Safety Principles and Philosophy
In the early stages of fast reactor development, the potential for accidents in liquid sodium-cooled fast reactors (SFRs) was considered as a major safety issue, especially for severe accidents. Severe accidents occur when the reactor is “unprotected,” which means that the reactor protection systems fail to scram the reactor. The consequences could include fuel melting, fuel relocation, and power excursions, up to and including energetic disassembly of the reactor core and the potential failure of the containment building. For fast reactors, these accidents are typically in one of three categories: loss of cooling (unprotected loss of flow [ULOF]), loss of normal heat removal (unprotected loss of heat sink [ULOHS]), and reactivity addition (unprotected transient overpower [UTOP]). Such events were the major focus of the licensing reactor. This combination of circumstances made the reactor designers to study and develop more advanced fast neutron reactors and concepts, including the concept of “inherent” safety, that is, design choices and features that do not require an active system to function, but are the result of inherent fundamental physical processes such as thermal expansion or gravity, which do not have a probability of failure.