nonfissionable
adjective
non·fis·sion·able ˌnän-ˈfi-shə-nə-bəl 
-zhə-;
-ˈfish-nə-,
-ˈfizh-
: not capable of undergoing fission : not fissionable
nonfissionable uranium
Word History
First Known Use
1946, in the meaning defined above