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Jiro comments on Pascal's Muggle (short version) - Less Wrong
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The original giant heap of uranium bricks with k=1.0006 (CP-1 the first pile) - was that chain reaction all due to U235? Maybe the spontaneous fissions are mostly U235, but are the further fissions mostly neutrons hitting U235? This doesn't correspond with my mental model of a pile like that - surely the 2-3 neutrons per fission would mostly hit U238 rather than U235. I also know there were graphite bricks in the pile and graphite bricks are for having slow neutrons being captured by U238.
Let's suppose U235 didn't exist any more. We couldn't build a huge heap of pure U238 uranium bricks, and throw in a small number of neutrons from somewhere else (radium?) to get things started?
EDIT: Okay, I just read something else about slow neutrons being less likely to be absorbed by U238, so maybe the whole pile is just the tiny fraction of natural U235 with the U238 accomplishing nothing? This would indeed surprise me, but I guess then the case can be made for all access to chain reactions bottlenecking through U235. Still seems a bit suspicious and I would like to ask some physicist who isn't frantically trying to avoid hindsight bias how things look in retrospect.
EDIT2: Just read a third thing about slow neutrons being more easily captured by U238 again.
U-235 has a higher neutron absorption cross-section than U-238, so more U-238 than U-235 doesn't necessarily mean more neutrons hitting U-238 than U-235.
How much higher? Natural uranium, which is what they used in CP-1, is over 99% U238.
Wikipedia under "Neutron cross section" lists U-235 as having a capture cross-section of 60 and a fission cross section of 300, while U-238 has a capture cross-section of 2. This is for thermal neutrons (the cross-section depends on the neutron speed).
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.)
I wonder how low is the probability of obtaining at least 1 such sufficiently stable fissile isotope, if you change fundamental physical constants a little, preserving stars and life (and not making Earth blow up). It may be very low, actually, seeing it as U235 does have unusually long half life for a fissile isotope.
EDIT: Scratch that, your post is the right response.
I've currently got a Discussion post running to figure out how much this generalizes.