Thermal Hydraulic Design Aspects of an Integral Pressurized Water Reactor

This work presents a customized RELAP5-3D nodalization designed specifically for an iPWR (integral Pressurized Water Reactor) operating under natural circulation conditions. Given the absence of appropriate correlations in the used SYS-TH (system-thermal hydraulic) code for HCSGs (Helical Coil Steam Generators), tuning of pressure drops and HTC (Heat Transfer Coefficient) has been necessary on both sides of these heat exchangers. Following the successful simulation of steady state conditions at full power, a comprehensive start-up procedure has been implemented, comprising power increase ramps and steady-state assessments. During these simulations, DWOs (Density Wave Oscillations) in the secondary side at low power conditions have been encountered, prompting exploration of mitigation strategies leveraging the understanding of Ishii-Zuber dimensionless numbers, namely the Phase Change Number and the Subcooling Number. Thereafter, a power uprate under primary forced circulation is pursued. The integrated linear heat generation rate of the hottest rod is increased to 55 kW/m, followed by the scaling of the reactor power accordingly. The angular speed of the implemented primary pumps is regulated using an integral controller. Results indicate reasonable superheating in the secondary side and satisfactory subcooling at the core outlet. Given the reactor's small core dimensions and the significant power increase achieved, fuel cycle duration would be substantially shortened unless fuel enrichment is increased.