Abstract After the two major nuclear accidents in history considered to be beyond-design-basis accidents (BDBA) – the Chernobyl Reactor Four explosion in Ukraine (1986) and the Fukushima Daiichi accident in Japan (2011), the United States Nuclear Regulatory Commission and other countries have included in the Final Safety Analysis Report (FSAR), a new chapter (19) dedicated to the Probabilistic Safety Assessment (PSA) and Severe Accident Analysis (SAA), with significant damage to the reactor core. The Safety Analysis Report (SAR) is the most important document used by a regulatory body to assess the adequacy of plant safety at all stages of the life time of a Nuclear Power Plant (NPP), and presents the licensing basis of a plant. This study aims at comparing the progression of severe accidents during a Total Loss of Feed Water Accident (TLOFW) and a TLOFW combined with An Anticipated Transient Without SCRAM (ATWS) reactivity accident for a Pressurized Water Reactor (PWR) type NPP. This paper takes into account the results of the PSA Level 1 for the reference NPP. The calculations were performed with the deterministic computer code MELCOR version 2.2 used to analyze severe accident progression in NPP with a Light Water Reactor (LWR). The paper presents a hydrogen risk assessment – the assessment is deterministic, s a prerequisite before performing PSA Level 2. The hydrogen source is determined, too. Saphiro's diagram is applied to investigate the containment atmosphere condition (inert or combustible) for two scenarios. The main results of the study reveal that the containment integrity of the reference PWR NPP when both accidents occurred is maintained. However, the core is severely damaged. A future work will investigate mitigation strategies to limit the hydrogen risk during accident.

中文翻译：

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使用 MELCOR 对 PWR 进行彻底的严重事故分析

摘要 在历史上被认为是超设计基准事故（BDBA）的两次重大核事故——乌克兰切尔诺贝利四号反应堆爆炸（1986 年）和日本福岛第一核电站事故（2011 年）之后，美国核管理委员会和其他国家在最终安全分析报告 (FSAR) 中包括了一个新的章节 (19)，专门用于概率安全评估 (PSA) 和严重事故分析 (SAA)，对反应堆堆芯造成严重损坏。安全分析报告 (SAR) 是监管机构用来评估核电厂 (NPP) 寿期所有阶段的电厂安全充分性的最重要文件，并提供了电厂的许可基础。本研究旨在比较压水反应堆 (PWR) 型核电厂在给水完全损失事故 (TLOFW) 和 TLOFW 与预期瞬态无 SCRAM (ATWS) 反应性事故相结合期间严重事故的进展情况。本文考虑了参考 NPP 的 PSA Level 1 结果。计算是使用确定性计算机代码 MELCOR 2.2 版进行的，该代码用于分析具有轻水反应堆 (LWR) 的核电厂的严重事故进展。该论文提出了氢风险评估——评估是确定性的，这是执行 PSA 级别 2 之前的先决条件。氢源也已确定。Saphiro 的图表用于研究两种情况下的安全壳气氛条件（惰性或可燃性）。研究的主要结果表明，当两起事故发生时，参考压水堆核电厂的安全壳完整性得以保持。然而，核心被严重损坏。未来的工作将研究缓解策略以限制事故期间的氢气风险。