Calculation of ablation instabilities during MCCI with the TIM model

•The Transient Interface Model is used to investigate MCCI ablation instabilities.•The ablation instability leads to concrete ablation in preferential directions.•The range of instability conditions is represented on a pseudo binary phase diagram.•An ablation instability may occur in the initial sta...

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Bibliographic Details
Published in:Nuclear engineering and design 2023-06, Vol.407, p.112269, Article 112269
Main Authors: Seiler, J.M., Seiler, N.
Format: Article
Language:English
Subjects:
Ablation instability
CCI2
CCI3 tests
Chemical Physics
MCCI
Molten Core-Concrete Interaction
Physics
TIM model
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Summary:•The Transient Interface Model is used to investigate MCCI ablation instabilities.•The ablation instability leads to concrete ablation in preferential directions.•The range of instability conditions is represented on a pseudo binary phase diagram.•An ablation instability may occur in the initial stage of the MCCI transient. 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 (∼1 m in a reactor pit of 6 m inner diameter).
ISSN:0029-5493
1872-759X
DOI:10.1016/j.nucengdes.2023.112269