What caused these Reactors to start?

• The fission process
• Uranium isotopes today
• Uranium isotopes 2000 million years ago
• Natural reactor requirements
• Natural Reactor Zone 15
• Fission or waste products
• Oklo as a Breeder reactor
• Things to do and think about
  

Click on these images for more detail.

	The Fission Process: 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: 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 years ago still exists today. This graphic shows the relative number of U-235 and U-238 atoms present in the earth 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 (ie 3%)
	Uranium Isotopes 2000 million years ago: 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: 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: 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 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 isotope 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: 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 form 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.

Things to do and think about

1. Where does the energy come from in the nuclear fission reaction?
2. How many neutrons are produced by an individual nuclear fission reaction?
3. The following experiment requires that you obtain i) a large amount of small grains of something, preferably in 2 distinctive but uniform colours and ii) two large transparent containers. (Sugar is one possibility and clean sand is another). Both can be obtained in a range of colours and sand can also be dyed;
   • count or measure out ~100000 grains of one colour (colour A) and place them into one of the transparent containers, put that container aside - it will be used for comparison purposes.
   • count or measure out ~720 grains of one colour (colour A) and ~100000 of colour (B), (make sure you have enough grains and a big enough container) and place these into the second transparent container.
   • Vigorously shake the container and then count how many of colour A you can see amongst the colour B from the outside of the container.
   • Repeat the same for ~3000 of A in ~100000 of B. Place the containers side by side and stand back about 1 metre and see if you can see any colour difference between the two containers.
   • Keep adding A to B in lots of ~3000, shake, stand back and compare the containers until a colour difference appears. How many sand grains of A are required to make a difference?
   • How many times smaller are atoms of U than your grains of sand or sugar?
4. What is the typical geological environment for a natural nuclear reactor to occur?
5. What evidence is there today that the Oklo fossil reactors BRED additional U-235 from plutonium?

If you get stuck or you are curious to find out more email the author.