在核電廠發生事故的初期階段，為了確保核能安全，須考慮超越核電廠設計基準事故的發生。雖然發生超越設計基準事故的機率非常低，但這樣的事故如果發生可能會導致嚴重的後果。

第四座核能電廠─台灣電力公司所屬龍門核電廠為台灣第一座進步型沸水式反應器，被選為本研究中模擬的對象，其全域模擬器用來產生測試資料，並幫助研究方法之建立。下列為本研究考慮之常見的初始事件：喪失冷卻水事故(Loss of coolant accidents)、喪失全飼水事故(Loss of all feedwater)、喪失場外電源事故(loss of offsite power)、電廠全黑事故(Station blackout)。

本研究將兩種取向的方法做結合，首先電廠損害狀態藉由徵兆取向的參數，決定了初始事件，當核電廠發生事故時，顯示燈變化的各種組合，可反應為不同的初始事件，藉由模擬器模擬各種事故，即可預先得知每個初始事件將會顯示何種燈號的組合及時間發生的序列。同時模擬器也提供了安全系統的狀態，可提供圍阻體中各組件可用與否的狀態來協助事件樹中上方事件的判斷。辨識電廠損害狀態後，將選定的初始事件與相對應之事件樹連結，而使用正確的事件樹，便可藉由先前機率風險評估的結果得到爐心熔毀頻率。利用徵兆取向與事件取向之結合的方法，對核電廠事故預警及應變方法便能有效地幫助。

During early stage of a nuclear power plant accident, consideration of beyond design basis accidents is an essential component of the defense in depth approach to assure nuclear safety. The probability of occurrence of a beyond design basis accident is very low, but such an accident may lead to significant consequences.

Taiwan's fourth nuclear power plants, the Lungmen nuclear power station (LNPS), which is an advanced boiling water reactor (ABWR) with fully digitized instrumentation and control (I&C) system is chosen as the target plant. The LNPS full scope engineering simulator is used to generate the testing data for method development. The following common initiating events are considered in this study: Loss of coolant accidents (LOCA), Loss of all feedwater (TLOFW), Loss of offsite power (LOOP), Station blackout (SBO).

This study combines symptom-oriented and event-oriented approach for early warning and decision making. First, the Plant Damage States, which is symptom-oriented, identify the initiating event. It also gives the information of safety system‘s status. Initiating event and safety system‘s status are used in Event Tree (ET), which is event–oriented, to determine accident sequences by using probabilistic risk assessment (PRA) technique. With accident sequences, the core melt frequency is easy to be calculated. Studies have indicated that the combination of the symptom-oriented approach and the event-oriented approach can be helpful for nuclear power plant accident early warning and management.

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[2] H. S. Chang, K. S. Kang & S. H. Chang, “Development of Severe Accident Management Supporting Systems Using Quantified Containment Event Trees”, Reliability Engineering & System Safety, Volume 48, Issue 3, pp. 205–216, 1995.

[3] C. M. Lee, W. C. Tsai, K. W. Wong & T. Pang, “Simulator for The New Generation of Nuclear Power Plants – the Lungmen ABWR Simulator”, American Nuclear Society, LaGrange Park, IL, Knoxville, Tennessee, April 5-9, 2009.

[4] International Atomic Energy Agency, “Procedures for Conducting Probabilistic Safety Assessments of Nuclear Power Plants (Level 2)”, SAFETY SERIES No. 50-P-8, 1995.

[5] Icove and Ruggles, “Basics of Fault Tree and Event Tree Analysis”, USNRC: EDU10-002, 2011.

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