Wordnet 3.0
ADJECTIVE (1)
1. 
 capable of undergoing nuclear fission; 
- Example: "a fissionable nucleous"- Example: "fissionable material"[syn: fissionable, fissile]
The Collaborative International Dictionary of English v.0.48:
fissionable \fis"sion*able\, a.
   Capable of undergoing nuclear fission; -- a property of
   certain isotopes of elements with heavy nuclei.
   Syn: fissile.
        [PJC]
WordNet (r) 3.0 (2006):
fissionable
    adj 1: capable of undergoing nuclear fission; "a fissionable
           nucleous"; "fissionable material" [syn: fissionable,
           fissile] [ant: nonfissionable]