Study of debris-generated core blockage scenarios during loss of coolant accidents using RELAP5-3D

Two RELAP5-3D models of a typical four-loop pressurized water reactor were prepared to simulate the reactor system response during loss of coolant accident (LOCA) scenarios of different break sizes and locations, under hypothesized debris-generated core blockage conditions. Three break sizes consisting of 2-in., 6-in., and double-ended guillotine (DEG) were selected as representative cases for small, medium and large break sizes, respectively. Simulations were performed to analyze the behavior of the system during a cold leg break and a hot leg break, assuming that all safety systems were available during the phases of the accident. A simpler model was used to perform the simulations up to the long-term cooling phase of the accident, under a full core and core bypass blockage condition. The simulation results help in identifying critical scenarios which, under such circumstances, may lead to core damage. One critical scenario was selected and analyzed with a more detailed core nodalization using RELAP5-3D multi-dimensional components, under different core blockage schemes, including partial core blockage, showing the ability of the cooling water to remove the decay heat from the core under such conditions.