Steam generator cooling post LOCA with ECCS failure in NuScale reactor using RELAP5

NuScale is an integral pressurized water reactor (PWR) that applies natural circulation mechanism in its safety system. During a loss of coolant accident (LOCA), its emergency core cooling system (ECCS) uses the containment wall to cool the reactor core by opening valves. If the ECCS fails to function, one alternative to prevent the core damage is by utilizing the steam generator (SG) to remove heat from the primary system. In this study, a simulation is carried out to investigate whether the SG design has sufficient capacity for such cooling. The simulation is performed using the RELAP5 code by modeling the reactor system, safety system, and control system at the input. The model is validated using steady-state and transient calculation data from the NuScale design certification application (DCA) document and independent analyst publication. The scenario was a pipe break at the chemical and volume control system (CVCS) injection line, followed by failure of the ECCS. Activation of the SG feed water is carried out 10 minutes after the ECCS failure is recognized. The simulation result shows that the SG cooling is capable of bringing the pressure and temperature in the primary system down and maintain a stable water level at a position above the top active fuel. However, due to the limitation ofRELAP5 in modeling the helical SG, this simulation result needs further verification, especially in condensation cooling.