Algorithms of three-dimensional concrete ablation front tracking (CAFT) and crust growth

•A 3D PLIC based algorithm was proposed to solve concrete ablation in complex cavity geometries.•The 3D ablation algorithm was combined with the 1D heat transfer model of the crust growth.•The energy mismatch caused by the combination of the 3D-1D approaches was shown to be small enough.•The applicability of the proposed method was discussed for a complex cavity geometry. A 3D MCCI code, CORCAAB, has been developed to account for uncertainty factors in the long-term molten corium–concrete interaction including changes in the three-dimensional cavity contour, which affect the convective heat transfer of the molten debris and late ablation behaviors. In this work, a 3D concrete ablation front tracking algorithm was formulated using the piecewise linear interface calculation method. The ablation and crust growth rates are governed by the local heat transfer across the 1D boundary layer that consists of molten debris, crust, slag, and concrete. This combination of the 3D and 1D methods led to the energy mismatch that was shown to be small enough even under a significantly large change of the cavity contour shape; thus, the proposed algorithm is compatible with the energy conservation law of the entire MCCI code. In order to test the efficacy and restriction of the proposed ablation algorithm, a simple rectangular cavity connected with a double sump on the cavity floor was adopted. Ablation surface formations including saddle points with multiple curvatures were confirmed to be adequate. A moderate dependence on the cell size was observed in predicting the disappearance of the thin walls separating the two partitions.