Comment on Pascal's Muggle (short version)

Wed, 08 May 2013 19:53:56 GMT

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).

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