The fuzzy reasoning is used to model the fractional release of fission products under severe accidents conditions comparing with the data from out of pile experiments conducted in special annealing furnaces, performed on specimens of spent nuclear fuel with high burn up. The physical phenomena of interest are migration of fission products from the severe damaged fuel matrix and its microstructure grains with associated phenomena of sweeping of fission products from inter-granular and intra-granular sites, the increase of grain size on the account of annihilation of smaller grains during high temperature annealing, and a coalescence of pores. These phenomena can be viewed and imaged as global mass transfer of imaginary packages carrying the fission products with time. So incorporating the Package Flow Model PFM to represent the migration of the imaginary fission product packages of grains, bubbles or, pores with adjusted size during the annealing tests. The predicted values have shown a good agreement with the experimental data set from ORNL VI and HI tests, CRL experiments and, CEA experiments.