keep core coolability in the T3UTERDG sequence. Three times the raw-water injection rate is the minimum flow rate to flood the dry-well water level above MDSL and cool down the corium on the dry-well floor in the T3UTERDGX sequence. A raw-water system can be. used as a mitigating measure, especially in an SBO. The RPV should be depressurized as quickly as possible if a raw-water system is the only mitigation measure in the accident. It is worthwhile to increase the raw-water flow rate to cool down the debris in the dry well after RPV breach.