Large-scale boiling experiments of the flooded cavity concept for in-vessel core retention

This paper presents results of ex-vessel boiling experiments performed in the CYBL (cylindrical boiling) facility, a reactor-scale facility which has a tank-within-a-tank design simulating the reactor vessel and the reactor cavity. Experiments with uniform and edge-peaked heat flux distributions up to 20 W/cm2 across the vessel bottom are performed. Boiling outside the reactor vessel is found to be subcooled nucleate boiling. The subcooling is mainly due to the gravity head which results from flooding the sides of the reactor vessel. The boiling process exhibits a cyclic pattern with four distinct phases: direct liquid/solid contact, bubble nucleation and growth, coalescence, and vapor mass dispersion (ejection). The results suggest that the flooded cavity in a passive PWR like the AP-600 should be capable of cooling the RPV in the central region of the lower head. 9 figs., 1 tab., 14 refs