I’m still suspicious that Fermi could truly not have done better than “ten percent”, and wonder if people are trying a little too hard not to give in to hindsight bias and overfitting, at the cost of failing to learn heuristics that could indeed generalize. Agreed that if +chain reaction implied a new fact of physics in the sense that it tells you about a previously unknown heavy element which emits free neutrons and is splittable, the standard heuristic “does the failure of this prediction tell us a new fact of physics” does not work in the vanilla sense. This doesn’t mean that a fair estimate of the probability of at least one not-yet-examined element having the desired properties would have been ten percent. Chain reactions were not just barely possible for a large barely-critical fission plant 50 years later, rather they were soon achieved at a prompt supercritical grade adequate for nuclear explosions by two distinct pathways of U-235 refinement and P-239 breeding, both of which admittedly required effort, but was the putting-in of that effort unpredictable? But this should be continued in the other post rather than here.
rather they were soon achieved at a prompt supercritical grade adequate for nuclear explosions by two distinct pathways of U-235 refinement and P-239 breeding
They could realistically only breed enough Pu239 by starting with U235 fueled reactor. Everything that you can do in 1945 goes through U235 , which we have only because it has unusually long half life (350x the next stablest fissile isotope, Np-237) . On top of that, they didn’t even know that fission released prompt secondary neutrons at all—those could of simply remained in the fission products and convert to protons via beta decay.
I know they got a critical reaction with a big heap of unrefined uranium. This makes no mention of uranium needing to be isotopically refined for plutonium production on the Manhattan Project. As you are generally a great big troll, I am afraid I cannot trust anything you say about isotopic refinement having been used or required without further references, but I will not actually downvote yet in case you’re not lying. Got a cite?
The only proven and practical source for the large quantities of neutrons needed to make plutonium at a reasonable speed is a nuclear reactor in which a controlled but self-sustaining 235 U fission chain reaction takes place.
Discussion of the particle accelerator route which would enable the bootstrapping of a non U-235 route eventually (producing a critical mass of 10+ kg of plutonium using superconductors and huge amounts of energy and accelerator time), but only with much increased difficulty:
Wilson died in 2000 but a paper he wrote on this topic in 1976 has now found its way onto the arXiv and it highlights some thought-provoking ideas.
At the time, Wilson was director of Fermilab where he was building an accelerator called the Energy Doubler/Saver, which employed superconducting magnets to steer a beam of high energy protons in a giant circle. These protons were to have energies of up to 1000 GeV.
The Energy Doubler was special because it was the first time superconductivity had been used on a large scale, something that had significant implications for the amount of juice required to make the thing work. “One consequence of the application of superconductivity to accelerator construction is that the power consumption of accelerators will become much smaller,” said Wilson. And that raised an interesting prospect.
Imagine the protons in this accelerator are sent into a block of uranium. Each proton might then be expected to generate a shower of some 60,000 neutrons in the material and most of these would go on to be absorbed by the nuclei to form 60,000 plutonium atoms. When burned in a nuclear reactor, each plutonium atom produces 0.2 GeV of fission energy. So 60,000 of them would produce 12,000 GeV.
Using this back-of-an-envelope calculation, Wilson worked out that a single 1000 GeV proton could lead to the release of 12,000 GeV of fission energy. Of course, this neglects all the messy fine details in which large amounts of energy can be lost. For example, it takes some 20MW of power to produce an 0.2MW beam in the Energy Doubler.
But even with those kinds of losses, it certainly seems worthwhile to study the process in more detail to see if overall energy production is possible.
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.
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?
U238 is essentially only fissioned by fast neutrons (only fast neutrons are not dramatically more likely to be simply captured than to fission it), and overall tends to capture neutrons without being fissioned (that’s how you get Pu-239: U-238 absorbs a neutron, becomes U-239, then after one beta decay becomes Neptunium-239, then after another beta-decay, Plutonium-239).
So, while it does fission and does release neutrons when it fissions, it doesn’t sustain chain reaction. Fission doesn’t imply chain reaction even with secondary neutrons.
Fortunately U238 has small enough capture cross section that you can make a reactor work with natural uranium, but only if you use graphite to slow neutrons down. You need graphite because http://www.whatisnuclear.com/articles/fast_reactor.html (scroll down to graphs, note that U235 fission cross section increases faster with decrease in neutron energy than U238 capture cross section so even though both are larger at lower energies, u235 fission wins over u238 capture. Also note nice almost-fractal peaks and valleys (resonance) which very much get in your way when you try to figure anything out from real data. This is a true extreme miracle of actual human rationality that this stuff was figured out sufficiently to build anything).
surely the 2-3 neutrons per fission would mostly hit U238 rather than U235
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.
