Abstract The reactor water level measurement system in Fukushima was based on measuring the pressure difference between two water-filled pipes that were connected to the reactor at different elevations. A Japanese accident investigation committee has presented a hypothesis that a leak of superheated steam from the unit 2 reactor to the drywell caused evaporation of the water in the measurement system, causing the indicated water level to be much higher than the actual level. This article presents an analysis of the measurement system with the MELCOR code. The results support the hypothesis of a small leak, starting 78.5 h after the earthquake. The best agreement with the measurements was obtained with 8 cm2 leak area. The leak was probably located close to the water level measurement system, so that it caused localized heating in the drywell. The leak increased the fission product release to the environment because it bypassed the suppression pool. The calculated cesium release from unit 2 to the environment was 2.1% of the core inventory. Combined with the author’s earlier MELCOR calculations of units 1 and 3, the current estimate of the total Cs-137 release from all three accident units at Fukushima is 14.3 PBq.

中文翻译：

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福岛2号机组事故反应堆水位测量分析

摘要 福岛反应堆水位测量系统基于测量连接反应堆不同高度的两个充水管道之间的压力差。日本事故调查委员会提出了一种假设，即过热蒸汽从 2 号机组反应堆泄漏到干井导致测量系统中的水蒸发，导致指示水位远高于实际水位。本文介绍了对带有 MELCOR 代码的测量系统的分析。结果支持了小泄漏的假设，发生在地震后 78.5 小时。8 cm2 泄漏面积获得了与测量结果的最佳一致性。泄漏点可能位于靠近水位测量系统的位置，从而导致干井局部加热。泄漏增加了裂变产物释放到环境中，因为它绕过了抑制池。计算出的从单元 2 释放到环境中的铯是核心库存的 2.1%。结合作者早期 MELCOR 对 1 和 3 机组的计算，目前对福岛所有三个事故机组释放的 Cs-137 总量的估计为 14.3 PBq。