Posted on 11/23/2004 4:11:14 PM PST by LibWhacker
The world's only known natural nuclear reactor, which decommissioned itself over two billion years ago, could provide insights into how modern nuclear plants can operate more safely.
The site, in Gabon, West Africa, ran for 150million years without blowing up, and storing its own waste in a safe manner.
The reactor was a natural deposit of uranium. Today, and for the last two billion years, natural uranium will not undergo nuclear reactions, because it contains too little of the fissionable isotope, uranium-235 (U235).
But in the distant past, U235 was more abundant, comprising 3% of the total amount - the approximate concentration of enriched uranium used in nuclear fuel today. The Gabon deposit also contained, by a quirk of geology, a mixture of minerals which acted as a neutron moderator, slowing the neutron flux enough to allow the fission process to take place.
In a nuclear reactor, it takes large numbers of specialists and serious application of high technology to prevent reactions from running away. 'The big question we addressed was: when the uranium reached criticality, why didn't it blow up?' says Alexander Meschik of Washington University in St Louis, Missouri.
The answer, it appears, is that the site functioned like a geyser.
The energy generated by the nuclear reaction boiled the groundwater around the deposit. Water is a natural neutron moderator, so as it was converted into steam, it stopped absorbing neutrons and shut down the chain reaction. As the rocks cooled down, the steam condensed, and the presence of water once again slowed the neutrons down and restarted the chain reaction. Meschik calculates that the reactor operated for about half an hour at a time, then shut down for two and a half hours.
Meschik deduced this by analysing the other neutron moderator in the deposit, a 'mineral assembly' containing lanthanum, cerium, strontium and calcium and known as alumophosphate. This also acted as a waste storage medium, the researchers found; it absorbed the isotopes of xenon which were formed by the fission of the U235.
Xenon is extremely rare on Earth and is a characteristic marker of a fission process. It occurs in nine isotopes, and it was the analysis of the relative abundances of these which gave the researchers the clue to the way the reactor operated.
The find could provide insight into how to operate industrial reactors more safely. 'This is very impressive, to think that this natural system not only went critical, it also safely stored the waste,' Meschik says. 'Just using the fact that the water boiled at the reactor site might give contemporary nuclear reactor researchers ideas on how to operate more safely and efficiently.'
Very interesting post. Thank you.
And you can, of course, extract energy from steam as well. Interesting story. Thanks for posting.
Does this sound backwards to you as it does to me?
If it stopped absorbing Neutrons as water was deminished that would SPEED UP the reaction.
In water moderated reactors, like this one. Water temperature has an inverse coefficient of reactivity.
at least that is how it was explained to me in school. Which is why nuke boats run a little better with cold primary coolant.
Nothing that wasn't studied in beginning Nuclear Engineering classes decades ago.
Yep, sounds backward to me, too. And I can't pretend to understand it. Thought it was fascinating, though, that there was a natural fission reactor on Earth that ran safely, unattended for 150 million years!
You are absolutely correct. More later.
What a bunch of crap! I safely stored the waste products for 2 BILLION years! Guess that was enough time for them to decay to NOTHING! We in the nucular (spelling intentional) have alway said that the Romans had nuclear power because they solidified their waste (mostly Co-60) in concrete drums (like we do today), then the drums rusted away. Now we call them columns. (and yes, I'm kidding)
As for "and storing its own waste in a safe manner.", others I've read (ages ago) have hypothesized the site as one of the main drivers of the initial diversification of life through the halo of higher than normal radiation induced mutation around the structure...
Depends on the type of reactor the dynamics of the natural core. Funny thing is, natural cores act have a tendency to act the opposite of what was just described, i.e, + void coefficient of reactivity. Some have the opposite effect.
The thing seems to have stopped seriously radiating waaaayy before humans evolved.
I'm not sure, but if the water absorbs an atoms neutrons, it destabilizes the atomic weight of the atom losing the neutron and that then creates the reaction with another atom. That's why as it steams out, the reaction settles don.
DOH! Burned by spellcheck!
How do we know it ran safely? Like, who was monitoring it? No animals died from its radiation? No fish suffered cancer?
This is (pun intended) a very old story, but to use it as evidence of anything is kind of silly.
Wow!! The Creator is amazing. Theres nothing new under the sun. Everything man does God has already done.
Interesting article, however, not new information. This mode of operation, to my knowledge, has been assumed since the discovery of the site.
With regard to commercial applications, the measure of the effect of water density on criticality is called Alpha T. A negative Alpha T implies that a decrease in water density promotes shutdown (as described here). A positive Alpha T implies the opposite. All US and Western commercial reactors have always, by law, been required to have a negative Alpha T. The Soviet RBMK reactor (think Chernobyl) OTOH had a positive Alpha T.
The principle described therefore has always been a bedrock of Western commercial reactor design.
Nuclear waste is radioactive in terms of being very harmful to human health for 100,000 years. It doesn't return to the background radiation that natural uranium has for upwards of 1,000,000 years.
If we dispose of it in Yucca mountain for example, who will be able to read the operations manual in 100,000?
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