Evaluation of dryout region in a heap-like debris bed formed in an APR1400 pre-flooded reactor cavity

Abstract

This study evaluates how large a debris bed becomes dryout during ex-vessel cooling in a pre-flooded reactor cavity if a postulated severe accident occurs. It is assumed that all generated energy is transferred to water liquid by boiling if the water liquid fraction is more than assumed critical liquid fraction. Heap-like debris beds with a repose angle of 25°, 35° and 45° are analyzed, and the effective particle diameters are assumed to be 0.65 mm, 0.9 mm and 1.65 mm with 0.37 porosity, respectively. The system pressures are considered from 1 bar to 5 bar. In all cases, dryout occurs in the debris beds and the proportion of dryout region accounts for 29.2 % to 92.2 %. The larger the effective diameter, the smaller the dryout region; and the larger the system pressure, the smaller the dryout region. It is also evaluated that as the repose angle increases, the dryout region decreases. As the repose angle increases, the angle at which flowing water faces the debris bed becomes steep, so more coolant can be injected into the debris bed and less coolant flows along the slope of the debris bed.