Even if catastrophic criticality excursions are prevented by a judicious choice of the different reactivity coefficients as well as of efficient active measures, possible serious accidents, such a that of Three Miles 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 shut-down. 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 16 Mw after 1 day and 9 Mw after 5 days[28]. 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 failure of the cooling fluid circulation system are possible and their probabilities depend very much on the type of reactor. Since sub-critical 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.