I understand that the Earth is kept warm in part by radioactive decay. But with the Earth being largely sort-of-fluid inside,wouldn’t the heavier, radioactive elements migrate to the core and achieve a critical mass? Thus releasing heaps of energy like at Oklo?
I doubt it. To get a chain reaction from enriched uranium[sup]1[/sup], you need a moderator, i.e., a substance capable of slowing down the fast neutrons released from a fission event so they can react with other uranium nuclei. In the case of Oklo, the moderator was groundwater that seeped in to the ore. It’s believed that the natural reactor operated in cycles: groundwater seeps in, which moderates the neutrons, which starts the reaction, which heats up the ore, which drives out the groundwater, which stops the reaction, which cools off the ore, which allows the groundwater to seep in again. So deep in the earth’s core, the lack of water (or other moderators) would make a natural uranium pile unlikely to occur.
[sup]1[/sup] The uranium present on earth two billion years ago was “enriched” in the sense that the natural abundance of [sup]235[/sup]U was about four or five times higher than it is now — it decays with a half-life of about 700 million years. So by our standards, it would be “enriched.”
I was wondering if molten iron might be the moderator - some reactors use molten sodium.
Also worth pointing out that the effects of gravity grow fainter the deeper you go inside the substance of the Earth, for all intents and purposes, because all the mass of the earth that’s ‘above’ the spherical layer you’re at will cancel out entirely. Heavy iron does tend to concentrate in the core of the Earth, or the rest of the Earth tends to remain in concentric layers around the heavy core. But there’s very little gravitational pressure that would tend to force even heavier, denser impurities towards ‘the core of the core.’
side bar, please: what is the Oklo thing you’re talking about? some sort of natural reactor?
Liquid sodium is used as a coolant in fast neutron reactors like the Fast Breeder Reactor, Integral Fast Reactor, and Liquid Metal Cooled Reactor precisely because it is not a neutron moderator. Moderated (“thermal”) reactors like a Pressurized Water Reactor, Pressurized Heavy Water Reactor (PHWR or CANDU), Boiling Water Reactor, Advanced Gas Cooled Reactor, et cetera, use a moderator to slow neutrons down so they can be captured. The moderator used is often pyrolytic graphite because of its stability and robustness and the coolant is water or helium, but in heavy water (deuterium enriched) reactors the coolant is also the moderator.
I’ve never considered or seen discussion on using liquid iron as a moderator but I suspect it would work poorly for that purpose owing to the relatively low ratio of neutron cross-section to molar mass.
Even at 1G gravity, can you purify uranium or plutonium by just melting some ore and taking what settles at the bottom? Because that seems to be the equivalent of what the OP is suggesting.
A “nuclear core” hypothesis has been proposed by Marvin Herndon, but it certainly isn’t widely accepted.
A 2002 article from Discover magazine:
And, of course, Wikipedia:
Pretty much. Basically, if you get rich enough uranium ore and enough groundwater to moderate the neutrons released when the U-235 decays, you can get critical mass in uranium deposits. This couldn’t happen in natural uranium ore today, because too much of the U-235 has decayed over the lifetime of the Earth; but two billion years ago, when there was more U-235 kicking around, it apparently occurred in uranium deposits in Gabon.
Well, that’s not either of the major ways that the Manhattan Project settled on to purify uranium. Since that’s a really simple and obvious method to try, I’m fairly confident in assuming that the Project correctly ruled it out as a way to produce uranium sufficiently enriched for bomb use in a human timescale.
That doesn’t mean it’s not possible on geologic timescales, though.
so would this Oklo natural reactor have been detectable from the surface? would it have been like a radioactive Old Faithful? the wiki link says it had a cycle of a couple of hours. any idea how long it lasted for?