Preliminary accident analysis of Flexblue underwater reactor - 15149

Flexblue is a sub-sea-based, transportable, small modular reactor delivering 160 MWe. Immersion provides the reactor with an infinite heat sink - the ocean - around the metallic hull. The reference design includes a loop-type PWR with 2 horizontal steam generators. The safety systems are designed to operate passively: safety functions are fulfilled without operator action and external electrical input. Residual heat is removed through 4 natural circulation loops: two primary heat exchangers immersed in safety tanks cooled by seawater and two emergency condensers immersed in seawater. In case of a primary piping break, a two-train safety injection system is actuated. Each train includes a core makeup tank, an accumulator and a safety tank at low pressure. To assess the capability of these features to remove residual heat, the reactor and its safety systems have been modelled using thermal-hydraulics code ATHLET with conservative assumptions. The results of simulated transients for three typical PWR accidents are presented: a turbine trip with station black out, a large break loss of coolant accident and a small break loss of coolant accident. The analyses show that the safety criteria are respected and that the reactor quickly reaches a safe shutdown state without operator action and external power. (authors)