Transient analyses on loss of heat sink and overpower transient of natural circulation LBE-cooled fast reactor

Two transient analyses on a natural circulation LBE-cooled fast reactor were performed by using a Neutronics and Thermal-hydraulics Coupled Simulation Program NTC-2D. The two transients were unprotected loss of heat sink (ULOHS) and unprotected overpower transient (UTOP). For ULOHS, as the reactor is driven by natural circulation, the coolant mass flow rate decreased, but the simulation results showed that the reactor shutdown by itself and the peak temperatures of fuel and cladding were far smaller than the safety limits. For UTOP, the results showed that the coolant, cladding and fuel temperatures in the reactor increased and the more heat was taken away by the increase of the coolant mass flow rate. Finally, a new steady state was re-established by natural circulation.