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.
In a natural uranium fuelled reactor, the actual fuel (at startup) is U235, present in the natural uranium at a concentration of about 0.7%. No U235 in nature = no easy plutonium.
Those two “distinct” pathways both rely on properties of 1 highly unusual nucleus, U235, which is both easy to fission, and stable enough to still be around after ~5 billion years. How unusually stable is it? Well, from the date of, say, 40 million years since explosion of the supernova that formed Solar system, it was for all intents and purposes the only one such isotope left. (Every other fissile isotope was gone not because of fizzling or spontaneous fission but because of alpha decay and such)
I’m still suspicious that Fermi could truly not have done better than “ten percent”, and wonder if people are trying a little too hard not to give in to hindsight bias and overfitting, at the cost of failing to learn heuristics that could indeed generalize. Agreed that if +chain reaction implied a new fact of physics in the sense that it tells you about a previously unknown heavy element which emits free neutrons and is splittable, the standard heuristic “does the failure of this prediction tell us a new fact of physics” does not work in the vanilla sense. This doesn’t mean that a fair estimate of the probability of at least one not-yet-examined element having the desired properties would have been ten percent. Chain reactions were not just barely possible for a large barely-critical fission plant 50 years later, rather they were soon achieved at a prompt supercritical grade adequate for nuclear explosions by two distinct pathways of U-235 refinement and P-239 breeding, both of which admittedly required effort, but was the putting-in of that effort unpredictable? But this should be continued in the other post rather than here.
They could realistically only breed enough Pu239 by starting with U235 fueled reactor. Everything that you can do in 1945 goes through U235 , which we have only because it has unusually long half life (350x the next stablest fissile isotope, Np-237) . On top of that, they didn’t even know that fission released prompt secondary neutrons at all—those could of simply remained in the fission products and convert to protons via beta decay.
I know they got a critical reaction with a big heap of unrefined uranium. This makes no mention of uranium needing to be isotopically refined for plutonium production on the Manhattan Project. As you are generally a great big troll, I am afraid I cannot trust anything you say about isotopic refinement having been used or required without further references, but I will not actually downvote yet in case you’re not lying. Got a cite?
Federation of American Scientists:
Discussion of the particle accelerator route which would enable the bootstrapping of a non U-235 route eventually (producing a critical mass of 10+ kg of plutonium using superconductors and huge amounts of energy and accelerator time), but only with much increased difficulty:
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.
U238 is essentially only fissioned by fast neutrons (only fast neutrons are not dramatically more likely to be simply captured than to fission it), and overall tends to capture neutrons without being fissioned (that’s how you get Pu-239: U-238 absorbs a neutron, becomes U-239, then after one beta decay becomes Neptunium-239, then after another beta-decay, Plutonium-239).
So, while it does fission and does release neutrons when it fissions, it doesn’t sustain chain reaction. Fission doesn’t imply chain reaction even with secondary neutrons.
Fortunately U238 has small enough capture cross section that you can make a reactor work with natural uranium, but only if you use graphite to slow neutrons down. You need graphite because http://www.whatisnuclear.com/articles/fast_reactor.html (scroll down to graphs, note that U235 fission cross section increases faster with decrease in neutron energy than U238 capture cross section so even though both are larger at lower energies, u235 fission wins over u238 capture. Also note nice almost-fractal peaks and valleys (resonance) which very much get in your way when you try to figure anything out from real data. This is a true extreme miracle of actual human rationality that this stuff was figured out sufficiently to build anything).
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.
In a natural uranium fuelled reactor, the actual fuel (at startup) is U235, present in the natural uranium at a concentration of about 0.7%. No U235 in nature = no easy plutonium.
Those two “distinct” pathways both rely on properties of 1 highly unusual nucleus, U235, which is both easy to fission, and stable enough to still be around after ~5 billion years. How unusually stable is it? Well, from the date of, say, 40 million years since explosion of the supernova that formed Solar system, it was for all intents and purposes the only one such isotope left. (Every other fissile isotope was gone not because of fizzling or spontaneous fission but because of alpha decay and such)
I explained it in greater detail here .