Who discovered these Natural Fossil Reactors?

• Some Surprising Predictions
• Discovered the Oklo Reactors
• Man Made Fission Reactors
• Enrico Fermi
• Things to do and think about

Some Surprising Predictions:

Less than 15 years after the first man made fission reactor scientists were thinking about the possibility of naturally occurring nuclear reactors. The first detailed published study by Professor Paul Kuroda (1956), a Japanese physicist, who determined the detailed requirements for any likely natural reactors.

In what has become to be recognized as one of the best set of predictions in nuclear physics Paul Kuroda laid out in detail the;

1. Approximate age range for a natural reactor:
2. The U concentration
3. ²³⁵U/²³⁸U ratio requirements
4. The natural reactor shape requirements:

Despite this detailed work, Paul Kuroda could not find a match for his natural reactor model amongst the earth's then known U ores. A small but critical detail that Paul Kuroda overlooked was the possibility that water could act as a useful moderator and that certain U ores could be porous enough to contain sufficient water to moderate the neutrons and sustain the reaction.

During the late 1950's and early 1960's, the ²³⁵U/²³⁵U ratio in hundreds of U ores from around the world were measured to detect any change in this ratio. Any reduction in this ratio would indicated that some ²³⁵U had fissioned some time in the past. Of the hundreds of ores investigated, none had a ²³⁵U/²³⁵U ratio outside the generally accepted value of 0.007202 +/- 0.00006.

Discovery of the Oklo Fission Reactors

The Oklo natural fossil reactors were discovered on June 2 1972 by a French analyst (Bougzigues) whilst working at the Pierrelatte nuclear fuel processing plant. During routine mass spectrometry measurements of the value of ²³⁵U/²³⁵U ratio in U ore samples he observed a tiny change in the ratio (0.00717, compared to a normal value of 0.00720). So precisely known is this ratio that this small difference was sufficient to suggest something strange had occurred.

At first it was thought that some used nuclear fuel had inadvertently slipped into the processing plant. However ,this was quickly ruled out because of the lack of intense radiation normally found in such "spent" fuel.

Other possibilities raised included spent fuel from an extra-terrestrial space ship or U from an ancient nuclear waste site perhaps even from a past civilization. A careful check on the source materials traced the U ore back to a very high concentration U deposit present in a minesite in Gabon, a country in South West Africa.

A detailed investigation detected the presence of all the conditions necessary for, and large quantities of ancient (no longer radioactive) fission product waste embedded in the natural U ore, confirming that natural nuclear fission reactions had taken place at Oklo some 2000 million years ago.

The First Man Made Fission Reactors

When the world's first man made nuclear fission reactor achieved criticallity on the 2nd of December 1942 the scientists and technicians who laboured away for a considerable period of time did not realize that nature had beaten them by some 2000 million years. This acheivement was carried out in secret during WWII as part of the development (known as the Manhattan project) of the atomic bomb. Led by the Nobel prize winning physicist (Enrico Fermi, lower leftmost in the photo) the team assembled the world's first man made self sustaining fission reactor inside a squash court underneath on of the grandstands of the University of Chicago sports stadium.

The similarity between this and the Oklo reactor was that both used natural uranium as a nuclear fuel but Oklo was largely a water moderated reactor whereas the Chicago reactor was carbon (graphite) moderated. Fermi's rector used cadmium rods to control the reaction while the Oklo reactors were not "controlled" in the conventional sense but rather lost moderator through overheating which slowed down (and probabably even temporarily stopped) the reaction over periods of anywhere between days and years.

Enrico Fermi

The author cannot let this opportunity go by without acknowledging the special abilities and efforts of Enrico Fermi, the leader of the Manhattan project team that established the first man made reactor in 1942. Enrico Fermi was a remarkable individual and physicist. During the construction of the first "Atomic pile" (reactor) Fermi personally supervised the placement of the U fuel spheres and was able to predict (thankfully!) to exactly when the reactor would undergo self sustaining fission.

Fermi was equally at home with classical literature, in a physics classroom (as he was a consumate teacher), in the lab (performing either a mundane task or one that required considerable manual skill) or working on the latest theoretical aspects of physics.

After years of working with relatively primitive equipment at the University of Rome, Fermi was awarded the Nobel Prize for physics in 1938 for the production of transuranic elements by neutron irradiation.

Fermi's greatest assest was perhaps his ability to simplify seemingly complex problems to a point where he could solve them on a table napkin or "back of the envelope". Many physics students today also attempt to emulate and practice this extremely useful skill. Fermi died in 1954 in his 53rd year, who knows what he would . The element Fermium (atomic number 100) was later named in his honour.

Things to do and think about

1. Who predicted the existence of and who discovered the first natural fission reactors?
2. What are the critical factors needed to start a nuclear (natural or man made) fission reactor?
3. Why is the ²³⁵U/²³⁵U ratio an important indicator for the presence of a natural nuclear reactor?
4. What other factor (beside the U isotope ratio) confirmed the natural nature of the nuclear reactions at Oklo?
5. How many years are there between:
   • the operation of the Oklo "natural" and first "man made" fission reactors?
   • the first man made natural reactor and the prediction of natural fission reactors
   • the prediction and the discovery of natural fission reactors
6. What role did Enrico Fermi play in the development of the first man made nuclear reactor?
7. The actual shape of fission reactors (man made or natural) is critical to their operation. This is because, to sustain the chain reaction, it is important to retain as many neutrons as possible inside the reactor. To illustrate this;
   • draw on a piece of graph paper as many shapes as you can (rectangles, squares, circles, ovals etc) each having approximately the same area.
   • Determine the area (A) of each shape (count up the number of small squares inside the shapes
   • Now measure or calculate the perimeter or distance (d) around the outside of each shape.
   • Calculate the area (A) to perimeter (d) ratio (A/d).
   • Shapes with the highest A/d ratio will be the most efficient for a fission reactor - can you think why this would be so?
   • The above example is for a 2 dimensional structure but what would be the case for a 3 dimensional structure?
8. In the year 5025, USS Enterprise astronauts orbit the planet Taurus V and detect large amounts of concentrated U undergoing fission. Based on this, what can you say about the age, geology and life forms on Taurus V ?

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