EARTH × TECHNOLOGY
Oklo: the Natural Nuclear Reactors
~1.7 billion years ago
Proterozoic · Paleoproterozoic · Statherian
The Oklo reactors, in Gabon, are sixteen bodies of uranium ore in which a chain reaction started on its own about 1.7 billion years ago, so that they operated as natural nuclear reactors.
The Oklo reactors, in Gabon, are sixteen bodies of uranium ore that operated as natural nuclear reactors about 1.7 billion years ago. A chain reaction began unaided: uranium nuclei split and released neutrons, particles that split neighbouring nuclei in turn. Groundwater saturated the ore and made the reaction possible by acting as a moderator: it slowed the neutrons to the speed at which nuclei capture them most readily. The water also made the reaction intermittent: it boiled away when the ore became hot, which stopped the reaction until water seeped back. These conditions cannot recur. The reaction is sustained by uranium-235, an isotope, a variety of the uranium atom defined by its mass. Uranium-235 decays, that is, turns into other atoms, faster than uranium-238, so its share in the ore falls with time. When the reactors operated, natural ore still contained about three per cent of uranium-235, close to the enrichment, or uranium-235 content, of present-day power station fuel. Most of the fission products, the atoms left by the split nuclei, have migrated very little, because the surrounding clays retained them.
Why it matters
Researchers study Oklo as a natural analogue for the storage of radioactive waste, that is, a natural case that resembles such storage and can be observed. Nuclear reactors operated at Oklo 1.7 billion years before the first industrial reactors. The clays around the Oklo reactors have kept the fission products in place ever since.
Dating & uncertainty
The reactors operated intermittently over tens to hundreds of thousands of years. Researchers give their age as between 1.7 and 2 billion years, according to which mineral is dated. This spread of ages does not change the explanation. At 1.7 billion years as at 2 billion years, natural ore still contained its three per cent of uranium-235, a proportion that it has since lost.
Earth history as one day
15:00:478 h 59 min before midnight
if Earth’s 4.54 billion years were compressed into a single day