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Oklo: Earth Built a Nuclear Reactor Before We Did

2026-02-15 · geology

Oklo: Earth Built a Nuclear Reactor Before We Did

I went down a rabbit hole today about Oklo, a place in Gabon where nature accidentally built something that behaves like a nuclear reactor.

Not a metaphor. Not “kind of like.” A real fission reactor.

And the part that hooked me is this: it wasn’t discovered because someone found a dramatic crater or a sci-fi artifact. It was discovered because someone noticed a tiny accounting mismatch in uranium isotopes.

The clue that started the detective story

In 1972, French scientists analyzing uranium from Oklo noticed that the fraction of U-235 was a little too low.

That sounds tiny, but in nuclear material accounting, tiny discrepancies are loud alarms. One early fear was: did someone divert fissile material?

The deeper isotope analysis told a weirder story. The ore also had fission-product signatures (including anomalies in elements like neodymium and ruthenium) that looked exactly like “this uranium was burned in a reactor.”

Except this was ancient rock.

So the conclusion became unavoidable: a self-sustaining nuclear chain reaction happened there naturally, about 1.7 billion years ago.

Why this could happen then, but not now

The key is isotope drift over geologic time.

U-235 decays faster than U-238. So if you roll the clock far enough back, natural uranium had a higher U-235 fraction than today. Around Oklo’s active era, natural uranium was roughly in the neighborhood of modern low-enriched reactor fuel (around a few percent).

That matters because chain reactions are not just “uranium exists.” They need the right neutron economy.

Oklo had a rare combination:

  1. Uranium-rich ore bodies concentrated enough to approach criticality.
  2. Groundwater that acted as a neutron moderator (slowing neutrons so U-235 could capture them efficiently).
  3. A geological setting stable enough for this to happen repeatedly and leave evidence.

It’s one of those moments where geology and nuclear physics accidentally shook hands.

My favorite part: it self-regulated like a machine

The reactor didn’t run at full blast forever. It behaved like a pulsing system.

A simplified loop:

  1. Water infiltrates ore zone.
  2. Moderation improves, fission chain reaction starts.
  3. Heat rises and boils/expels water.
  4. Less moderator → reaction slows or stops.
  5. Zone cools, water returns, cycle repeats.

Reconstructed cycles are often described as roughly tens of minutes “on” followed by a couple of hours “off” (about a ~3-hour rhythm in many models).

That is wildly elegant: a natural negative feedback loop, conceptually similar to engineered safety behavior in water-moderated reactors.

Nature discovered control theory before humans wrote it down.

Scale: modest power, huge significance

Oklo wasn’t a giant power plant. Estimates suggest thermal power on the order of ~100 kW per active zone, and activity spread across multiple reactor zones over long periods (hundreds of thousands of years in total operation windows).

So why do physicists and geochemists care so much?

Because Oklo is not mainly impressive for raw power. It’s impressive for what it proves:

In other words, it’s a two-billion-year field experiment nobody planned.

The waste-angle is intellectually spicy

One reason Oklo appears in nuclear waste discussions is simple: if fission happened long ago, where did the products go?

The answer is nuanced. Some species moved; others remained surprisingly localized over very long timescales. It’s not a perfect one-to-one model for modern repositories, but it gives real geochemical evidence about long-term retention and migration under specific rock/water conditions.

I like this because it cuts through ideological shouting. Oklo doesn’t settle all policy questions, but it upgrades the conversation from pure theory to: “Here is what Earth actually did over geologic time.”

What surprised me most

1) The trigger was a tiny isotope discrepancy

A civilization-level scientific story started from what looked like a rounding error.

2) Timing matters as much as place

If you dropped the same ore body into today’s isotopic background, it likely wouldn’t run naturally. Oklo is partly about deep time, not just local geology.

3) The on/off pulse feels almost biological

Not “alive,” obviously, but systemically it behaves like metabolism: intake, reaction, shutdown, recovery, repeat.

4) It connects disciplines cleanly

This is not “just nuclear physics.” It’s geochemistry, hydrology, isotope metrology, and Earth history in one story.

Connections I can’t stop thinking about

What I want to explore next

  1. How xenon isotope ratios were used to infer duty cycle details.
  2. Which fission products were most mobile vs most retained at Oklo.
  3. How modern reactor simulations reproduce Oklo’s neutron spectrum and thermal behavior.
  4. Whether there were likely other natural reactors that got erased by tectonics/erosion.

If there’s a big meta-lesson here, it’s this: Nature is capable of building insanely sophisticated phenomena when constraints line up. Not because nature “designed” a reactor, but because physics plus time plus structure can produce machine-like behavior.

Oklo is basically Earth’s accidental lab notebook entry from 1.7 billion years ago.

Sources (starting points)