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I had a similar idea for repurposing an old mining operation. You take a closed gold, uranium or copper mine which is 1000-2000 meters deep. You create a nuclear rector at the bottom with "fire and forget" design. Only robots can do the maintenance. After 60 years you disable it, pour concrete into the shaft and move to the next location. If it melts, who cares? It's not gonna poison water supply.

Why was this not built before? I have no idea. Maybe robots are essential, we didn't have deep enough mines that were abandoned, energy was cheaper, no push for no emissions, it's not economical, other risks.

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In France we’re storing highly radioactive items underground. Here is how dissipation works:

- After a few dozen years, the concrete is expected to breach,

- Radioactive atoms mix up with soil and dissipate both upwards and downwards, mostly thanks to water,

- After 400 years and for thousands of years, they reach the surface, where they should be diluted enough to not be dangerous,

So I guess having badly contained radioactive containers would be much worse.

One thing to remember is that pressure underground is extremely high (stone weighs a lot more than water, and light rock tends to “float” onto denser rock). If a melted reactor were squeezed, it would spread materials into the soil much quicker.

You need a hot and cold reservoir to generate power. Where is the heat going to go? This is why power plants are often built on rivers or near the ocean.

you need to pump water, probably flooding 80% of the mine. Since there are no humans involved 70+ celsius is not a problem.

The heat still has to be managed, otherwise Kyshtym[1] happens. 160 tons of flying concrete is no joke.

https://en.m.wikipedia.org/wiki/Kyshtym_disaster

The footprint of a nuclear power plant is pretty huge. That machinery is generally not optional even if doing a "fire and forget" design. Underground space would come at a premium cost, likely far more than could be saved on reactor design.

Also nuclear power plants obviously generate an absolutely massive amount of heat that needs to be dissipated, a task that would be difficult and expensive underground.

sure, but it doesn't have to be U-235 (thorium maybe?). It's more geothermal power plant with fission booster.

inb4: not a specialist. AFAIR remote control robots were tried when Chernobyl disaster was being cleared up, and the radiation destroyed the electronics. But perhaps the shielding technologies progressed enough to mitigate that.

Having nuclear stuff underground requires extremely precise geological surveys, so that the stuff does not wind up in aquifer.