Coolant flow rate instability during extended station blackout accident in NuScale SMR: Two approaches for improving flow stability

Highlights: • NuScale reactor performance during the station blackout accident is evaluated in this research. • Mass flow rate of the primary fluctuates in some stages and it may lead to potential structural damage to reactor components. • Two different approaches are proposed to mitigate flow instabilities in the primary loop. • It is observed that core decay heat can be removed smoothly in the long term with consideration of these systems in design. Nowadays, small modular reactors have become an appropriate alternative option for replacement with most fossil fuel plants and also in some cases with reactors of past generations. Significant improvements take place in this technology in terms of safety, initial capital cost, compatibility with electricity demand, and usage for thermal processes. One approach to achieve a higher level of safety in these reactors has been the utilization of natural circulation in the primary system as the main mechanism for heat removal from the core in steady-state and accident conditions. Several SMRs such as NuScale, CAREM-25, ABV-6M, and IMR are designed to work with this method. Utilizing natural circulation in the primary system eliminates the risk of pump-related problems and causes significant simplification and improvements in design; however, under some circumstances, it becomes unstable. Exposure of the reactor components to these flow instabilities and flow-induced vibrations, in turn, is a potential source for further component failures. Especially tubes of steam generators and fuel rods are susceptible elements for damage in this situation. In this research, response of the NuScale SMR to station blackout (SBO) accident is calculated. It is observed that after approximately 6 hours from the beginning of the accident, flow begins to oscillate in the primary system. After analyzing SBO for the reference design, two new methods are suggested to make flow stable during the accident. These include utilizing a safety injection tank and using riser valves. The performance of the reactor is investigated in the long term, considering these systems. The strategies guarantee stable flow and decay heat removal during the coping time required by NRC regulations, even if the emergency core cooling system fails to actuate. The results of this research can be used for the design improvement of IPWRs with the natural circulation mechanism in the primary loop.