High burnup (definition)

The energy generated during reactor operation from the fissile material contained in the fuel elements relative to the fuel mass used, is called burnup and has units of megawatt-days per kilogram of uranium. As the burnup increases, the remaining quantity of fissile atomic nuclei decreases while the amount of radioactive fission products increases. Therefore, “spent” fuel elements are replaced after several years’ use during annual overhauls. The higher the burnup, the longer the fuel can remain in the reactor core, with technical and regulatory limits being defined for this relationship.