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Fool grade plutonium

Page history last edited by Andrew Alder 13 years, 1 month ago

Fool grade plutonium is plutonium with more than 7% but no more than 19% Pu-240, and the rest almost entirely Pu-239.


Plutonium with this composition can go bang, but isn't likely to. That is to say, in theory you can make a bomb with it, and the Yanks did in 1962, but it's a lot cheaper and easier to make a weapons grade plute bomb from scratch than to make a bomb of this stuff, even if you got it for free. Weapons grade plute is no more than 7% Pu-240.


The problem is, Pu-239 is the stuff that makes the bomb go bang. But Pu-240 (and to a lesser extent Pu-242) is bad plute from a bomb maker's point of view. It tends to make it go fizzle instead, and unpredictably, like maybe as you're building the thing. As the point of making a bomb is normally to kill someone other than yourself, this will not do. The Yanks tried at one stage to build a plant to separate out the good plute from the bad plute, and gave up. Again, it turns out it's a lot easier to make good plute from scratch.


There's a third grade of plute that is even harder to set off (in theory you can make the kitchen sink go critical if you harness the energy of a few black holes to do it, so you need to be careful about saying impossible). This is reactor grade, which has more than 19% bad plute.


Or at least it has had since 1976, and therein lies a fascinating story.


So why get excited?


It's important to know the difference, otherwise you waste a lot of time and money trying to stop people from making bombs which won't work anyway, and while you're busy trying to stop them from doing things that won't work, just maybe they'll do something else which will.


Weapons plute is made in reactors designed to make it. Reactor plute is made in power stations. Same chemical element. Different isotopic composition. Or to put it in small words, it is not the same stuff.


So why three grades?


Well, up until 1976, there were only two. The predecessors of the US DOE (the AEC and later ERDA) used to call anything with more than 7% bad plute reactor grade. This was logical; Nobody is going to make a bomb with any of this stuff, as we said above. It's all good reactor fuel, and all good for nothing else.


In 1976, ERDA decided to split what they'd previously called reactor grade into two grades, and call one of them reactor grade. So we have a reactor grade (1962 definition), which is anything less than weapon grade, and a reactor grade (1977 definition), which is what comes out of US power stations. Neither is useful for making bombs.


At least, nobody is going to make a bomb that they want to use against their enemies out of any of this stuff. But in 1962, as we also said above, the Yanks made a "low yield" bomb out of less-than-weapon-grade plute, made in a power station, for God knows what reason, and successfully test exploded it, probably because it wasn't good for anything else.


Exactly what sort of plutonium was this? Good question. One more people should ask. Well, they couldn't make it from US power station plute, because firstly that was illegal by their own laws, and secondly they didn't have any facility for reprocessing the stuff from power station fuel anyway, and thirdly their power stations are nearly all the sort that produce reactor grade (1977 definition) plute anyway, and nobody has ever built a bomb from that stuff (even for God knows what reason) and nobody is ever likely to (or from the kitchen sink for that matter). So they got some plute from the Brits instead. The Brits made it in their Magnox reactors, which were designed to produce both electricity and the plute for the British bombs, in nearly any grade you like to ask for. Neither the Yanks nor the Brits have ever said what the composition of this particular lot was, but blind freddy and a couple of unauthorised sources all agree it was a lot less than 19% bad plute.


But it wasn't weapons grade, and it was made in a power station. And that, in politics, is itself an explosive combination.


Then, in 1977, Jimmy Carter decided he wanted to ban the reprocessing of the spent fuel from the Yanks' power stations. There are lots of reasons you might want to do such a stupid thing... You might want to waste perfectly good fuel by burying it, so that you have to buy a lot more uranium (and that's good if you're a uranium miner), or you might want to increase the amount of waste you need to bury and the amount of time you need to worry about it (and that's good if you want to argue that nuclear power is expensive and bad for the environment), or you might want to leave deposits of plutonium for future generations to dig up and make their own bombs - the plute grade improves with age, plute-239 has the longest half-life, so the others just go away and leave it there (and that's bad).


The most likely reason to want to ban reprocessing is just that you and/or your supporters hate the nuclear industry, and want to tie them up in any sort of restriction you can, and the hell with the consequences.


But Carter wanted a good reason. And there are some very good reasons for not reprocessing. The obvious one is cost - in 1977, and up until the present day, there is a glut of uranium, so it's cheaper to use new uranium than recycled uranium. But this thinking leads down a slippery slope. The obvious next question is, if it's not economic, why would anyone want to recycle nuclear fuel anyway? And it turns out it's largely for idealistic reasons: Reduced waste, reduced environmental footprint, and so on. This then raises the next question: Shouldn't the environmental movement then demand that uranium be recycled, rather than opposing it? And this a Question You Must Not Ask.


No, blind freddy (remember him?) said that cost wasn't a good reason to give for the decision to ban reprocessing. The next most obvious one is do to with timing... it's a lot easier to reprocess fuel after a few decades, when it's less radioactive so the plant needs less shielding. And some of the fission products are noble gases such as xenon and krypton, which you can't capture chemically, so they go up the reprocessing plant chimney if you do it now. But they decay in those few decades. So it makes sense to store the fuel as an asset... nuclear waste an asset? Hold it right there, this is another Forbidden Idea.


When it became obvious that none of the good reasons were, um, were the right sort of good reasons, Carter seems to have wondered, "is anyone stupid enough to think that the stuff that we made go bang in 1962 is the same stuff that we make in our own power stations?" (He was a nuclear engineer, so he knew better. But he was also a politician.)


And blind freddy (remember him?) said "it's worth a try".


So, they declassified a little of the information from the 1962 test. They said it was reactor grade, which by 1962 definitions it was, and by 1977 definitions it wasn't. When asked which definition they were using, they said "that information is classified, and regarded as being not in the public interest to release". Clever! Clever enough to fool anyone?




And that, dear Josephine, is why we call it fool grade plutonium. It's sometimes pronounced fuel grade, but it's not a lot better as reactor fuel than reactor grade (1977 definition) stuff is. So that's not nearly so good a name for it.


It's the stuff you can in theory make go bang, but wouldn't use in any serious bomb project, as opposed to weapons grade, which is useful for bombs, and reactor grade (1977 definition), which is what you find in spent PWR and BWR fuel. It's an in-between grade that they invented in 1976, for reasons that are also regarded as not being in the public interest to release. Even blind freddy (remember him?) doesn't know. God might.


But more important, it isn't the stuff in spent PWR or BWR fuel, but the widespread and mistaken belief that it might be is to this day the official reason that the Yanks don't recycle their PWR and BWR fuel, exactly as Carter wanted.

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