Abstract
On March 11th of 2011, a massive earthquake hit the north-east coast of Japan. The earthquake induced a tsunami caused extensive damages on Fukushima Daiichi nuclear power Plant (NPP). The plant is operated by the Tokyo Electric Power Company (TEPCO) and comprises six Boiling Water Reactors (BWRs). Upon the attack of the earthquake, Unit 1~3 are in the full power operation mode and Unit 4~ 6 are shutdown for maintenance. The attack of earthquake and tsunami caused a complete loss of AC power of the plant. In the present study, the accident scenarios of Units 2 and 3 are simulated using MAAP5 code. The surrogate plant used in the analysis is Chinshan Nuclear Power Station of Taiwan Power Company, which also employs BWR IV reactor and Mark I containment. The reactor building of the plant is modeled in detail in order to catch the pressure load due to containment venting and hydrogen burns. For unit-2 reactor, the results show that the running time of Reactor Core Isolation Cooling injection has a very significant impact on the hydrogen burns. If the accident scenarios were progressed as described in the official reports of Japanese Government and INPO, the hydrogen burns in the torus room would not occur. The steam generated from the continued operation is high enough to prohibit the hydrogen burn. It can be demonstrated using MAAP code that hydrogen burns are possible when RCIC tripped at about 54.4 hours after accident was initiated. For unit-3 reactor, the results show that hydrogen burns is hardly to occur due to the high density of steam produced with hydrogen. However, it is suspected that the reactor was damaged due to the pressure spike upon the relocation of molten core from core region to vessel lower plenum in the present simulation. It can be demonstrated using MAAP code that a pressure spike up to 253 kPa due to steam produced by relocation of molten core.