nonfissionable

from WordNet (r) 3.0 (2006)

nonfissionable
    adj 1: not capable of undergoing fission [ant: {fissile},
           {fissionable}]

from The Collaborative International Dictionary of English v.0.48

nonfissionable \nonfissionable\ adj. (Physics)
   not capable of undergoing fission[4]; -- of atoms or chemical
   elements. Opposite of {fissionable}.
   [WordNet 1.5]