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 disc ussed. In the present study hypothetical accidents are simulated for t hese reactors, i.e., unprotected simultaneous ULOF (total loss of prim ary primping system) and UTOP (rod run out transient over power) accid ents, caused by the simultaneous withdrawal of ah control rods. The pr oposed designs have some important safety characteristics such as low reactivity swing (only 0.2-0.25$), and negative coolant void coefficie nt over whole burnup period. Effectively negative value of all compone nts of reactivity during an accident is observed. The safety performan ces of the balance, pancake, and tall slender type of core, each of th em 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 withou t the help of reactor scram and without the need of operator actions. The margin to coolant boiling and fuel melting is large. From the para metric survey results it is found that primary pump coastdown half tim e and relative position of primary side outlet of SG to the inlet of t he core can effectively influence the maximum coolant, cladding, and p ellet temperature during accidents.