An ancient nuclear reactor that formed naturally in Gabon 2 billion years ago didn't pump power smoothly--it pulsed, researchers now report. Their findings clarify how this unusual reactor worked and may lead to better long-term storage of radioactive waste.
Physicists discovered the remains of the naturally formed nuclear reactor in 1972 in the Oklo uranium mine in Gabon. Several similar geological formations exist in the region. All formed from the same essential ingredients: a relatively pure plug of uranium with a high concentration of the radioactive isotope U-235; plus organic compounds, graphite, or water to slow down the neutrons produced by the uranium so they have time to split another uranium atom instead of zooming away. Such a reactor could not form nowadays, because U-235 is too rare, but 2 billion years ago, the isotope made up about 3% of naturally occurring uranium--the concentration needed to sustain fission in a reactor.
While trying to figure out how the reactor worked, physicist Alex Meshik and colleagues at Washington University in St. Louis, Missouri, discovered a strange pattern of xenon isotopes--byproducts of uranium fission--trapped in aluminum phosphate minerals around the reactor. In a fission reactor, U-235 can produce xenon either by means of short-lived intermediate atoms or longer-lived ones. The researchers noticed that xenon isotopes from the short-lived intermediaries were in surprisingly short supply. That suggested to them that the reactor cycled on and off. If a short-lived intermediary formed during an "on" cycle, they reasoned, heat from the reactor would have caused the xenon gas it produced to dissipate into the air. In contrast, a longer-lived intermediary would have been more likely to stick around till the reactor was off, producing xenon that became trapped in the aluminum phosphate before the next on cycle. The researchers' calculations suggest that when the reactor was on, it pumped out an average of 100 kilowatts, much of it as heat that boiled off the moderating water (probably from a nearby river) in about 30 minutes. With no water, the reactor shut off and cooled down for about 2.5 hours until more water collected, the researchers report in the 29 October issue of Physical Review Letters."The amazing capability of aluminophosphate to capture fission gases" is an important insight, Meshik says, because it suggests that the material could be used to prevent radioactive gases from escaping modern reactors--something that's been difficult to do. Donald Bogard, an isotope geochemist at Johnson Space Center in Houston, Texas, agrees. The Oklo reactor's aluminum phosphates "might have some important implications" for how we store radioactive waste, Bogard says.