Comment by acidburnNSA | Hacker News Reader

Comment by acidburnNSA | Hacker News Reader

The Oklo reactor(s) in Gabon are such a wonder. I have to be careful to say "The world's first man-made chain nuclear reaction was achieved in a squash court at the University of Chicago in 1942."

While Oklo georeactors are relatively well-known now, have you all heard of the crazier hypothesized georeactors? Like that the Moon may have formed in a nuclear excursion? [1]

[1] https://phys.org/news/2010-01-moon-nuclear-explosion.html

Or the case for a terracentric georeactor (uranium is dense and would go down...) [2]

[2] https://www.pnas.org/content/100/6/3047

Uranium-235 decays faster than U-238. The current natural enrichment is 0.711%. 4 billion years ago it was 25%, well above levels necessary to make a chain reaction even in far less than ideal conditions.

Somewhat related, it also continues to blow my mind that about half of the internal heat budget of Earth comes from radioactive decay of primordial nuclides (while the other have is just what's left over from residual cooling) [3]. So geothermal energy is really just a gigantic Radioisotope Thermal Generator.

[3] https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget...

Geologist here! Earth's internal heat budget [1] is a really cool subject since it is surprisingly hard to constrain, yet influences the operation of pretty much the entire solid Earth system (plate tectonics is entirely driven by secular cooling and radiogenic heat production).

Even using the current best estimate that about half of the current 42-46 TW geothermal heat flux is primordial and half is radiogenic, there's a bit of a mismatch if you try to extrapolate back in time* the solution to which may involve either a hotter core than previously expected, anomalously high present heat flux, or more errors in the way we're currently modelling heat loss through the mantle.

*To put it simply, imagine stepping back 1 Myr from the present. The ~half of the heat lost over that period that was _not_ radiogenic came from cooling the core and mantle, so the core and mantle should have been hotter 1 Myr ago. But if the mantle was hotter, it should have been less viscous -> convecting faster -> losing heat even faster in the past. If you keep extrapolating like this, it doesn't take too long for the extrapolated mantle to become physically implausible -- only about 2 or 3 Gyr out of the 4.56 Gyr we have to work with. There are various ways to solve this problem, but we don't know which solution or combination of solutions is what really happened.

Unfortunately there isn't a really good recent review paper on this, but for anyone interested here's a scattering of citations [2-5] that have most of the components to piece together.

[1] https://en.wikipedia.org/wiki/Earth's_internal_heat_budget

[2] doi.org/10.1029/JB085iB05p02517

[3] doi.org/10.1029/2003GL016982

[4] doi.org/10.1016/j.pepi.2020.106457

[5] scholar.google.com/scholar?cluster=7130495919742279897&hl=en

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