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Researchers describe how natural nuclear reactor worked


To operate a nuclear power plant like Three Mile Island, hundreds of highly trained employees must work in concert to generate power from safe fission, all the while containing dangerous nuclear wastes.

On the other hand, it’s been known for 30 years that Mother Nature once did nuclear chain reactions by her lonesome. Now, Researchers at Washington University in St. Louis have analyzed the isotopic structure of noble gases produced in fission in a sample from the only known natural nuclear chain reaction site in the world in Gabon, West Africa, and have found how she does the trick. Picture Old Faithful.

Analyzing a tiny fragment of rock, less than one-eight of an inch, taken from the Gabon site, Alexander Meshik, Ph.D., Washington University senior research scientist in physics, has calculated that the precise isotopic structure of xenon in the sample reveals an operation that worked like a geyser. The reactor, active two billion years ago, worked on a 30-minute reaction cycle, accompanied by a two-and-a-half hour dormant period, or cool down.

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.

Tony Fitzpatrick | EurekAlert!
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