I'm surprised. I guess CP-1 could've been, in effect, mostly empty space filled with U-235 dust. And I'll go ahead and agree that if all non-particle-accelerator pathways to chain reactions bottlenecked through U-235 then Fermi may have been correct to say 10% (though it is still not totally clear why 10% would've been a better estimate than 2% or 50%, but I'm not Fermi). This would then form only the second case I can think of offhand where erroneous scientific pessimism was not in defiance of laws or evidence already known. (The other one is Kelvin's careful calculation that the Sun was probably around 60 million years old, which was wrong, but because of new physics - albeit plausibly in a situation where new physics could've rightly been expected, and where there was evidence from geology. Everything else I can think of offhand is "You can't have a train going at 35mph, people will suffocate!" or "You can't build nanomachines!" so you can see why my priors made me suspicious of Fermi.)