Transient interface temperature on a vertical surface in multi-component solid-liquid systems with volume heating. Application to various severe accident situations
Résumé
The question of the transient interface conditions in various severe accident situations, involving multicomponent and multi-phase material mixtures has until now remained largely unresolved. The interface temperature controls melt temperature and transient heat-flux distribution. The present paper proposes a new approach to the transient interface temperature, drawing on the numerous past efforts. A single relation is proposed which applies to various severe accident situations: (i) thermal-hydraulic steady state for in-vessel retention (IVR); (ii) transient solidification; (iii) transient concrete ablation (MCCI); and (iv) refractory material (ceramic) ablation (core-catcher retention, liner design). This approach yields the following important conclusions: - When a thermal-hydraulic steady state can be reached (external cooling, in case of IVR and melt stabilization in an externally cooled core-catcher), the melt-solid interface temperature tends towards the liquidus temperature corresponding to the melt composition. - During the ablation transient: the interface temperature is lower than the melt liquidus temperature if the wall-melting temperature is less than the melt liquidus temperature (MCCI), the interface temperature is higher than the melt liquidus temperature if the melting temperature of the wall exceeds the melt liquidus (oxidic corium-refractory ceramic interaction). The new interface model also suggests that simultaneous ablation of two similar walls (for instance: two concrete walls) may be subject to instabilities which can potentially result in complete arrest of the ablation of one of the walls. In the case of simultaneous ablation of two walls of different nature, the wall with the lowest melting temperature is preferentially ablated during the transient phase.