In the Oct. 29, 2004 issue of Physical Review Letters, Meshik and his Washington University collaborators write: “This similarity (to a geyser) suggests that a half an hour after the onset of the chain reaction, unbounded water was converted to steam, decreasing the thermal neutron flux and making the reactor sub-critical. It took at least two-and-a-half hours for the reactor to cool down until fission Xe (xenon) began to retain. Then the water returned to the reactor zone, providing neutron moderation and once again establishing a self-sustaining chain.”
Prior to this calculation, it was known that the natural nuclear reactor operated two billion years ago for 150 million years at an average power of 100 kilowatts. The Washington University team solved the mystery of how the reactor worked and why it didn’t blow up.
Meshik and his collaborators, Charles Hohenberg, Ph.D., Washington University professor of physics, and Olga Pravdivtseva, Ph.D., senior research scientist in physics, used a selective laser combined with sensitive, ion-counting mass spectrometry to concentrate on the sample’s moderator, a uranium-free mineral assembly of lanthanum, cerium, strontium and calcium called alumophosphate. The xenon found and analyzed provides the story of this ancient natural nuclear reactor. Meshik and his colleagues inferred from the xenon analysis the mode of operation and also the method of safely storing nuclear wastes, particularly fission xenon and krypton.
“This is very impressive, to think this natural system not only went critical, it also safely stored the waste,” said Meshik. “Nature is much smarter than we are. Nature is the first genius. We have all kinds of problems with modern-day nuclear reactors. This reactor is so independent, with no electronics, no models. Just using the fact that water boiled at the reactor site might give contemporary nuclear reactor researchers ideas on how to operate more safely and efficiently.”
In 1952, the late Paul Kuroda predicted that if the right conditions existed, a natural nuclear reactor system could go critical. Twenty years later, noticing that uranium ore from the Oklo mine was depleted in 235 Uranium , it was discovered that the site had once been a natural nuclear reaction system.
“The big question we addressed was: When it reached criticality, why didn’t it blow up?” Meshik said. “We found the answer in the xenon.”
Critical means that a fissionable material has enough mass to sustain a reaction. There were two major theories on how the reactor operated. One held that the system burned up highly neutron-absorbing impurities such as rare earth isotopes or boron, and because of that the system shut down regularly, and different parts of the reactor might have operated at different times. The other involved the role of water acting as a neutron moderator. As the temperature of the reactor went up, water was converted to steam, reducing the neutron thermalisation and shutting down the chain reaction. The chain reaction re-started only when the reactor cooled down and the water increased again.
Analysis of the xenon, the largest concentration of xenon ever found in any natural material, confirmed the water method. It also revealed the role of alumophosphate as the system’s waste absorber.
Xenon is extremely rare on earth and very characteristic of the fission process. Chemically inert, the element has nine isotopes and is abundant in many nuclear processes.
“You get a big diagnostic fingerprint with xenon, and it’s easy to purify,” said Hohenberg, who noted the importance of alumophosphate in the natural nuclear reactor.
“More krypton 85, a major waste from modern nuclear reactors, is getting piped into the atmosphere each year,” he said. “Maybe this natural mode can suggest a safer solution.”
Can there be a natural nuclear reactor in actual operation today?
“Today even the largest and richest uranium deposit cannot become a reactor because the present concentration of 235 U is too low – only about 0.72 percent,” said Meshik. “However, because 235 U decays much faster than 238 U, in the past, 235 U was more abundant. For example, two billion years ago 235 U was five times higher, about three percent, approximately the concentration of enriched uranium used in modern commercial reactors.”
Another vital condition for self-sustaining nuclear reaction is the high content of a moderator to slow the neutrons, Meshik said. Water, carbon, most organic compounds, silicon dioxide, calcium oxide and magnesium oxide all are natural neutron moderators. Also, the concentrations of neutron absorbents – iron, potassium, beryllium, and especially gadolinium, samarium, europium, cadmium and boron – should be low.
“Only when all of these requirements are met can a self-sustaining chain reaction occur,” Meshik said.
Given my own speculations that such natural reactors were part of the abiogenic processes that resulted in the creation of original terrestrial life, this piece opens up a whole range of possibilities.
Alumophosphate, eh? I wonder whether this has anything to do with the origins of organic phosphates, including nucleic acids. The geyser notion is really neat- it would allow gross mixing to occur that I’d never thought of, as well as also setting the stage for filtrational separation as the materials resettled after the production of steam.
It would be very interesting to know how common such natural reactors might have been even earlier in earth’s history when the relative ratio of 235/238 U was even higher, and other isotopes (U and other elements) might have also played some role.
Variants on the famous Miller-Urey experiment abound, but I doubt if anyone has tried the radio-route to life. Just too counterintuitive I guess (radioactivity=death), but remember that microfossils have OFTEN been found associated with uranium deposits from this time depth.
codemaniac
One of the links (to http://www.pureinsight.org) contains this paragraph:
I know you had probably just googled the link up Ricky for a general page to link to, but … aargh.
I have to be less busy these days and more careful when I’m voting so that I catch these editorial deals during voting.
What was it a civilization of, blue-green algae? Thanks mtigges for pointing out my laugh for the day :-D
I did not realize the Falun Gong were interested in science.
Didn’t catch that. Funny, tho. I’ll try to be more careful, too. As Sweetwind can tell you, sometimes I accidentally link to some pretty nutty stuff without realizing it…
…the article reads quite reasonably until that final paragraph. I’m impressed mtigges caught it!
I wanted to ask my co-worker (a PhD in nuclear physics) about Oklo, so we went to that link … it was the most general description linked in the story. So, he had obviously heard of this before, but hadn’t heard the new result regarding why it never exploded. Anyway, we’re reading this thing … and both of us get to the end, and go “wait a minute, what was that??” It was such an otherwise sensibly written article, that both of us had to reread that paragraph to make sure we’d actually read what reached our brain. It was really strange.
I think it should be recognized that some of Meshik’s remarks are just as questionable as those at the end of the Oklo page discussed above. When P. Kuroda predicted natural reactors (in the sense that although their remains already existed, no-one ever noticed until long after Kuroda said what to look for), I believe he did so as a competent reactor kineticist.
As such he would have had no difficulty understanding why the reactors he was predicting would not blow up; it would have surprised him, I guess, that anyone would ask this*. He would have understood the negative feedbacks liquid water moderation imposes. These include Doppler broadening of resonances, cf. this searchable textbook chapter (PDF) (!). (A good search target would be
). Plus of course there is the more readily understood negative feedback of the water’s boiling. I think this was mentioned in the Scientific American article by Cowan, no relation, that is referenced from this non-wallyjabber web page. Note its inclusion of this phrase–
Emphasis mine. Andrew Karam understands that increasing temperature slows down water-moderated reactors.
In connection with boiling, how can we characterize this Meshikism: “Just using the fact that water boiled at the reactor site might give contemporary nuclear reactor researchers ideas on how to operate more safely and efficiently”? Well, in light of the fact that something like a third of the reactors in the world are boiling-water reactors, stupid seems to be the right word.
— Graham Cowan
Boron: A Better Energy Carrier than Hydrogen?
* He forgot to predict the vanished ancient supercivilization, too, didn’t he.