There had always been four possible mechanisms for an explosive chain reaction in uranium: (1) slow-neutron fission of U238; (2) fast-neutron fission of U238; (3) slow-neutron fission of U235; and (4) fast-neutron fission of U235. Bohr’s logical distinction between U238 and thorium on the one hand and U235 on the other ruled out (1): U238 was not fissioned by slow neutrons. (2) was inefficient because of scattering and the parasitic effects of the capture resonance of U238. (3) was possibly applicable to power production but too slow for a practical weapon. But what about (4)? Apparently no
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