Principles of Fast Fission

In the thermal reactor a moderator of some sort is used to slow down the neutrons so that fission can take place at thermal speeds. In the case of a reactor using U-235 along with U-238, it is just possible to produce a self-sustaining chain reaction. This is because U-238 is only fissionable by high energy neutrons and the inelastic scattering cross section of U-238 is dominant at high energies. Therefore the fast neutrons produced on fission have a very small chance of producing another fission