Even if catastrophic criticality excursions are prevented by a judicious choice of the different reactivity coefficients, combined with efficient active measures, possible serious accidents, such as that of Three Mile Island, may be caused by a defective extraction of the residual heat produced in the fuel by the radioactivity of the fission fragments after reactor shut­down. Immediately after shut-down the residual heat amounts to 7% of the heat produced at full power. This means that a 1 GWe reactor (3 GWth) produces 200 MWth of residual heat after shut-down. This value drops to 16MWth after 1 day and 9 MWth after 5 days [37]. In principle, if the coolant is still present and the circulating system active, this residual heat is easily disposed of. However, both loss of coolant (LOCA) and a cooling fluid circulation system failure are possible and their probabilities depend on the type of reactor. Since subcritical assemblies of hybrid reactors are not different, in this respect, from critical assemblies, we discuss the properties of the most representative reactor types, as far as heat extraction is concerned.