Who discovered these Natural Fossil Reactors?

Some Surprising Predictions

Part of Figure 38 from US Patent 2,708,656, ‘Neutronic Reactor’, awarded to Enrico Fermi and Leo Szilard.
[Source: Wikipedia CCL]

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:

• Approximate age range for a natural reactor
• The U concentration
• ²³⁵U/²³⁸U ratio requirements
• The natural reactor shape requirements.

Some Surprising Predictions

On the way to a 1964 AGU Meeting in Washington, DC. From left: Oliver Manuel, DD Bogard, Marvin Rowe and Professor Paul Kuroda.
[Picture: Dr. Pentti Kauranen of the University of Kuopio, Finland. Source: Essays in Nuclear, Geo- and Cosmochemistry, Proceedings of the 1987 SW Regional ACS Society]

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 radioactive remains of a natural nuclear fission reaction that happened 1.7 billion years ago in Gabon, Africa, were held in place by the surrounding geology.
[Source: Wikipedia CCL]

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

Atomic Scientists, 20th Anniversary Reunion, 1962. The group who participated with Enrico Fermi in the December 2, 1942 experiment to create the world’s first fission reactor.
[Source: Wikipedia GFDL]

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 achievement 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 the team assembled the world’s first man made self sustaining fission reactor inside a squash court underneath the grandstands of the University of Chicago sports stadium.

Both reactors 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

Enrico Fermi, Italian-American physicist.
[Source: Wikipedia CCL]

Enrico Fermi was a remarkable individual and physicist. During the construction of the first ‘Atomic pile’ (reactor) he personally supervised the placement of the U fuel spheres and was able to predict (thankfully!) 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.