Abstract
Probabilistic Safety Assessment (PSA) is a powerful tool to assess the safety of nuclear power plants (NPP). The purpose of the study is to evaulate the impact of Ultimate Response Guidelines (URGs) on the Level-1 PSA of Kuosheng NPP of Taiwan Power Company. After the core damage accidient of Fukushima Daiichi Nuclear Power Plant, Ultimate Response Guidelines (URGs) were developed by Taiwan Power Company to help operators mitigating the consequences of events similar to the Fukushima accident. The purpose of URG is to prevent the peak cladding temperature during the accident from rising above 1500℉, which is the imitation temperature of the release of the gap activity. URG is activated in a seismic event, an event of Station Blackout or an event of losing water injection to core. According to URG, the emergency response staff of the plant is required to implement movable or portable A.C. power and alternate water source within a specific time after the motivation of the accident. If the continuous removal of decay heat can not be assured, operators are directed to depressurize the vessel in order to bring the non-conventional low pressure water keeping the core covered. The decay heat is removed via containment venting. The actions may damage the economic values of the plant due to the injection of dirty or sea water into the core. It can be expected that URG can lower the Core Damage Frequency (CDF) of events similar to Fukushima accident. In the present study, the impact of URG on the CDF of Kuosheng NPP of Taiwan Power Company is assessed. The plant employs Boiling Water Reactor (BWR6) designed by General Electric. The plant specific PSA model of the plant is adopted on the study. The thermal-hydraulic responses of the system during the assicdent are simulated using RELAP5-3D code. WinNUPRA3.0 code is used in performing PSA analysis. The results show that in a Station Blackout event, the URG can reduce CDF by 53.4% when the RCIC (Reactor Core Isolation Cooling system) operates 24 hours. For a sequence similar to Fukushima accident, the CDF can be reduced by two orders of magnitude. Nevertheless, the URG has no impact on the sequence with failure of RCIC injection. In such an event, the time to core uncover is too short to implement any extra mitigation actions beyond the design basis.