Oklo-phenomenon - is this a Natural nuclear reactor?

[gjonesy]

I read a very interesting article about this u235 deposit found in Africa. Everything I have read about it suggest that some of the components needed for this to naturally occur can not exist without human intervention. The water involved in the nuclear reaction must be extremely pure. Even a few parts per million of contaminant will “poison” the reaction, bringing it to a halt.

is there any possible natural explanation?http://www.scientificamerican.com/article/ancient-nuclear-reactor/

https://www.deepdyve.com/lp/springer-journals/the-oklo-phenomenon-RA0Na6j0gD

thanks for you input, any ideas on this would be greatly appreciated.

 

[mfb]

Even a few parts per million of contaminant will “poison” the reaction, bringing it to a halt.

Please give a (credible!) source for that claim. And another source discussing why this should be an issue.

is there any possible natural explanation?

Obviously, as the reaction happened. There are always some details left to explore, so what? For every natural phenomenon there is a crackpot site claiming this cannot be natural, that is irrelevant.

Note that natural nuclear reactions are a rare phenomenon. There are many uranium sources, existing over billions of years. Oklo at this specific time is not everything that ever existed.

 

[QuantumPion]

The water involved in the nuclear reaction must be extremely pure. Even a few parts per million of contaminant will “poison” the reaction, bringing it to a halt.

I don't see why thus would be the case. Regular power plants, with similar U-235 enrichments, run at much higher power levels with 1000+ ppm of boron, plus whatever other materials are in the reactor structure and coolant. There are only a few natural elements which are strong neutron absorbers - boron, silver, gadolinium, etc. Most abundant light elements like silicon, carbon, aluminum, etc., which is what the minerals at Oklo are made of, are not.

 

[gjonesy]

I believe what the article was referring to was a reaction with unenriched u235 and u238. Pure heavy water.

Wikipedia › wiki › Heavy_water

Can this type of water occur in nature.

 

[mfb]

Pure heavy water? That doesn't exist in nature. It is not necessary.
The uranium back then was "enriched": the fraction of U-235 is going down over time as it has a shorter lifetime than U-238. 2 billion years ago the fraction was similar to the ratio today's reactors use.

 

[gjonesy]

Ok just so i can get a better understanding myself, the confusion over this phenomena is due to the decay of the radioactive materials naturally over time? My initial take was that this phenomena mistaken for something else that had actually occurred. Misidentification of the processes or another undicovered plausible explanation all together.

 

[QuantumPion]

No, what they found at Oklo was a U-235 concentration significantly lower than any other uranium deposits, along with telltale traces of fission product decay products.

When you talk about decay of radioactive materials, there are two different things going on. First, the amount of U-235 is gradually decreasing over time because it has a shorter half life than U-238. While today only 0.7% of uranium is U-235, a couple hundred million years ago there was much more, enough to allow natural fission reactors with normal water to occur. Second, the other aspect of radioactive materials is reference to the telltale sign of fission occurring the past, due to the presence of certain elements and isotopes which only are produced by radioactive decay of fission products.

 

[gjonesy]

Well all the information on topics like this although interesting can contain less than accurate information. That's why these forums are good for painting more accurate picture of things of this nature.

thanks

 

[SteamKing]

No, what they found at Oklo was a U-235 concentration significantly lower than any other uranium deposits, along with telltale traces of fission product decay products.

When you talk about decay of radioactive materials, there are two different things going on. First, the amount of U-235 is gradually decreasing over time because it has a shorter half life than U-238. While today only 0.7% of uranium is U-235, a couple hundred million years ago there was much more, enough to allow natural fission reactors with normal water to occur. Second, the other aspect of radioactive materials is reference to the telltale sign of fission occurring the past, due to the presence of certain elements and isotopes which only are produced by radioactive decay of fission products.

To be sure, the reactor at Oklo is thought to have gone critical about 2 billion years ago. The half-life of U-235 is about 700 million years, so there was a much richer proportion of U-235 in naturally occurring uranium then than today's 0.7%.

 

[gjonesy]

The French scientists that discovered the discrepancies in the U235 said it was 0.717 % but should have been 0.720 % seems like a insignificant difference.

note: I'm not a physicist so I don't know the importance of these ratios. I'm not even sure of the accuracy of the information.

 

[mfb]

The French scientists that discovered the discrepancies in the U235 said it was 0.717 % but should have been 0.720 % seems like a insignificant difference.

This just means that only a small fraction of the available U-235 fissioned.

2 billion years ago, the natural fraction of U-235 everywhere was about 5.600%. In Oklo, some U-235 fissioned, in this particular sample the fraction went down to 5.577%. The chain reaction stopped, the uranium then decayed as everywhere, and today the observed fractions are 0.720% outside Oklo, and 0.717% in this particular sample. In other places, more uranium was split, but the idea is the same.

Note: the 2 billion, 5.600% and 5.577% numbers are not very accurate, but the ratio is quite precise.

 

[SteamKing]

The French scientists that discovered the discrepancies in the U235 said it was 0.717 % but should have been 0.720 % seems like a insignificant difference.

note: I'm not a physicist so I don't know the importance of these ratios. I'm not even sure of the accuracy of the information.

I think it indicates that at the Oklo mine, the concentration of U-235 found at the site was slightly lower than would have occurred had the amount of the U-235 isotope not been affected by having some of it fission into other elements.

 

[nikkkom]

The French scientists that discovered the discrepancies in the U235 said it was 0.717 % but should have been 0.720 % seems like a insignificant difference.

It is significant.

Isotope ratios of most elements are very stable across the planet, because isotopes are chemically nearly identical, and material was pretty thoroughly mixed during our planet's creation.

A variation in isotope ratios has to have an explanation. Some variations are caused by production of a specific isotope via nuclear decay (radioisotope rock dating method uses this). For light elements, slight variation may be induced by chemical separation (e.g. water vapor has a bit less deuterium % than the water it was produced from).

Uranium's isotope ratios are checked not only by scientists, but also by security/intelligence agencies, in order to detect clandestine enrichment programs. "Bad guys" need to leak just a tiny, tiny amount of their uranuim to be caught.