Several designs of light water reactors consider melt fragmentation and cooling of corium debris bed in a deep pool as important part of their severe accident management strategies. Traditional approach to assessment of debris coolability is based on the bed dryout criterion. However, this is the most conservative criterion which doesn’t take into account possibility of debris temperature stabilization in steam cooling regime. In this work numerical simulations of cooling of a decay heat-releasing porous debris bed in a water pool are carried out for the conditions where local dryout of porous material occurs. It is shown that the temperature of solid material in the dry zone can be stabilized if sufficient vapor flow is generated in the wetted part of the debris bed beneath the dry zone. A simple one-dimensional model which connects the maximum temperature and the relative size of the dry zone is proposed and verified against the numerical simulations with DECOSIM code for different shapes of the debris beds relevant to severe accident conditions in a Nordic type boiling water reactor (BWR). On the basis of this model, a criterion is obtained which defines the critical relative height of the dry zone corresponding to specific temperature of debris material which can be considered as a safety limit (e.g. start of zirconium oxidation, remelting of metallic debris or oxidic corium, etc.). The criterion allows one to evaluate the safety margins and degree of conservatism introduced by the dryout-based approach to assessment of debris coolability.

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