Analysis of the Thermal Hydraulic Consequences Following Common Mode Pump Seizure in a Nuclear Power Plant

In nuclear power plants, the primary heat transport system (PHTS) is designed according to ASME Section 3 Division 1. The PHTS together with Primary Coolant Pumps (PCPs) is designed so that the integrity under safe shutdown earthquake (SSE) is maintained. However, the operability under SSE may not be ensured. There might be a possibility of seizure of PCPs under SSE. The pump seizure has two thermal hydraulic effects. The flow sharply reduces and the kinetic energy of the fluid gets converted into pressure pulses. The magnitude of the pulse depends on the locked pump rotor flow resistance. The reduction in the flow affects the heat transfer in the core and the steam generator in different ways. This mismatch between the heat received and rejected by the PHTS results in high system pressure inspite of relief valves. This paper presents and discusses the aforementioned two effects for the case of a PHWR. The study shows that the peak pressure reached during the transient is much higher than the operating pressure. The system integrity may be affected if the peak pressure is significantly higher than the design pressure.