ABSTRACT To suggest efficient process of the fuel debris treatment after the retrieval from the Fukushima Daiichi Nuclear Power Plant (1F), thorough investigation is indispensable on potential source of U in the fuel debris. The present study aims to estimate chemical forms of U in the in-vessel fuel debris, especially in the minor phases such as metallic phases, by performing the thermodynamic calculation under various conditions considering material relocation and changing environment during the accident progression in the 1F Unit 2. Input conditions for the thermodynamic calculation such as composition, temperature, and oxygen amount were set mainly based on the transient change in the core material distribution which had been calculated with severe accident analysis codes such as MAAP and MELCOR. The result showed that chemical form of U varied depending on the local amount of Fe and O. In regions of low steel content, the U-containing metallic phase was dominated by α-(Zr,U)(O), while regions of high steel content were dominated by Fe2(Zr,U) (Laves phase). A few percent of U transferred to the metallic phases were highly expected under reducing conditions. Therefore, those metallic phases should be one of the potential sources of U.

中文翻译：

---

通过与受损反应堆压力容器中堆芯材料分布相关的热力学计算评估铀的化学形态

摘要 为了提出福岛第一核电站（1F）回收后燃料碎片处理的有效过程，对燃料碎片中U的潜在来源进行彻底调查是必不可少的。本研究旨在通过在 1F 机组中考虑材料重新定位和环境变化的各种条件下进行热力学计算，估计船内燃料碎片中 U 的化学形态，尤其是金属相等微量相。 2. 热力学计算的输入条件，如成分、温度和氧气量，主要根据堆芯材料分布的瞬态变化进行设置，这些变化是用MAAP和MELCOR等严重事故分析程序计算出来的。结果表明，U的化学形态随局部Fe和O含量的不同而变化。在钢含量低的区域，含U的金属相以α-(Zr,U)(O)为主，而在钢含量高的区域，含U金属相以α-(Zr,U)(O)为主。钢含量以 Fe2(Zr,U)（Laves 相）为主。在还原条件下，预计会有百分之几的 U 转移到金属相中。因此，这些金属相应该是 U 的潜在来源之一。