NUCLEAR FUEL is a material which can be used or consumed by nuclear fusion or fission to derive NUCLEAR ENERGY
The only Fissionable nuclear fuel occurring in nature is URANIUM , of which is 99.3% U-238 and 0.7% is U-235 and U-234 is only a trace. The other fissionable materials are Pu-239 and U-233 which can be produced artificially from U-238 and Th-232.
NUCLEAR FUEL IS SAID TO BE EFFICIENT IF IT PRODUCES A LOT O F ENERGY AND A VERY SMALL BIT OF POLLUTANT.