Early core degradation determines the amount of hydrogen generated by cladding oxidation as well as the temperature, the mass, and the composition of corium that further relocates into the lower head of reactor pressure vessel (RPV), which is essential for the effectiveness analysis of in-vessel retention (IVR) and hydrogen recombiners. In this paper, the mechanisms of controlling phenomena in the early phase of core degradation are analysed at first. Then, numerical models adopted to calculate (1) core heating up, (2) cladding oxidation, (3) dissolution between molten zirconium and fuel pellets, and (4) formation of a molten pool in the core active section are presented. Compared with integral codes for severe accident analysis (such as MAAP and MELCOR), the models in this paper are established at the fuel pin level and the calculation is performed in 3D, which can capture the detail local phenomena during the core degradation and eliminate the average effect due to equivalent rings used in integral codes. In addition, most of the control equations in this paper are calculated by implicit schemes, which can improve the accuracy and stability of the calculation. In the simulation, the calculation oxidation is calculated by using the oxygen diffusion model, while the dissolution is calculated with Kim, Hayward, Hofmann, and IBRAE models to perform uncertainty analysis. For the validation, the cladding oxidation model is verified by Olander theoretical cases in the conditions of both steam-rich and steam-starved. The dissolution models are validated by the RIAR experiment. The code is overall verified by Phebus FPT0 on the integral phase of core early degradation. According to the simulation results, it can be inferred that the dissolution reaction between the molten zirconium and fuel pellets is the main reason for the melting of UO₂ at low temperature. In the case of starved steam, part of the fuel pellets can melt down even at 2248 K and relocate to the bottom of the core, which is much lower than the melting point of UO₂ (3113 K).

中文翻译：

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严重事故下岩心早期降解阶段的数值模拟与验证

堆芯的早期降解决定了包壳氧化产生的氢气量，以及温度，质量和进一步迁移到反应堆压力容器（RPV）下部头部的皮质成分，这对于分析反应堆的有效性至关重要。 -血管保留（IVR）和氢重组子。本文首先分析了岩心退化早期的现象控制机制。然后，给出了用于计算（1）堆芯加热，（2）包层氧化，（3）熔融锆和燃料粒料之间的溶解以及（4）在堆芯活动区中形成熔池的数值模型。与用于严重事故分析的完整代码（例如MAAP和MELCOR）相比，本文中的模型是在加油销处建立的，并且以3D进行计算，可以捕获堆芯退化期间的局部细节现象，并消除了积分代码中使用的等效环带来的平均影响。另外，本文的大多数控制方程均采用隐式方案进行计算，可以提高计算的准确性和稳定性。在模拟中，使用氧扩散模型计算氧化，而使用Kim，Hayward，Hofmann和IBRAE模型计算溶解度，以进行不确定性分析。为了验证，通过Olander理论实例在富蒸汽和缺乏蒸汽的条件下验证了包壳氧化模型。溶出模型通过RIAR实验验证。Phebus FPT0在核心早期降级的积分阶段对代码进行了全面验证。根据模拟结果，可以推断出熔融锆与燃料颗粒之间的溶解反应是UO熔融的主要原因。₂在低温下。在缺少蒸汽的情况下，即使在2248 K时，部分燃料芯块也可能融化并重新定位到堆芯底部，这比UO ₂的熔点（3113 K）低得多。