Severe accident analysis of the Qinshan Nuclear Power Plant and evaluation of boundary conditions for ex-vessel heat transfer

Severe accident analysis is critical for the operation and licensing of new generation reactor designs as well as for the prevention and management of severe accidents. The Severe Accident Management Guidelines (SAMGs) are developed and implemented for prevention and mitigation measures of a severe accident. Retention of corium in the reactor vessel via ex-vessel cooling is one of the most promising mitigation strategies. Its success requires a thorough analysis of the reactor core following a severe accident to determine the boundary conditions for external cavity flooding. In this study, two loss of coolant accident cases at the Qinshan nuclear power plant are analyzed using RELAP5/SCDAP simulations. Passive accumulators are credited in the first case (Case-1), but not in the second (Case-2). The important severe accident events are studied for the plant and the sequence of events and melt conditions are captured. Hydrogen generation has been estimated to be 120 kg in Case-1 and 108 kg in Case-2. In both instances, a molten pool is formed. The corium reached a maximum temperature of 3016 K and 3006 K, respectively in Case-1 and Case-2. The metallic/oxidic layer properties and heat transfer coefficient of the corium pool as well as the ex-vessel heat transfer boundary conditions are evaluated analytically for the reference plant. The analytically determined temperatures of oxidic and metallic phases in the corium pool are 3006 K and 1919 K respectively. The upward and sideward heat fluxes (vessel wall) are determined to be 298.3 kW/m2 and 730.75 kW/m2 respectively, while the heat transfer coefficients for oxidic and metallic phases are 2.80 W/m2-K and 4.42 W/m2-K respectively.