Comments on the Frisch-Peierls Estimate of the Critical Mass of a Uranium Fission Bomb

The Frisch-Peierls memorandum of March 1940 opened the way to nuclear weapons through its conclusion that the critical mass for an explosive chain reaction in ²³⁵U could be as small as 0.6 kg. This gross underestimate was based on the conjecture that the cross section for the fission of ²³⁵U with fast neutrons was 10 barns. Though this value turned out to be far too high, it is argued here that the choice made was not unreasonable at the time.