Simulation of the loss of RHR during midloop operations and the role of steam generators in decay heat removal

Loss of residual heat removal (RHR) during midloop operations was simulated using the RELAP5/MOD3 thermal-hydraulic code for a typical four-loop pressurized water reactor (PWR) under reduced inventory level. Two cases are considered here: one for an intact reactor coolant system with no vents and the other for an open system with a vent in the pressurizer. The effect of air on the transients was studied, unlike the RETRAN analysis of core boiling during midloop operations performed by Fujita and Rice, which did not analyze the presence of air in the system. The steam generators have water in the secondary covering the U-tubes. The system gets pressurized once water starts boiling in the core with higher system pressures for the vent-closed case. Reflux condensation occurs in the U-tubes aiding decay heat removal and preventing complete uncovery of the core. Sudden pressurization of the hot leg and vessel upper head causes the reactor vessel to act as a manometer reducing the core level and raising the downcomer level. Fuel centerline and clad temperatures are below safety limits throughout the transients