so on top of that 1%-ish gallium (which stabilizes delta phase, the least dense one, that collapses under pressure to alpha, the most dense one, which allows slightly less compression to be used, and this means more compact weapons), which would need to be separated before that plutonium gets turned into MOX, i understand that most of american plutonium stockpiles are somewhere around 94% 239Pu, and some are 97%+. i guess they’re selling the worst stuff, which would make little difference for reactors, and if there’s anything that american military is good at, it’s logistics. on production side, the very shortened story is that we start with uranium and irradiate it, which gives

238U (n,gamma) 239U (beta) 239Np (beta) 239Pu

and that’s it. but it really is a bit more complicated, because first, nothing stops that just made plutonium from reacting further, so some of it will fission which means that it can’t be recovered, but it also means more neutrons, which will also make some plutonium, so it’s not all bad. but not all plutonium will fission, and so there’s entire series of reactions:

239Pu (n,gamma) 240Pu (n,gamma) 241Pu (n,gamma) 242Pu (n,gamma) 243Pu

at this point reaction stops because 243Pu is very short-lived, and it decays into 243Am. at the same time, if neutrons react without leaving fuel pin, they won’t be slowed down much, which means another reaction is possible (but much less likely):

242Pu (n,2n) 241Pu (n,2n) 240Pu (n,2n) 239Pu (n,2n) 238Pu

there’s also 238U (n,2n) 237U (beta) 237Np (n,gamma) 238Np (beta) 238Pu

of these, 238Pu and 240Pu have unacceptably high neutron emission rate, which means that if there’s too much of these nuclear weapon is likely to predetonate. because the most important use of plutonium in modern advanced thermonuclear weapons is in primary, this means that it could be so that entire weapon fails to function if there’s too much of these contaminants (low-kt or even sub-kt yield instead of, say, 350kt). of these, 238Pu and 241Pu have short halflives, which means that plutonium containing these will heat up with considerable power. this can damage explosives bonded to it. 240Pu is additionally a radiation hazard because of neutrons emitted, but it’s only really relevant for submarine crews. this is why these weapons use the better 97%+ grade plutonium, and additionally some of that 97% grade was made in order to blend with some older, worse quality stocks

there’s remarkably few parameters that can be used in order to steer these reactions in the way we want. about the only relevant one here would be neutron temperature, which is really chosen at reactor design stage and increasing it means that neutron capture is less likely. this makes fission more likely, more neutrons are present and more plutonium can be formed. this also turns reactor into fast reactor which are notoriously hard to build and iirc only russia and india operate large fast reactors today. short of that, about the only way to prevent 239Pu from reacting further is to take it out of there, which means low burnup and only tiny amounts can be recovered per run. from what i understand, the choice of 94%-ish 239Pu content is end effect of massive optimization problem focused on how to make a pit at the lowest price. this includes all the (expensive, slightly dangerous) labour it takes in reprocessing fuel and how required pit mass increases with lower quality plutonium

little of that matters when running a powerplant. some isotopes being neutron emitters are actually an advantage because it makes startup smoother. the longer fuel stays in reactor the less fissile plutonium there is as a result, and the more advanced reactors allowing higher burnups make plutonium recovery less attractive, or, in other words, most of benefit of recovering plutonium in order to put it in new fuel can be realized just by leaving fuel in reactor for longer time in the first place. in regular light water reactor, steady state develops where plutonium is formed as fast as it is consumed, and it accounts for about third of energy released when that steady state sets in. this also means that about third of fissile uranium can be replaced by fissile plutonium with no modifications to reactor

on top of weapons use, some countries do reprocessing anyway as a matter of policy as a hedge against future shortages of uranium. some of these schemes require fast reactors which can burn these isotopes useless in regular reactor and make fresh, weapons grade almost pure 239Pu, which limits countries that can make it work, by diplomatic means, only to already established nuclear powers, and labour costs needed for its operation limit it to currently only india and russia, and formerly france. this is because at any burnup, in best case plutonium can be only separated once from light water reactor fuel and used in light water reactor. after that, plutonium quality is too low to be useful this way

in comparison to reactor grade plutonium, weapons grade plutonium would allow to make more fuel per kg of this material, but also it would be astronomically expensive compared to regular stuff, which is already unprofitable for power generation in countries with western labour costs, unless it’s a fire sale. the second problem is that if more than this 1/3 of fissile isotopes in fresh fuel is plutonium, then generally reactors would need recertification or maybe completely new design, because plutonium gives less delayed neutrons which are critical for smooth control of reactor power. this is, mind you, in context of country that practically stopped building new nuclear powerplants. if small reactors are on the table, then these naturally need higher amount of fissile isotopes, like 20-30%, and something tells me people involved would like to try this

another thing to note here is that there’s an old american policy in place that forbids (domestic) plutonium recovery for alleged (international) nonproliferation reasons. i don’t believe this is the case, it makes zero sense, and instead i think that nuclear industry wanted a way out of that money pit without state backing, because plutonium recovery and reuse for power generation is more expensive than just buying more uranium, sustainability of this energy source and volume needed for nuclear waste be damned. they could get away with this because military weapons grade plutonium stockpiles were assessed as sufficient. now they’re pawning heirloom nukes to monorail salesmen instead