What stops all the uranium in the earths molten layers from sinking to the core, reaching critical mass and detonating? I know most uranium isn't fissible and the U238 has to be centerfuged out, but wouldn't billions of years being liquid + gravity be able to effectively do that as well?
Uranium is found in the earth's crust and oceans. I think most of it has been deposited by meteors. There is a mountain, I think in France, where it is believed a natural nuclear reaction took place for centuries. It just generated heat.
The long explanation: https://arxiv.org/ftp/astro-ph/papers/0606/0606634.pdf The short explanation: In very low uranium concentrations, such as those found in the earth's core, molten uranium will stay dissolved in molten iron. Thus, it doesn't separate out, much in the way that salt doesn't separate out of salt water at the bottom of the ocean.
Additionally, the gravity at the core is nearly zero. I wasn't even thinking of that! Noting that gravity is what causes things to separate by density.
Because uranium is a more chemically reactive element, so stays combined with oxygen. Together it ends up being a lower density than iron. So it's believed to float just above the core.
Uranium is not, in and of itself, explosive. Its density lends itself to the chain reaction we associate with a nuclear reaction, but that reaction is unlikely to occur in the environs within the Earth.
I think it's interesting that heat is an inhibitor in a nuclear reaction. That is, if a mass of uranium sitting on your coffee table is exactly critical, it could be made less than critical by raising the thermostat. (Don't try this at home.)
Yes, if it was precisely on the edge, that would be true. In the more general case, it's not the temperature of the uranium that matters, as uranium expands very little with temperature. It's the temperature of the water around the uranium that really matters. For sustained fission to occur, the chain reaction has to keep going. The chain reaction is: Fission occurs, producing gamma rays, atom fragments and free fast neutrons Free neutrons bounce off the hydrogen atoms in water molecules and slow down. The slow neutrons can then be absorbed by other uranium nuclei, causing more fission. If the water heats up, the water becomes less dense, it becomes less effective at slowing neutrons, and the fission reaction tapers off. That factor is used as one type of control method in most pressurized water nuclear reactors. It's what happened over thousands of years in that "natural reactor" at Oklo, Gabon. The "reactor" there would cycle as groundwater heated up and cooled down. https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor There was also an interesting nuclear accident in Japan. Techs mixed too much uranium into water, and the mixture went critical instantly. That heated the water, so the reaction stopped, then the water would cool, and it would go critical again https://en.wikipedia.org/wiki/Tokaimura_nuclear_accident