Spent nuclear fuel

Used nuclear fuel is a complex mixture of the fission products, uranium, plutonium and the transplutonium metals. In fuel which has been used at high temperature in power reactors it is common for the fuel to be heterogeneous; often the fuel will contain nanoparticles of platinum group metals such as palladium. Also the fuel may well have cracked, swelled and been used close to its melting point. Despite the fact that the used fuel can be cracked, it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice.
[t] Oxide fuel under accident conditions
Main article: Nuclear fuel response to reactor accidents

Two main modes of release exist, the fission products can be vaporised or small particles of the fuel can be dispersed.