Answer:
Nuclear energy is produced by fission of new fuel, which is only very slightly radioactive to start with (it is safe to handle unprotected). It becomes radioactive during the reactor's operation, and at the end of life the radioactivity of the fuel contributes some 5% or so of the output heat, the rest is due to fission directly. There are some uses for radioactive isotopes to produce electricity directly but these are only used in satellites for small instrument type supplies.