> In 1939, Enrico Fermi expressed 90% confidence [0] that creating a self-sustaining nuclear reaction with Uranium was impossible. And, if you're working with U238, it basically is! But it turns out that it's possible to separate out U235 in sufficient quantities to use that instead.

You are moving goalposts. You mentioned in the first place "fission weapons" and now you take a quote about "nuclear fission reactor" which is a whole different thing.