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.
Proceedings of the Iowa Academy of Science
©1958 Iowa Academy of Science, Inc.
"Principles of Fast Fission,"
Proceedings of the Iowa Academy of Science, 65(1), 348-351.
Available at: https://scholarworks.uni.edu/pias/vol65/iss1/51