MOX fuel

Mixed-oxide fuel (or MOX fuel) is nuclear fuel that contains an oxide of more than one element. Most nuclear fuel that is not of the MOX type only contains uranium dioxide. In most cases, the additional oxide will be plutonium dioxide, sometimes thorium dioxide. Any reactor that uses uranium-based fuel will produce plutonium dioxide. Mox fuel consists of about 93% uranium, and about 7% of thorium or plutonium dioxide.
One of the benefits of using MOX-type fuel is that the plutonium dioxide that is produced, can be re-used for energy generation. Certain types of nuclear power plants need plutonium to work properly.