Abstract During a core disruptive accident in a sodium-cooled fast reactor, the molten fuel or steel is solidified into debris particles that form a debris bed in the lower plenum. When the debris bed starts boiling, the flow of the coolant and vapor together relocates and flattens the debris particles. This process is called debris relocation. The thickness of the debris bed and its porosity have significant influence on the cooling ability of the fuel debris in the lower plenum. Therefore, it is necessary to evaluate the transient changes in the shape and thickness of the debris bed in the relocation behavior for accident analysis. To simulate the relocation behavior, a debris-relocation model based on the COMMEN code was developed in this study. The debris-relocation model is based on the shear strength mechanism, which is widely applied in soil mechanics. Shear strength is a function of the density, position, and shape characteristics of particles. The debris bed is relocated only when the shear stress of the global debris particles is greater than the shear strength. By incorporating the relocation model into the COMMEN code, the relocation transient processes of different particle beds in a nitrogen-injection experiment were simulated, and the results were compared with experimental results. The analysis indicates that the proposed debris-relocation model can effectively predict the occurrence of debris-bed relocation and reasonably simulate its transient process, although future improvement is necessary.

中文翻译：

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钠冷快堆堆芯破坏事故分析中碎片颗粒重定位模型的建立

摘要 在钠冷快堆发生堆芯破坏事故时，熔融燃料或钢凝固成碎片颗粒，在下腔室形成碎片床。当碎屑床开始沸腾时，冷却剂和蒸汽的流动一起重新定位并压平碎屑颗粒。这个过程称为碎片重定位。碎片床层的厚度及其孔隙度对下腔室中燃料碎片的冷却能力有显着影响。因此，有必要对搬迁行为中碎片层形状和厚度的瞬态变化进行评估，以进行事故分析。为了模拟重定位行为，本研究开发了基于 COMMEN 代码的碎片重定位模型。碎片重定位模型基于抗剪强度机制，广泛应用于土力学。剪切强度是颗粒密度、位置和形状特征的函数。只有当整体碎片颗粒的剪切应力大于剪切强度时，才会重新定位碎片床。通过将重定位模型纳入COMMEN程序，模拟了注氮实验中不同颗粒床的重定位瞬态过程，并与实验结果进行了比较。分析表明，所提出的碎片迁移模型可以有效地预测碎片床迁移的发生并合理模拟其瞬态过程，但仍需进一步改进。只有当整体碎片颗粒的剪切应力大于剪切强度时，才会重新定位碎片床。通过将重定位模型纳入COMMEN程序，模拟了注氮实验中不同颗粒床的重定位瞬态过程，并与实验结果进行了比较。分析表明，所提出的碎片迁移模型可以有效地预测碎片床迁移的发生并合理模拟其瞬态过程，但仍需进一步改进。只有当整体碎片颗粒的剪切应力大于剪切强度时，才会重新定位碎片床。通过将重定位模型纳入COMMEN程序，模拟了注氮实验中不同颗粒床的重定位瞬态过程，并与实验结果进行了比较。分析表明，所提出的碎片迁移模型可以有效预测碎片床迁移的发生并合理模拟其瞬态过程，但未来仍需改进。并将结果与​​实验结果进行比较。分析表明，所提出的碎片迁移模型可以有效预测碎片床迁移的发生并合理模拟其瞬态过程，但未来仍需改进。并将结果与​​实验结果进行比较。分析表明，所提出的碎片迁移模型可以有效预测碎片床迁移的发生并合理模拟其瞬态过程，但未来仍需改进。