Mechanism for two-phase cooling in a narrow gap

In connection with possible light water reactor in-vessel cooling following core degradation, the formation of gaps and associated cooling mechanisms are of interest. At the high temperatures of interest, i.e., 500 to 1100 degrees C, the film boiling liquid droplet regime would appear of primary importance. In atmospheric pressure laboratory experiments performed by Henry and Hammersley, an average heat removal rate of ∼2500 W was measured in a 2-mm annular gap. Single-phase natural convection (all steam) cannot explain this heat removal rate; i.e., latent heat of vaporization must be involved. This brings into question a characteristic dimension related to possible gap formation and the ability for water to enter the gap