fissionable

fissiokelpoinen Capable of undergoing nuclear fission; fissile.
Any fissile or fissionable substance.
1956, Bulletin of the Atomic Scientists (volume 12, number 4, page 106)

If one nation seized the fissionables within its borders, this would “serve as a danger signal” to other nations. The wrongdoer's gain would be short-lived because other nations would take defensive action in kind.