Submitted by L0RD_E t3_11kx8rs in askscience
marcusregulus t1_jbay1yq wrote
Reply to comment by drhunny in How do plutonium based atom bombs work? What chemical reactions happen that make them explode? by L0RD_E
My understanding of the issue of a critical mass and nuclear fission with Plutonium is that you need Pu239. The nuclear reaction to create Pu239 also creates Pu240. Pu240, being only one mass number difference is very hard to separate from Pu239. A gun design is not fast enough to initiate a fission reaction with Pu, thus an implosion design is needed.
Basically, to generate the heat, pressure, and neutron flux to ignite a fusion reaction, takes a fission reaction first.
drhunny t1_jbbc5d2 wrote
That's mostly correct. You can't breed Pu-239 without also breeding a little Pu-240, and you basically can't get it out of the Pu-239. Pu-240 has a very high spontaneous fission rate (atoms occasionally just fall apart, often releasing a neutron or two). At the levels present in a few kilograms of Pu, there's a random neutron every microsecond or so.
For a gun-type design, it spends a fraction of a millisecond in a configuration where a chain reaction is possible but generates a dud. And the Pu-240 spontaneous fission makes it very likely that such a dud chain reaction will happen.
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