本次研究利用MAAP 5.0.0 Code 模擬日本福島核能一廠1二號機及三號機311事故。由於日本福島電廠二號機及三號機與台灣電力公司金山廠核能一廠同為BWR4型沸水式反應器搭配Mark I 型圍阻體之設計，本研究依據台電公司金山核能一廠的設計參數，建構反應器廠房MAAP5 輸入模式，依此分析日本311事故中，二號機與三號機的氫氣行為。模擬時，依據參考文獻之事故重要事件時序，例如注水時間、洩壓時間等，設定MAAP5程式的運算條件，調整未知參數，如注水流量，洩壓有效口徑，使MAAP5程式模擬結果貼近前述報告中的實際事故演變數據。最後分析氫氣爆炸條件與計算結果的比較二號機模擬結果顯示氫氣產生量遠不足以發生氫氣爆炸。當假設RCIC取水口由CST改為圍阻體抑壓池時，會因取水溫度過高而自動跳脫；模擬結果顯示，爐心會因長期缺水而融毀，但是氫氣爆炸發生時間遠早於官方公布之事故序列的時間。本研究將RCIC跳脫時間示為輸入參數，依MAAP5的模擬結果找出RCIC合理的跳脫時間。模擬結果顯示，RCIC跳脫時間在54.4小時，早期喪失注水，造成爐心熔毀、壓力槽失效、爐心融渣與混凝土發生反應、產生大量氫氣與一氧化碳，在此情況下發生爆炸時間會出現在86.37小時。僅比實際電廠發生氫爆時間早1小時，推測RCIC實際跳脫時間應該是在54.4小時前後一段時間內。根據表列之事故序列，三號機反應器廠房因氫氣爆炸而解體。模擬結果顯示，MAAP5程式計算之氫氣產生量、與所預測之氫氣再圍阻體與反應器廠房個區間的分佈，不足以讓反應器廠房發生氫氣爆炸事故。根據MAAP5的模擬結果，推測三號機廠房崩毀是因為融熔爐心掉落壓力槽下區間時，產生大量水蒸汽造成高壓所導致。在微調海水注水流量之後，水蒸汽爆炸發生時間為67.2小時，十分接近三號機反應器廠房損毀時間。

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.

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