Results of LHI tests and associated analyses on in-vessel debris coolability

A series of tests designated as LHI was performed in order to acquire findings on debris cooling mechanisms in the lower plenum. A molten UO2 mixture as a debris simulant was gravitationally slumped into a water-filled lower head model. Wall temperatures increased sharply after the contact with the debris followed by cooling to the initial temperature. A gap cooling model, which deals with gap growth at the interface between the debris and the lower head and water penetration into the gap, was incorporated into LIDEC code in conjunction with other key models. It was confirmed through experimental and TMI-2 accident analyses that LIDEC code was capable of reproducing the thermal transient of the lower head wall. The maximum coolable debris mass was evaluated to be 30 through 40 tons and approximately 4 tons for thermal-hydraulic conditions of the TMI-2 accident and Zion station blackout sequence, respectively. With the evaluated maximum coolable debris mass and the water injection probability, the RPV internal water is supposed to have limited effectiveness for the in-vessel debris cooling especially in low pressure sequences. (author)