What caused these reactors to start?

The Fission Process

Nuclear fission.

Fission is the splitting of an atomic nucleus. The easiest nuclei to split are very heavy nuclei like Uranium 235 (²³⁵U) and Plutonium 239 (²³⁹Pu) which if they absorb a small sub-atomic particle like a neutron can split into two fission fragments (or fission products) and produce 2 or 3 neutrons.

The ejected neutrons can in turn be absorbed by other U nuclei to produce even more fission events (a chain reaction). This self sustaining reaction can be controlled as is done in a man made nuclear fission reactor where control rods (made of neutron absorbing materials such as the metal cadmium) are inserted into reactors. If a runaway reaction does take place a nuclear explosion can occur – but this did not take place at Oklo where the reactions were also self regulated.

Uranium Isotopes Today

The Ranger uranuim mine in the Kakadu National Park, Australia

Uranium exists in nature in the form of two isotopes: ²³⁵U and ²³⁸U. Both isotopes are radioactive but have such long half lives that about half of the Uranium that was incorporated into the original earth (and also the rest of the solar system) 4500 million years ago still exists today. For every 100,000 atoms of U only 720 are ²³⁵U atoms. Since ²³⁵U is the isotope of U that is easiest to fission most man made reactors require ‘enriched U’ – U in which the relative amount of ²³⁵U is increased to about 3000 atoms per 100,000 atoms (i.e. 3%).

Uranium Isotopes 2000 million years ago

Stromatolites, like these from Shark Bay in Western Australia, have been around for 3500 million years.

At Oklo, as on the rest of the earth and solar system, 2000 million years ago the relative abundance of U-235 was 3000 atoms per 100,000 atoms.

This is one of the major reasons why nuclear fission started.

Natural fission reactors cannot form today because there is insufficient ²³⁵U in natural U. There are also several other important factors which must be satisfied before natural fission reactions will commence.

Natural reactor requirements

The requirements for a natural reactor.

Besides a natural enrichment of ²³⁵U compared to ²³⁸U, a natural reactor requires 4 other important parameters to be satisfied:

• A high overall concentration of U.
• A low concentration of neutron absorbers.
• A high concentration of a moderator.
• A minimum or critical size to sustain the fission reactions.

Reactor Zone 15

Zone 15 with evidence for a fossil reactor.

Of the seventeen known fossil reactors, 9 have been completely mined out. Reactor zone 15 is the only reactor which is accessible underground through a tunnel bored into the existing mine pit. The remains of fossil reactor 15 are clearly visible as the light grey/yellow coloured rock which is mostly Uranium oxide. The light coloured streaks in the rocks above the reactor is quartz which has been crystalized from the (hot) underground waters circulating around during and after the reactor”s operating lifetime.

The fission products

Fission product isotopes produced by the fission of three different heavy nuclei.

The mass numbers of the fission products is typically in the range from 85 to 150 atomic mass units. This graph shows the % amount of a limited range of fission product isotopes produced by the fission of three different heavy nuclei. Each type of fissioning nucleus produces a slightly different fission yield. By comparing the Absolute Cumulative fission yields measured at Oklo with those measured in modern nuclear reactors it has been possible to show that Oklo was fissioning both ²³⁸U and ²³⁹Pu as well as ²³⁵U . Since there is was no ²³⁹Pu present on the earth when it was formed the Oklo Reactors must have ‘bred’ the ²³⁹Pu itself.

Natural Breeder Reactors

The production of ²³⁹Pu and ²³⁸U from local ²³⁸U at Oklo.

This diagram shows how it was possible for the Oklo Reactors to breed ²³⁹Pu and ²³⁸U from local ²³⁸U.

Inititally the fission and resulting neutrons come from the fission of ²³⁵U. However, the presence of very high abundance of ²³⁸U absorbs some of the neutrons to become ²³⁹U. This in turn decays by beta decay to Neptunium 239 and the ²³⁹Pu. The Resulting Pu 239 then fissions but there is another twist to the story. The natural reactors operated for so long that the ²³⁹Pu had sufficient time to decay by alpha decay to ²³⁵U. Thus the natural reactors were true ‘Breeder’ reactors, fissioning in some cases more ²³⁵U than originally existed in the reactors.