During the severe accident which can lead to core melt, particle bed may be formed in vessel or on the cavity floor by quenching of corium. If the heat removal from the particle bed is insufficient, reactor vessel or reactor cavity floor may be attacked by re-melted corium.
Therefore, the prediction of DHF with high accuracy is important for the design and safety analysis for severe accident.
The DHF is affected by many parameters such as, geometry of particle bed, system pressure, coolant properties, bottom flow, etc. Until now, considerable experimental or theoretical studies have been performed. However, the useful DHF correlation for various kinds of conditions, specially low pressure and low bottom flow conditions, has not been recommended. In this regard, the existing correlations have been assessed and the new correlation that can predict DHF under low pressure and low bottom flow conditions, has been developed.
In the other hand, during the quenching process of corium, violent fuel-coolant interaction, named steam explosion, can be occured. In this process, film boiling heat transfer will occur by the great temperature difference between corium and coolant. Therefore, to exactly estimate the heat transfer rate to the coolant, film boiling heat transfer coefficient should be identified, especially on spheres.
Especially, in severe accident condition, corium and water may interact with each other under single- and two-phase conditions with some relative velocity between them. In this regard, a series of correlations of film boiling heat transfer on spheres under single- and two-phase flow conditions have been developed based on heat transfer correlations without phase change. The correlations have been developed for the conditions of saturated single-phase flow, subcooled single-phase flow, upward two-phase flow and downward two-phase flow conditions, respectively. They predict well the film boiling heat transfer coefficients within ±20 % of error bounds.