The Collaborative International Dictionary of English v.0.48:

nuclear fission \nuclear fission\ n.
   A nuclear reaction in which a heavy atomic nucleus splits
   into smaller nuclei with the simultaneous release of energy.

   Syn: fission.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

nuclear fission
    n 1: a nuclear reaction in which a massive nucleus splits into
         smaller nuclei with the simultaneous release of energy
         [syn: fission, nuclear fission]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "nuclear fission":
   alpha decay, atom-chipping, atom-smashing, atomic disintegration,
   atomic reaction, atomization, beta decay, bombardment, breeding,
   bullet, chain reaction, cleavage, disintegration series,
   dissociation, exchange reaction, fission, fission reaction,
   gamma decay, ionization, neutron reaction, nonreversible reaction,
   nucleization, photodisintegration, proton gun, proton reaction,
   reversible reaction, splitting the atom, stimulation, target,
   thermonuclear reaction