Mixed oxide fuel is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel
A used MOX, which has 63 GW days (thermal) of burnup and has been examined with a scanning electron microscope using electron microprobe attachment. The lighter the pixel in the right hand side the higher the plutonium content of the material at that spot