LONG-TERM SUMP NATURAL CIRCULATION IN A SUBMERGED SMALL MODULAR REACTOR

This work investigates the performances of the passive safety systems of a submerged and transportable Small Modular Reactor (SMR) after a Loss Of Coolant Accident (LOCA). The focus of the activity concerns the long-term period, addressing the feasibility of the “depressurized and flooded” safe state, i.e. a targeted situation where the reactor containment is flooded by the injection of water from a large safety tank. Decay heat is removed by sump natural circulation and rejected through the metal containment to the surrounding water, which acts as an infinite heat sink. Following the accidental event and the operation of the safety injection systems, the safe state is expected to provide a continuous and efficient cooling of the fuel rods, ensuring a potentially unlimited grace period with no electrical input or human intervention required. The study employs a numerical approach and simulations are performed using the 1D system code RELAP5. The nodalization process identifies three macro-components, i.e., the reactor pressure vessel, the reactor containment and the safety tank, connected by the recirculation lines. Conservative boundary and initial conditions are set to simulate the transient starting at 7h30’ after the scram of the reactor. Results assess the effectiveness of the sump natural circulation under the reference conditions. The main outcomes also show the good potentialities of the heat exchange systems, highlighting the safe operation of the passive safety systems for at least 21 days after the reactor scram. The sensitivity analysis identifies the nodalization of the reactor containment as a modeling and numerical issue, deserving further investigation. KEYWORDS