Calculation of ablation instabilities during MCCI with the TIM model

The Transient Interface Model (TIM) (Seiler and Combeau, 2014; Seiler and Jamet, 2019) dedicated to Molten Core-Concrete Interaction simulation is used to investigate ablation instabilities. The ablation instability leads to concrete ablation in preferential directions, potentially jeopardizing the mitigation criterion based on residual concrete thickness. The ablation instability occurs when all the corium/concrete interfaces do not undergo the same ablation regime. These regimes are specific of the interface temperature; in the homogeneous Low Interface Temperature (LIT) regime, the interface temperature is close to the liquidus temperature of concrete matrix, whereas in the homogeneous High Interface Temperature (HIT) regime, the interface as well as the pool temperatures are close to the pool liquidus temperature. When these different ablation regimes are present simultaneously (non-homogeneous situation), only the surfaces under LIT regime are ablated. It is shown that ablation instabilities are favored at low power dissipation and at small test scale. Calculations suggest that the limit between HIT and LIT dominant regimes can be represented in the pseudo-binary phase diagram. The surfaces initially under HIT regime (not ablated) may return to the LIT regime (with ablation) after a time delay which depends on the extension of the surface initially under HIT regime, on the value of the mass transfer coefficient and, to some extent, on the melt to interface heat transfer. Finally, the calculation at reactor scale with siliceous concrete indicates that an ablation instability may occur in the initial stage of the MCCI transient. Such ablation instability does not induce significant preferential ablation in the reactor configuration if it is limited to the horizontal surface of the reactor pit. If it affects the lateral walls, and due to the small thickness of the corium layer, a significant ablation depth could be reached (similar to 1 m in a reactor pit of 6 m inner diameter).

Automated summary

The Transient Interface Model (TIM) is used to study ablation instabilities, which can lead to preferential directions of concrete ablation and pose a risk to mitigation criteria. These instabilities occur when the ablation regimes of corium/concrete interfaces are not consistent. The different ablation regimes are determined by the interface temperature. The study suggests that ablation instabilities are more likely at low power dissipation and small scales.