EARTH × TECHNOLOGY

Nature's Nuclear Reactors

~1.7 billion years ago

Proterozoic · Paleoproterozoic · Statherian

In Gabon, sixteen bodies of uranium ore began working as nuclear reactors with no one to build them. A chain reaction started by itself: uranium atoms split, throwing off neutrons that went on to split the atoms next to them. Groundwater soaking the ore made this possible, slowing those neutrons just enough to keep the reaction going, and it set the tempo as well: when the ore ran hot the water boiled away and everything stopped, until it seeped back and everything began again. Nothing like it could happen today: uranium-235, the kind that feeds the reaction, decays faster than uranium-238, and two billion years ago natural ore still held about three per cent of it, close to what power-station fuel is enriched to now. Most of what those fissions produced has never gone far, sealed in by the clays that surround the reactors.

Why it matters

Nature ran nuclear reactors nearly two billion years before we did, and their waste has barely moved since. That is why we study them: as a dress rehearsal for burying our own.

Dating & uncertainty

The reactor zones ran intermittently over tens to hundreds of thousands of years, and their age is given anywhere between 1.7 and 2 billion years depending on which mineral is dated. The spread is harmless: at either end of it, natural ore still carried its three per cent of uranium-235, which it has long since lost.

Sources

1. Gauthier-Lafaye (2002), Comptes Rendus Physique · 2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon
2. Hidaka (2007), Journal of Nuclear and Radiochemical Sciences · Geochemical characteristics of an ancient nuclear reactor, Oklo
3. Bekker & Holland (2012), Earth and Planetary Science Letters · Oxygen overshoot and recovery during the early Paleoproterozoic

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