Oklo, the Earth's Two-billion-year-old only Known Natural Nuclear Reactor (2018)
20 comments
·June 20, 2025null
wjnc
This article could be so much better: How large are the estimated stores of ore that underwent natural fission? How much energy did it release and over how much time? When? Would this be noticable (and to whom)? So many questions, so little information.
I only know (or knew) high school physics, and when entering this in Claude I get an answer but am unable to verify the answer. Claude says 680 kWh gained per 0.03 grams of U-235 lost due to fission. I am left wondering into what the U-235 fizzed into (sorry, pun) and if I should take that into account.
Edit: There we go with modernity. I went to Claude instead of Wikipedia. Wikipedia at least has the answers. Thanks u/b800h. 100kW of heat on average. I can start filling in the blanks now.
adev_
The 'natural reactor' in Oklo has been discovered by some french researcher from the CEA in the 70s.
There is an entire scientific publication on the topic if it interests you:
https://www.sciencedirect.com/science/article/abs/pii/S00167...
the_arun
I wonder why Claude’s answers aren’t equal or better than Wikipedia - assuming Wikipedia is one of the training datasets. Is the temperature causing it to be probabilistic & other sources are carrying more weight?
danielbln
Wikipedia is the best first point of entry, but if you do use Claude, just tell it to do web search for you: https://claude.ai/share/73e67582-3e03-454b-aa12-e8906bd7b3fd
pfdietz
Uranium was very enriched back at the formation of the Earth, so for a given geometry it would have been much more reactive.
However, uranium ores are often formed due to redox processes, since U(VI) is much more soluble than U(IV). So maybe concentrations wouldn't have been as common back before the Great Oxygenation Event about 2.4 Gya. Still, that leaves ~600 Mya between that point and this reactor, which would be not quite one half life of U235.
Aardwolf
> All natural uranium today contains 0.720% of U-235.
That's related to the material of our solar system all coming from the same supernova explosion or similar, right? Does this apply to our entire milky way or just the solar system? What if parts collided with material of _other_ origins and some of that is on Earth, then there could be different mixes, right?
philipkglass
It's related to the age of when the uranium was formed:
https://world-nuclear.org/information-library/nuclear-fuel-c...
We can calculate the abundances of U-235 and U-238 at the time the Earth was formed. Knowing further that the production ratio of U-235 to U-238 in a supernova is about 1.65, we can calculate that if all of the uranium now in the solar system were made in a single supernova, this event must have occurred some 6.5 billion years ago.
This 'single stage' is, however, an oversimplification...
The really interesting thing is that phrase "the production ratio of U-235 to U-238 in a supernova is about 1.65"; the now-rare U-235 is actually more abundant than U-238 in the immediate debris of a supernova. It's just prolonged aging that has preserved more U-238 (half life 4.47 billion years) than U-235 (half life 0.704 billion years). If Earth had been formed with uranium that rich in U-235, there would have been Oklo events all over the place. Uranium wouldn't need isotopic enrichment to be used as fuel in light water reactors. Fission and radioactivity would probably have been discovered early in the 19th century, soon after the element itself was recognized, because any substantial quantity dissolved in aqueous solution would have gone critical.
lazide
It’s interesting to extrapolate that to the early earth - radioactive decay and fission interactions likely play a much larger role than we are able to reliably model. Okla is somewhat unique in that the formation survived for us to dig it up - most from that time would not.
mandevil
This is just in our little corner of the Milky Way, but not thought to be the result of just one supernova. I last looked into this about a decade ago so I might be behind the times, but at that time the most popular theory was that the cloud that became our Solar System was the result of thousands of supernova scattering and mixing atoms, across both the first two generations of stars (the Sun is considered to be a third-generation star), and that mixing is thought to be an important factor in making it complex enough to have rocky inner planets, gaseous outer planets, etc.
BurningFrog
Grok says: At Earth's formation ~4.5 billion years ago, natural uranium contained approximately 23.2% U-235
These numbers are probably only for the local corner of the galaxy. It depends on when the supernova(s) that created the uranium exploded.
_Algernon_
We all have access to Grok and other AI models, and we will ask it if we want it's bullshit hallucinations. There is no point polluting HN with this trash.
boothby
Once, I thought about implementing a project to make ai-crapified versions of major open source projects in order to poison the training data of future models. My error was thinking that people wouldn't be gullible enough to become breathless mouthpieces of LLMs despite their obvious flaws.
kkwteh
Maybe it’s a remnant from a nuclear ancient civilization.
julienchastang
A civilization (even perhaps extraterrestrial) that possessed nuclear energy? Unlikely, but still fun to think about! ;-)
eabeezxjc
Or a remnant of a nuclear war in a riotous time
saltserv
[dead]
The Oklo region has now-exhausted Uranium deposits.
From Wikipedia:
"Some of the mined uranium was found to have a lower concentration of uranium-235 than expected, as if it had already been in a nuclear reactor. When geologists investigated they also found products typical of a reactor. They concluded that the deposit had been in a reactor: a natural nuclear fission reactor, around 1.8 to 1.7 billion years BP – in the Paleoproterozoic Era during Precambrian times, during the Statherian period – and continued for a few hundred thousand years, probably averaging less than 100 kW of thermal power during that time. At that time the natural uranium had a concentration of about 3% 235U and could have reached criticality with natural water as neutron moderator allowed by the special geometry of the deposit."