Characteristics of spent fuel

Initially nuclear fuel is made up of a set of cylindrical ceramic pellets of U-238 oxide, with a variable degree of enrichment in U-235 (lower than 5%), placed inside tubes of a zirconium alloy and assembled in such a way as to form the structure that constitutes the fuel assembly.

During such time as the fuel is in the reactor, and as a result of the neutron flux to which it is subjected, neutron capture and nuclear fission reactions affecting part of the uranium and of the other radionuclides generated take place, giving rise to fission and activation products and to the generation of plutonium and minority actinides. A typical proportion in spent fuel is as follows:

• Uranium: 94.7% (0.7% being U-235)
• Fission products: 4.1%
• Minority actinides: 0.1%

The spent fuel from nuclear reactors, which looks just the same as fresh fuel, emits alpha, beta and gamma radiation and also generates heat as a result of radioactive decay.

The difference between spent and fresh fuel is their chemical composition. While new fuel contains only uranium oxide, after passing through the reactor the spent fuel presents a composition in which not only the initial uranium oxide but also practically all the elements in the periodic table are present.

The amounts and characteristics of the different components of irradiated fuel depend on its initial enrichment in U-235 and the degree of fuel burnup.