Abstract As a participant in the Benchmark Study of the Accident at Fukushima Daiichi phase 2 project, which aims to improve our understanding of the Fukushima Daiichi (1F) nuclear power plant conditions, the Institute of Applied Energy has performed a comprehensive evaluation and analyses of the 1F units using the SAMPSON code. The objective of this study is to clarify the accident progression and the distribution of debris/fission products in the plant as well as evaluate points of uncertainty in severe accident codes. The paper presents the results obtained for 1F Unit 3. Firstly, a likely accident scenario is evaluated taking into account all of the available measurements and recent information obtained from plant investigations. Subsequently, the SAMPSON code is applied to confirm the validity of the estimated phenomena and explain points of uncertainty in the the accident. The calculation is performed over the course of three weeks following the scram, which is representative of the time it takes to achieve stable plant conditions according to theories of thermal-hydraulics and fission product settling. The analysis confirms that a large portion of the core melted and relocated in the lower plenum of Unit 3. Thereafter, due to lack of cooling, the reactor pressure vessel failed, which caused the debris to be gradually transferred to the pedestal, where it interacted with concrete, generating flammable gases. It is estimated that a small amount of debris may have remained in the lower plenum in the form of a solidified crust, while a considerable portion relocated to the pedestal with limited spread to the dry well due to prolonged top-flooded conditions.

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SAMPSON 代码对福岛第一核电站 3 号机组的三周分析：对 BSAF-2 项目的贡献

摘要 作为福岛第一核电站二期项目基准研究的参与者，该项目旨在提高我们对福岛第一核电站（1F）核电站状况的了解，应用能源研究所对福岛第一核电站（1F）核电站状况进行了综合评估和分析。 1F 单元使用 SAMPSON 代码。这项研究的目的是澄清事故进展和核电厂碎片/裂变产物的分布，并评估严重事故规范中的不确定点。本文介绍了 1F 3 号机组获得的结果。首先，考虑到所有可用的测量结果和从工厂调查中获得的最新信息，对可能的事故情景进行了评估。随后，应用 SAMPSON 代码来确认估计现象的有效性并解释事故中的不确定点。计算是在紧急停堆之后的三周内进行的，这代表了根据热工水力和裂变产物沉降理论达到稳定工厂条件所需的时间。分析证实，大部分堆芯熔化并重新安置在 3 号机组的下腔室。此后，由于缺乏冷却，反应堆压力容器发生故障，导致碎片逐渐转移到基座，在那里相互作用与混凝土，产生可燃气体。据估计，少量碎片可能以凝固的地壳形式留在下部压力通风系统中，