Advancing inherent safety in small modular reactors: A CFD-based analysis of the PIUS concept

Abstract

Inherent safety for advanced small modular reactors has been significantly considered as one of the major targeted design features for the reliability and safety of nuclear reactors. As one of the candidates, the Process Inherent Ultimate Safety (PIUS) is noteworthy to enhance reactor safety representing accident mitigation performance without core meltdown as well as power excursion in emergency scenarios. These kinds of reactors can ensure the integrity of the nuclear core, components, and systems with the application of the PIUS concept. The analytical methodology was conducted to analyze the thermal-hydraulic behavior of the PIUS concept for the SMR scale by modeling a separate effect test loop with a computing method, such as a CFD tool, in this work. The model included key components, density locks, in which the thermal stratification phenomenon is actively generated. In normal operation, the density locks blocked the cold borated water flowing into the core region like valve functions. In transient cases, thermal stratification passively collapses since the momentum of the primary coolant dissipates or exceeds due to the coolant pump conditions. This passive injection of cold water triggers the reactor shutdown, and the primary system can be cooled by a long-term cooling process with global temperature behavior by natural circulation. The results indicate the feasibility of the PIUS concept with light-water-cooled technology for advanced SMR designs. This work is expected to provide research grounds for the ultimately safe SMR design with innovative safety technology.