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. The groundwater soaking the ore made this possible, slowing those neutrons just enough for the reaction to keep itself going; it set the tempo too, for whenever the ore ran hot the water boiled away and everything stopped, until it seeped back and everything started 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, precisely what power-station fuel is enriched to now. The products of those fissions have never left.

Why it matters

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

Dating & uncertainty

The reactor zones ran intermittently for 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. They could only work at all back then: uranium-235 decays faster than uranium-238, so natural ore was around three percent enriched, which is what reactor fuel is enriched to today.

Sources

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

See it on the timeline