Consequently, uranium-238 is a fissionable material but not a fissile material. Other naturally occurring isotopes are fissionable, but not fissile. Fissionable, non-fissile isotopes can be used as fission energy source even without a chain reaction. As a result, fissile materials (such as uranium-235) are a subset of fissionable materials. U-235 and non-fissile but fissionable and fertile U-238 are both used in the fission process. Reprocessing refers to the chemical separation of fissionable uranium and plutonium from spent fuel. Thorium is a fissionable material used in thorium-based nuclear power. ""Fissile"" is distinct from ""fissionable"". Nuclear power relies on fissionable material that can sustain a chain reaction with neutrons. In 1946, nuclear weapons physicists at Los Alamos were investigating neutron diffusion in fissionable material. Not all fissionable isotopes can sustain a chain reaction. Areva NC reprocesses the spent fissionable material. Liquid core nuclear engines are fueled by compounds of fissionable elements in liquid phase. The critical size must at least include enough fissionable material to reach critical mass. By 14 January 1964, enough fissionable U-235 had been successfully enriched from the Lanzhou plant.