Simulation of Loss of RHR During Midloop Operations and the Role of Steam Generators in Decay Heat Removal Using the RELAP5/MOD3 Code

Abstract

Loss of residual heat removal during midloop operations was simulated for a typical four-loop pressurized water reactor operated under reduced inventory level using the RELAP5/MOD3 thermal-hydraulic code. 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 presence of air in the reactor coolant system is modeled, and its effect on the transients is calculated. The steam generators are considered under wet layup with water in the secondary covering the U-tubes. The system is pressurized once the water starts boiling in the core. Higher system pressures are seen for the closed-vent case when compared with the open-vent case. Reflux condensation occurs in the steam generator U-tubes preventing complete uncovery of the core and aiding in decay heat removal. The total heat removed by the steam generators is one-third of that produced by the core. The hot leg and vessel upper head pressurization cause the reactor vessel to act as a manometer where the core level drops and the downcomer level rises. This phenomenon is seen at different transient times for the two cases. Since it occurs only for a brief period, the rest of the transient is unaffected. Fuel centerline and clad temperatures are observed to be below the accepted safety limits throughout both transients.