Abstract Transient analysis for possible prompt supercritical accidents of fuel debris in the Fukushima Daiichi Nuclear Power Station is quite important. However, unlike solution fuel systems, there is little knowledge about supercritical transient analysis in fuel debris systems. In particular, reactivity feedback effects, which may have a significant impact on the results of the analysis, are important and require further study. In particular, the impacts of radiolysis gas void and moderator boiling should be discussed. Thus, the purpose of this study is to clarify whether the reactivity feedback effects of radiolysis gas and boiling of the moderator impact the supercritical transient analysis in fuel debris systems. To accomplish this, we used a power profile obtained by the MIK code with the Doppler reactivity feedback effect; radiolysis gas analysis and heat transfer analysis were performed. For the radiolysis gas analysis, the AGNES2 model was modified to consider the difference between solution fuel and fuel debris systems. The heat transfer analysis used an OpenFOAM solver to perform conjugate heat transfer calculations. We found that the radiolysis gas void was negligible when probable G values, which are the generation number of molecules per absorbed energy, were used. In addition, the results showed that boiling could be also negligible under most conditions. However, we found that the boiling time may be earlier than the peak time of the power when the radius of the fuel debris particle is small. In this case, ignoring the boiling may give conservative results. These considerations should be included in future analyses.

中文翻译：

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反应性反馈对燃料碎片超临界瞬态分析的影响

摘要 福岛第一核电站燃料碎片可能发生的超临界超临界事故瞬态分析具有重要意义。然而，与解决方案燃料系统不同，对燃料碎片系统中的超临界瞬态分析知之甚少。特别是，可能对分析结果产生重大影响的反应性反馈效应很重要，需要进一步研究。尤其应该讨论辐解气体空隙和慢化剂沸腾的影响。因此，本研究的目的是阐明辐射分解气体和慢化剂沸腾的反应性反馈效应是否影响燃料碎片系统中的超临界瞬态分析。为了实现这一点，我们使用了由 MIK 代码获得的具有多普勒反应性反馈效应的功率分布图；进行了辐射分解气体分析和传热分析。对于辐解气体分析，修改了 AGNES2 模型以考虑溶液燃料和燃料碎片系统之间的差异。传热分析使用 OpenFOAM 求解器来执行共轭传热计算。我们发现，当使用可能的 G 值（每个吸收能量的分子产生数）时，辐射分解气体空隙可以忽略不计。此外，结果表明在大多数条件下沸腾也可以忽略不计。然而，我们发现，当燃料碎片颗粒的半径较小时，沸腾时间可能早于功率的峰值时间。在这种情况下，忽略沸腾可能会给出保守的结果。这些考虑因素应包括在未来的分析中。