In nuclear engineering, a fissile material is one that's capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. Thus the predominant neutron energy may be typified by either slow neutrons (for example a thermal system) or fast neutrons. Hence fissile materials can be used to fuel: A thermal reactor, with a neutron moderator A fast reactor, with no moderator A nuclear explosive Fissile vs fissionable… (More on Fissile)