Under all definitions above, uranium-238 (U) is fissionable, but not fissile. Neutrons produced by fission of U have lower energies than the original neutron (they behave as in an inelastic scattering), usually below 1 MeV (i.e., a speed of about 14,000 km/s), the fission threshold to cause subsequent fission of U, so fission of U does not sustain a nuclear chain reaction.