Thermohydraulic CFD study on the residual power's extraction in a IV Generation Sodium Cooled Fast Reactor : In 4th generation Sodium-cooled Fast Reactor a situation of unprotected loss of flow without scram can lead to a severe accident scenario with a partial melt down of the core. The mixture of steel and fuel (debris) is relocated in the core catcher. A 3-D CFD simulation, with NEPTUNE_CFD code coupled with SYRTHES (for the conduction in solids), has been performed so to analyse the behaviour of the reactor in this condition focusing on the role of two decay heat removal systems (one in-vessel and the other ex-vessel).