The nuclear fuel can swell during use, this is because of effects such as fission gas formation in the fuel and the damage which occurs to the lattice of the solid. The fission gases accumulate in the void that forms in the center of a fuel pellet as burnup increases.  As the void forms, the once-cylindrical pellet degrades into pieces.  The swelling of the fuel pellet can cause pellet-cladding interaction when it thermally expands to the inside of the cladding tubing.  The swollen fuel pellet imposes mechanical stresses upon the cladding. A document on the subject of the swelling of the fuel can be downloaded from the NASA web site.