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...