The Collaborative International Dictionary of English v.0.48:

nuclear energy \nuclear energy\ n.
   Energy derived from nuclear reactions; -- used at present
   especially of electrical power generated in atomic reactors,
   but encompassing also fusion energy.

   Syn: atomic energy, atomic power, nuclear energy, nuclear
        power.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear energy
    n 1: the energy released by a nuclear reaction [syn: atomic
         energy, nuclear energy]