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Comment by fritzo | Hacker News Reader

Geothermal energy can be seen as just natural nuclear energy, where the fission reactions are safely isolated deep underground.

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In like fashion, we can see wind, solar, and hydro as just natural nuclear energy, where the fusion reaction is safely isolated at eight light-minutes' distance through space.

We can see fossil fuel as nuclear energy, as the molecular bonds we break were formed by said fusion reactor millions of years ago

Oh, well, that is a clever twist! Light of the ancient sun, indeed.

The claim here is that 80% of it is nuclear and the other 20% heat comes from gravitational accretion

https://petrowiki.spe.org/Geothermal_energy

Doesn't most geothermal heat come from radioactive decay and gravitational potential energy of the planet as it formed, rather than reactions involving multiple particles? I'd have agreed if you'd said "radioactive substances" rather than "fission reactions".

Natural radioactive decay is also an example of fission - just not a fission chain reaction (… except when it is! https://en.m.wikipedia.org/wiki/Oklo )

Sure, minus fuel processing, disposing of processing waste, transportation of fuel, inspection of the vessel and plant, maintenance of the vessel and plant, storage and/or reprocessing of used fuel rods and transportation of used fuel rods.

I think the OP agrees?

Have we ever thought about building a nuclear reactor underground before? Seems a little insane, but I'm trying to think through the "why" part.

It would be awfully convenient if the way you "store" the nuclear waste is to just leave it in the reactor underground and then build another one!

Immediate problem that comes to mind: Groundwater contamination.

Also it being a pain to build (probably?).

Anybody aware of if anybody has explored this before?

So there are people working on that [1]. It's basically geothermal where the heat is generated by a nuclear reactor. If anything goes wrong, you just bury it. If it's geologically stable, not near a water table and a mile deep, then there's a certain logic to that.

[1]: https://eepower.com/news/underground-energy-how-this-mile-de...

Sounds neat, but isn't like the number 1 most important thing at a reactor keeping it cooled?

You can do that underground in Klei’s Oxygen Not Included — synthesize some supercoolant, run it through an aquatuner, eat up the waste heat with steam turbines, maybe throw a wheezewort someplace for good measure… but I might be missing some steps.

Water is generally used to cool nuclear reactors. And you're heating water so it's kind of a 2-for-1.

You could even have a separate cooling loop with a heat exchange with the water you're boiling to avoid any radioactive containment, I guess?

I don't know what the failure modes for this look like. These are small reactors. Is there enough fuel for it go critical? I would assume not. What happens if there's a containment breach? Will it start releasing radioactive steam?

The whole thing is still at a conceptual stage.

That idea was totally crazy. In order to save on pouring lots of concrete for a containment dome, you dig one mile in bedrock.

But one thing that everyone missed when the story came out was what was so special about 1 mile. Why not half a mile or 1.5 miles? The neat thing (which they did not bother to publicize) was that the pressure in a pressurized water reactor is exactly the pressure at the bottom of a column of water one mile high. So you do not need pumps for your reactors. You just let your water fall to the reactor, the reactor heats it and sends it back. It's quite elegant. Of course, the remaining engineering details are left to the reader. For example, what happens if some valve breaks 1 mile deep underground?

dimensionally it's hugely advantageous. a 1600-meter-thick containment dome (to use modern units rather than medieval ones) would be 17 billion cubic meters of concrete, which would cost about 700 billion dollars, at the price of 40 dollars per cubic meter i happened to see in the newspaper today. but a 300-mm-diameter hole 1600 meters into the bedrock only requires breaking up 110 cubic meters of bedrock. all things being equal, that's about 150 million times easier

(in practice people skimp on the containment instead of trying to build trillion-dollar containment domes)

as for water pressure, you can design pwrs to work over a significant range of water pressures

Reactors have to be refueled periodically. This might be much more difficult if the reactor is deep underground, or at least it might complicate the design. Unless the design and construction can be simple enough that it's cheaper to make a new one than refuel an existing one.

The plan for old HI nuclear material in USSR in a decommissioned plant was to "settle in place" for a hundred year (after that its mostly MI/LI that you can glass easily). If you have enough space, the "return to green" that is the goal in France (10-30% of the lifetime costs depending on who you listen to) is too expensive for little benefits.

Doesn't seem like a particularly helpful or practical way of categorizing fuel sources. Would you then place wind as a subset of solar, and solar as a subset of nuclear?

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