An isotope is said to be fissile if the nuclei can be split by thermal (slow-moving) neutrons. Nuclear reactors use fissile material as fuel. Uranium-233, uranium-235 and plutonium-239 are fissile.

An isotope that can be split only by fast-moving neutrons is fissionable

Fis"sile (?), a. [L. fissilis, fr. fissus, p.p. of findere to split. See Fissure.]

Capable of being split, cleft, or divided in the direction of the grain, like wood, or along natural planes of cleavage, like crystals.

This crystal is a pellucid, fissile stone. Sir I. Newton.

© Webster 1913.