Phenomenological studies on molten core-concrete interactions

Abstract

Recent assessments of risk due to severe accidents in light water reactors indicate that molten core-concrete interactions (MCCI) may dominate containment loads and performance, as well as the release of non-volatile fission product aerosols to the containment building. This program has investigated several important aspects of MCCIs in an effort to support the USNRC integral melt-concrete programs as well as the CORCON and VANESA computer code development and verification program. Among these are interlayer heat and mass transfer and liquid-liquid boiling processes. The issues of interlayer heat and mass transfer address phenomena that occur primarily interior to the molten core debris itself. Models have been developed to predict the onset of entrainment between liquid layers, the rate of entrainment, and the rate of heat transfer, both with and without entrainment. Application to the reactor case indicates that entrainment and mixing may or may not occur, depending upon the prevailing thermal hydraulic conditions in the melt. The issues of liquid-liquid film boiling address phenomena that occur primarily at the melt-coolant interface. Results from melt-water film boiling studies have suggested that the coolant heat flux increases above the flat plate limit with non-condensable gas injection from below. Steam explosions occur frequently; the frequency and magnitude vary with gas injection rate.