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Given that this guy was charged under a non-proliferation act it makes a massive difference whether it's Pu-238 or Pu-239.

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Why, because one has a critical mass of 9-10 kg and the other has a critical mass of 11 kg? You'd think it would matter a great deal more that the amount he obtained was apparently 35 nanograms, so he was about a hundred million samples short of a working reactor.

Pu-238 isn't usably fissile for weapons purposes (and apparently[0] isn't regulated as such (?)). Just look at the hero image on its Wikipedia entry[1]. A critical mass would have >5 kW of decay heat.

[0] https://www.nrc.gov/reading-rm/doc-collections/cfr/part071/p... ("Fissile material means the radionuclides uranium-233, uranium-235, plutonium-239, and plutonium-241")

[1] https://en.wikipedia.org/wiki/Plutonium-238 ("Plutonium-238 oxide pellet glowing from its decay heat")

https://en.wikipedia.org/wiki/General-purpose_heat_source#St...

Hmm, interesting! I stand corrected. Thank you!

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