Simplified Modeling of Natural Circulation during Station Blackout with MELCOR

Station blackout is typical high pressure beyond design basis accident, in which natural countercurrent circulation occurs. If all of the alternate current power sources failed, removal of decay heat from the core is possible by natural circulation through hot leg and steam generator. In this paper a simplified approach to the modeling of the natural circulation during SBO at APR1400 reactor is presented. The proposed model takes into account the physical characteristics of the reactor coolant system, including the core behaviour and passive safety systems, like Safety Injection Tanks, Pilot Operated Safety Relieve Valves and safety valves in the secondary side. Main goal of this work is to prove natural circulation benefit, that is delay of the core damage. The paper concludes with a discussion of the limitations of the model and potential improvements for future work in this field.