6 - Ex-vessel molten corium coolability

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During severe accident, when the molten corium is ejected out of the vessel, it falls into the reactor cavity. The melt will fragment and forms a porous debris bed. This debris bed, if not quenched, will again remelt due to the decay heat and forms a new melt pool. If there is no water in the reactor cavity, this situation will directly occur when melt falls in the cavity. The melt starts attacking the concrete basemat, which is termed as the molten coolant- concrete interaction (MCCI). The heat transfer from the hot melt to the concrete causes ablation of the concrete. Further, the melt starts reacting with the water vapor and may release hydrogen. This has many fold impacts as the chemical reactions add a lot of heat into the system, generation of noncondensable gases because of concrete decomposition into CO, CO2, H2 and water vapor; and pose a threat to containment integrity due to pressurization of gases and combustion or explosion. Besides, if basemat melt-through occurs, there is a potential danger of ground and water contamination by radiological waste. Since, the final barrier for radioactivity release is the containment, in any case, the containment barrier should not be breached. The ultimate aim of any severe accident management strategy is the management and termination of the accident as quickly as possible. A severe accident is said to be stabilized and terminated only when the core melt has been cooled and quenched and kept in the latter state for a long time and there is no release of radioactivity in the environment. However, the phenomenon of quenching of melt pool is much more complex involving multiphase multi-component heat, mass and momentum transfer associated with wide variation of thermo-physical properties of the melt. Hence, in order to devise a strategy for arresting further progression of severe accident, understanding of coolability in such scenarios is utmost important. In this chapter, corium coolabilities under different flooding conditions have been explained. Experimental insights from a series of experiments, modelling of corium coolability has also been presented.