QuickCheck: Can nuclear reactors occur naturally?


NUCLEAR reactors are seen as one of the greatest technological marvels and terrors of the 20th Century, with the harnessing of fission reactions - the splitting of the the atom - to generate heat and electrical power being seen as one of the major points of the late 1940s and the 1950s as a whole.

However, is it true that nature beat mankind to the punch with naturally-occurring 'nuclear reactors' emerging billions of years in the past?

Verdict:

TRUE

The remnants of a naturally-occurring fission reactor was in fact detected in Oklo in Gabon, Africa in the 1970s

Writing on it in a 2011 Scientific American article, marine geology and geophysics scientist Evelyn Mervine said that the Oklo fission reactions went on for several hundred thousand years, albeit intermittently on a start-and-stop basis.

As to how the reactions occurred, Mervine explains that this occurred after many small bits of uranium-bearing minerals formed in a sandstone layer in what is now Gabon's Franceville Basin.

She added that the materials were dispersed until the sandstone became infiltrated with oxidizing waters around two billion years ago.

"These oxidizing waters dissolved the uranium-bearing minerals and concentrated the uranium in several deposits towards the top of the sandstone layer," said Mervine, adding that uranium actually became extraordinarily well-concentrated.

"Fission of uranium could have begun when the uranium concentration reached 10%; the Gabon uranium deposits in which natural nuclear reactors developed contained about 25% to 60% uranium," said Mervine.

As for how the whole process operated, Mervine said that the final element was when water could flow down to meet with the naturally occurring uranium-235

"Water was able to percolate into the permeable sandstone containing the uranium deposits, and this water acted as the neutron moderator," said Mervine.

"There were also no significant quantities of neutron-absorbing elements to inhibit the self-sustaining fission reaction.

"All of this provided the perfect recipe for a natural nuclear fission reactor," she added.

A neutron moderator is a material that can slow down fast neutrons such as those produced by splitting elements like uranium-235 to a point that they make fission reactions more efficient.

It has to be said though that the power produced by such naturally-occurring fission reactors is far lower than man-made nuclear power sources, a fact pointed out by Mervine.

"The average power output of the Gabon reactors was about 100 kilowatts, which would power about 1,000 lightbulbs.

"As a comparison, commercial pressurized boiling water reactor nuclear power plants produce about 1,000 megawatts, which would power about ten million light bulbs," said Mervine.

And no, one does not need to fear the existence of such radiation sources in the present day as the relative density of fissile uranium has now decayed below that needed for a sustainable reaction.

References:

1. https://blogs.scientificamerican.com/guest-blog/natures-nuclear-reactors-the-2-billion-year-old-natural-fission-reactors-in-gabon-western-africa/

2. https://apod.nasa.gov/apod/ap021016.html

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