The reason nuclear weapons use Pu239 is because it is superior for that purpose due to the far easier acquisition thereof. That's an extremely narrow and very doubtful view of 'easier'. The burning of uranium (usually requiring some 1% of U235 enrichment anyway, so you still need a centrifuge complex etc.) to create plutonium does not make it easier to acquire, it makes it vastly more problematic, dangerous, expensive and polluting. All just to create warheads that are seriously more difficult and dangerous to fabricate, much more expensive, more difficult to handle, and more difficult to maintain and periodically rebuild. The US just retired most of them and tried to build warheads that were less of a pain in the arse, and they used a lot of HEU in them.Alternatively an implosive HEU munition doesn't have these serious impairments nor life-cycle costs or the endless safety and security issues (HEU cores are much harder to find and target).Plus its vastly easier to dilute and dispose of excess weapon-grade HEU. You could even dilute it back to 1/144th of stainless steel, and bury the ingots in an an old uranium mine, with zero ill or unnatural cumulative effects.But plutonium remains a serious problem for disposal as its radioactivity would require an enormous amount of dilution in other stable metals, in order to become as harmless and as inert as a 1/144th HEU stainless steel ingot.Weapon-grade plutonium could only be considered 'easier' to acquire, if money were no object, and you were ignoring everything that's so abundantly negative about it, due to being in a nuclear arms race--which unfortunately is what happened.But for a small and hopefully secret A-bomb program, HEU is always going to be a vastly less visible, expensive or risky option, and that is the point.
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