Safety aspect of long-life small safe power reactors

Safety aspects of several design options of long-life small safe fast power reactors using nitride fuel and lead-bismuth as coolant are discussed. In the present study hypothetical accidents are simulated for these reactors, i.e., unprotected simultaneous ULOF (total loss of primary pumping system) and UTOP (rod run out transient over power) accidents, caused by the simultaneous withdrawal of all control rods. The proposed designs have some important safety characteristics such as low reactivity swing (only O.2...{).2S$), and negative coolant void coefficient over whole burnup period. Effectively negative value of all components of reactivity during an accident is observed. The safety performances of the balance, pancake, and tall slender type of core, each of them satisfy reactivity and negative coolant void coefficient constraint, against the above accident are compared. The simulation results show that all of the design options can survive the above accidents without the help of reactor scram and without the need of operator actions. The margin to coolant boiling and fuel melting is large. From the parametric survey results it is found that primary pump coastdown half time and relative position of primary side outlet of SG to the inlet of the core can effectively influence the maximum coolant. cladding, and pellet temperature during accidents