This work illustrates 21) SIMMER-I1I and 3D SIMMER-IV calculations for fuel dispersion analyses during postulated accidents in the MYRRHA-FASTEF reactor. The aim was to identify some potential events which can involve a fuel release and the most affecting parameters for the recriticality concern. In the first part, a 3D SIMMER-IV model of MYRRH A reactor is presented. It includes a gas region above the reactor that allows the simulation of transients in natural circulation flow. We found that the flow pattern which establishes in the primary circuit affects considerably the fuel dispersion phenomena. The low mass flow rate prevents the redistribution of the fuel that accumulates in the upper plenum. This model was used also to simulate two reference cases in forced circulation, showing no significant differences with respect to the analogous cases performed with the previous model, which did not include the gas region. As 3D simulations required large CPU time, a SIMMER-III 2D model was likewise arranged for the long term analyses (up to 1-2 hours). The results highlighted some preferential accumulation points for the fuel particles in the long term Nevertheless, in this case the influence of stagnation zones observed in 3D simulations cannot be adequately represented by SIMMER-III. 3D SIMMER-IV model was also used for the simulation of the Unprotected Loss of Flow Accident (ULOF) and the Unprotected Loss of Heat Sink (IJLOHS) transients. The evolution of these transients did not show the likelihood of core degradation and release of fuel in the primary circuit within the time considered.