Analytic Predictions of Boiling Film Stability for Conditions Consistent with Light Water Reactor Degraded Core Conditions

Abstract

An improved understanding of boiling film dynamics as it relates to energetic steam explosions during degraded core conditions in light water reactors is developed. Several models have been developed and used to predict the characteristics of film boiling when a molten fuel drop suddenly comes into contact with water. An incompressible model and an approximate compressible model, utilizing Gilmore's equation are developed consistent with past works and are determined to have several shortfalls. To improve the treatment of compressibility effects, a model employing Lagrangian equations is developed. This improved model predicts that applying an external pressure pulse can make a stable film go unstable and decreasing water subcooling stabilizes film oscillations; both predictions are consistent with experimental observations. However, the improved model predicts stable film boiling at low melt temperatures that cannot support such boiling. Modeling Taylor surface instability effects at the water/steam interface indicates that the surface area change due to this surface instability can stabilize the film oscillations.