|Spent fuel pellet|
Irradiated fuel is one of the most highly complex physical-chemical systems imaginable. Fission fragments belong to some forty chemical species. They lose fission energy as they collide with atoms in the material, displacing them by a few hundred microns. Fuel undergoes various transformations inside the reactor which leave it in a state of potential imbalance. This cross-section of an irradiated fuel pellet shows the cracking caused by fission, high temperatures and the difference in temperature between the core and the outer edge.