Nuclear fission (natural reactor)

History

In 1972, routine measurements on uranium mined in Oklo (Gabon, Africa) showed a ²³⁵U concentration lower than the expected. Further investigations discovered uranium ore with a ²³⁵U concentration as low as 0.44%, which is almost 40% below the normal abundance of 0.72%.

As it turned out, it were the remnants of an extinct nuclear fission reactor, where self-sustaining nuclear reactions have occurred about 2 billion years ago. At that time, the fissile isotope ²³⁵U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors.

Neodymium and other elements were found with isotopic compositions different from what is usually found on earth. For example, Oklo contained less than 6% of ¹⁴²Nd while natural neodymium contains 27%. However, Oklo contained more of ¹⁴³Nd. The isotopic composition matched that produced by the fission of ²³⁵U (see Figure).

Related concepts

• cosmic radiation
• cosmogenic radionuclide
• natural decay chain
• nucleosynthesis
• primordial radionuclide