Integral Effects Tests and System Thermal-hydraulics Code Analyses on the Decay Heat Removal Systems of the STELLA-2 Facility

Selected integral effects test results conducted in the STELLA-2 facility on the decay heat removal systems (DHRS) are presented, along with numerical analyses with MARS-LMR system thermal-hydraulics code. STELLA-2 is a large-scale sodium test platform modeled after the Korean sodium-cooled fast reactor, PGSFR, with a 1/5 scale reduction in length. The facility aims to evaluate the overall plant dynamics and safety aspects of the reactor under long-term transient conditions. With equivalent geometrical representations, reactor transients can be investigated while maintaining the integral behavior and interactions of the prototypic reactor’s heat transport systems with minimal distortions. The facility has four individual DHRS loops, comprising two active and two passive loops differentiated by their cooling mechanisms for ultimate heat sinks. The selected tests focus on scenarios where the primary system after reactor shutdown relies solely on two DHRS loops, postulating a loss-of-offsite-power condition. Overall, the system code successfully captured the general transient behaviors in the individual loops, although deviations were observed in the peak temperature and the time required to reach it. It is believed that discrepancies in local temperature distributions, particularly in larger volumes, are attributed to the system code’s limitations in its discretization scheme and closure relations.