Abstract The existing gap cooling studies have shown the coolability of debris retained in the lower head of a vessel and effective mitigation of the rupture of a reactor vessel, but no plausible mechanism of the gap formation has been clearly identified. Several experiments of the gap formation, pretests of FARO-19, LAVA, ALPHA, and EC-FOREVER- 5 and 6, were reviewed to find out the mechanism of the gap formation. We confirmed that a pre-flooded condition, in which a simulant of a reactor vessel is filled with water before pouring a melt to the vessel, makes it possible to form the gap. We performed simple tests to pour a melt onto a plate under both pre-flooded and dry conditions discovering the gap formation under the pre-flooded condition. We proposed a model for the initial gap thickness considering the Inverse Leidenfrost effect. The initial gap thickness is estimated solving the balance equations for mass, momentum, and energy. For the estimation of the transient gap thickness, we considered the thermal fracture as well as the thermal contraction of the crust. For the estimation of the thermal fracture of a melt, we adopted Yeo and No’s study (Yeo and No, 2019) considering the crust cracks of a melt being cooled down under the flooded condition. The proposed model was validated against experimental data of the KAIST test, ALPHA tests performed at JAERI, and LAVA-6, LAVA-10, LMP200-1, and LMP200-2 tests performed at KAERI. The current model predicts the gap thicknesses much better than the existing approach with the linear thermal contraction model and the assumed initial gap size but no consideration of the crust crack formation. Also, it well agreed against the experimental data.

中文翻译：

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反应堆容器下封头内由真皮与水相互作用形成的环形间隙厚度建模

摘要 现有的间隙冷却研究表明，保留在容器下封头的碎片具有冷却能力，可以有效缓解反应堆容器的破裂，但尚未明确确定间隙形成的合理机制。回顾了间隙形成的几个实验，即 FARO-19、LAVA、ALPHA 和 EC-FOREVER-5 和 6 的预测试，以找出间隙形成的机制。我们证实，在将熔体倒入容器之前，反应堆容器的模拟物充满水的预淹没条件使得形成间隙成为可能。我们进行了简单的测试，在预充液和干燥条件下将熔体倒在板上，发现预充液条件下的间隙形成。我们提出了一个考虑逆莱顿弗罗斯特效应的初始间隙厚度模型。初始间隙厚度是通过求解质量、动量和能量的平衡方程来估计的。对于瞬态间隙厚度的估计，我们考虑了地壳的热断裂和热收缩。为了估计熔体的热断裂，我们采用了 Yeo 和 No 的研究（Yeo 和 No，2019），考虑了在淹没条件下冷却的熔体的地壳裂缝。所提出的模型针对 KAIST 测试、在 JAERI 进行的 ALPHA 测试以及在 KAERI 进行的 LAVA-6、LAVA-10、LMP200-1 和 LMP200-2 测试的实验数据进行了验证。当前模型对间隙厚度的预测比使用线性热收缩模型和假定初始间隙尺寸但未考虑地壳裂纹形成的现有方法好得多。还，