Safety aspects of heavy metal-cooled accelerator-driven waste

Abstract
Accelerator-driven, subcritical lead / bismuth cooled systems have several safety advantages. The critical accident initiators in such systems lead only to a relatively slow coolant heat-up that should be noticed by the reactor operators who will initiate a shutting down of the accelerator. This decreases the reactor power to decay heat levels. If the coolant temperature increase should go unnoticed, passive systems will lead to an automatic shutdown of the accelerator or a blocking of the proton beam. Emergency decay heat removal by natural air circulation cooling of the vessel outside is an attractive option for such a system. If no active or passive beam shut-off took place during a coolant overheating due to a significant Loss of Heat Sink (LOHS) accident, a core melt could eventually occur. Oxide fuel would probably mix with the heavy metal coolant with its high boiling point and circulate in the primary system in a coolable fashion. This type of scenario seems to have happened in a core melt accident in a Russian Alpha submarine with its lead / bismuth cooled critical reactor.