When did these Natural Fossil Reactors begin operating?

An Oklo Timeline

The Oklo minesite looking south.

Using a number of radioactive clocks the Oklo fossil reactors have been radioactively dated to be about 2000 million years old. The uranium in these reactors is thought to have come from the tiny amounts of uranium orginally scattered throughout the earth’s crustal rocks during its formation.

The history of the Oklo fossil reactors spans almost the entire history of the earth. ‘Oklotime’ can be divided into four stages:

1. U mobilization phase: Commenced ~3500 million years ago.
2. U ore/reactor formation: Started ~2800 million years ago.
3. Reactor operation: Commenced 2000 million years ago (for about a million years).
4. Waste movement: The last 2000 million years.

1. U mobilization phase (Commenced ~3500 million years ago)

Stromatolites, like these still found in Shark Bay (Western Australia), were in existence when the Earth’s oxygen atmoshphere was being created.

Uranium for these reactors are are thought to have come from the tiny amounts (ppm, parts per million) of uranium orginally scattered throughout the earths crust during its formation.

The change in the earths atmosphere brought about by the evolution of plants starting at about 3500 million years ago slowly changed the earth’s atmosphere from one contining major amounts of carbon dioxide into one containing significant amounts of oxygen. This increase in atmospheric oxygen slowly gave surface waters an ‘oxidizing’ nature which dissolved some of the trace amounts of U from the crustal rocks into streams and rivers.

2. U ore/reactor formation (Started ~2800 million years ago)

Oxygen rich rainwater leaching U and Fe from surface rocks and then depositing it in swamps and river deltas.

When rivers and stream flow into swamps and river deltas or oceans they slow down depositing sand, organic matter and other chemicals into what are called sediments, muds and oozes. These in turn react reacted with any soluble U and deposited it as a thin relatively low concentration layer over a large area. There are many U ore deposits on the earth today in the USA, Africa and Asia that have been formed in this way. Further details about U ore formation can be found in a paper on this subject at the Uranium Information Council.

Over millions of years some thousands of meters of sand were then deposited on top of the Uranium layer. Eventually, movements of the earths crust lifted the sand was converted in sandstone rocks, lifted above sea level and tilted over to an angle of about 45 degrees. Rainwater (now even more oxidizing) percolated down through cracks and fissures re-mobilized the U and ‘pushed’ it further underground. Eventually the soluble U encountered ‘reducing’ chemical conditions to redeposit some of the U in high concentration (greater than 50% UO₂) ‘blobs’ ranging from fist size lumps up to the size of a large car.

3. Reactor operation (Commenced 2000 million years ago)

A close up of reactor sites 7, 8 and 9.

Each reactor operated on an intermittent basis for a period ranging from a few years to hundreds of thousands of years. The total time period over which the reactors operated is thought to be about a million years.

4. Waste movement (The last 2000 million years)

The Gabonese highlands today.

Some two thousand millions years have passed since the reactors last operated. This is so long that all of the radioactive waste products (even those with million year half lives) have decayed away. In fact it can be shown that even for a radioactive waste products such as ¹²⁹Iodine. ¹³⁷Cesium, and ¹⁰⁷Palladium (half lives of 1.7 x 107, 3 x 106 and 7 x 106 years respectively) that even if all the U at Oklo had been converted to radioactive Iodine there would be less than one atom of this isotope left today.

Radioactive Clocks

Part of the laboratory at Curtin used for radiometric dating.

Radioactive clocks play important roles in deciphering the Oklo phenomenon. To date the age of the U deposit, uranium-lead, rubidium-strontium and samarium-neodymium clocks have been used. The fact that sufficient ²³⁵U relative to ²³⁸U was present in the reactors is related to the difference in the decay rates of these two isotopes.

Using Radioactive Clocks at Oklo

The zone 15 reactor site.

During the reactors lifetime a vast array of radioactive clocks were inititated by the fission reactions. Virtually every fission product isotopes produced was a radioactive clock. The majority of these isotopes are far too short to be of any real benefit in studying the Oklo phenomenon but there are still several dozen with half lives of several years to millions of years or more which have been used to determine many reactor parameters including:

• the time period over which each of the individual reactors operated;
• how much ²³⁵U each the individual reactors ‘bred’ from ²³⁸U;
• how long it took for some more mobile fission products to move out from the reactor zones; and
• how much of each of the fission products was retained inside the reactors.