No, actually the graphite core reactors are fast-neutron reactors, ideal for breeding Pu-239. The light-water reactors we generally use for power are slow-neutron reactors, and create much less Pu-239. That is good for non-proliferation reasons.
These reactors are
Yup, the Wigner energy needs to be released periodically. The Windscale reactor fire in Scotland in the '50s was a really nasty mess, a precursor to what the Chernobyl disaster could be.
I think the Pakistani device is U235. The Indians may have made some U235 devices, but I think they now have the Pu239 production capability.
If they're fast reactors, then what's the graphite for ? They're either thermally moderated, or they're a mixed mode. Power reactors (and keeping the reactor operating) uses thermal neutrons, but if it's operated as a "dual use" reactor then the fast flux is still appreciable and is used for breeding.
The USA uses light-water PWR's because Rickover's Navy bankrolled the development costs. Then the monopoly advantage steamrollered the rest of the world. The British designs went their own way for a couple of generations, but couldn't hold their own beyond the '70s. Only the French really have an indigenous nuke design industry remaining - everyone else is limited to license-building US designs, if they're lucky.
You're confusing temperature and void coeffcients. Temperature isn't a problem - the thing is massive, it only heats up slowly. The real gotcha is where water is used as a coolant - boiling can be rapid (especially with a sudden loss of pressure), and changes the reactivity in a way that the control system can't cope with.
What does Wigner energy have to do with a positive temperature coefficient ?
Windscale isn't in Scotland.
No relation at all.
Chernobyl was a stack of failures on top of each other. Bad operations, and a design that simply couldn't have a "small" accident. But it's far-fetched to draw major comparisons between it and either the Windscale air-cooled piles or the Windscale Pu-producing Magnoxes.
And by that fallacious argument, "Iraq was within reach of nuclear weapons, they just needed to get some fissiles"
FFS - Half of Usenet is within reach of nukes if someone hands them enough HEU.
The US, China and Pakistan went for uranium weapons first, but this is somewhat unusual. I doubt if a "rogue state" would try it now, as U enrichment plant is harder to hide than a reactor.
Windscale 1 was a _low_ temperature air cooled graphite pile. Chernobyl was a _high_ temperature water cooled graphite pile. The Wigner energy gets released from the graphite crystaline matrix at about 250 C. Wigner energy was not implicated in the Chernobyl event, although it could be of relevance during decommissioning (only to the extent that the graphite needs to be annealed in a non oxidising atmosphere.
Uranium isn't particularly good for a dirty bomb, you ideally want some isotopes that have a half-life of under a few years, and some nasty decay mode that will cause big biological effects.
Ground up spent fuel rods are good, as are some hospital radiological sources. It's really hard to make a dirty bomb that will cause large numbers of casualties. Oone that will cost many tens of millions to cleanup, and kill a few is much easier.
No kind. Uranium has a long half life, that means it isn't very radioactive. For high enough activity to be scary, you need a material with a short half life, and if it is bioaccumulated, so much the better.
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