Simulation of initiation and transition phases of unprotected transients in ESFR-SIMPLE SMR

A 360 MWth Small Modular Reactor based on the European Sodium Fast Reactor (ESFR) design is studied within the EURATOM ESFR-SIMPLE project. The reactor features a new core configuration with enhanced safety measures, including a near-zero coolant void reactivity. The performance of these measures is evaluated at KIT using SIMMER-III for simulations of unprotected loss of flow (ULOF) and unprotected transient of overpower (UTOP) scenarios. Benchmark results of KIT show good agreement with those of other partners and indicate no fuel pin failure for ULOF or UTOP cases with reactivity insertions up to 1.14 $. A higher reactivity insertion (1.5 $) may induce sodium boiling, leading to power oscillations, core melting, and re-criticality due to fuel relocation. The boiling-induced power oscillations, with a period of approximately 2 s, are attributed to alternating boiling and flooding in the upper fissile core and sodium plenum, consistent with mechanisms previously observed in ULOF analyses for a large ESFR design.