Nuclear Reactors in

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Nuclear Reactors in Nature The Oklo Phenomenon
Carolyn Bauer SUNY Oneonta July 27, 2006
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Fission
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How do we know that Oklo was the site of a
natural nuclear reactor?

• Ratios of U-235 to U-238
• Fission Products

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Abundances of U-235

• Accepted Isotopic concentration 0.7202
• Isotopic concentrations found at Oklo as low as
  0.440

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Fission Products

• Differences in Isotopic Concentrations of
• Neodymium
• Ruthenium
• Elevation in isotopic concentrations
• of U-235
• Existence of Plutonium

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Decay Scheme for U-238 to U-235

• 238U n ? 239U ? 239Np ß- ? 239Pu ß-? 235U a

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3 Basic conditions necessary for a reactor to
work

• Ore with a high Uranium content
• Uranium must be at least 3 U-235
• Existence of a moderator

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Isotopic compositon of U-235 Today
Isotopic compositon of U-235 2 billion years
agon
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Moderator

• Close in size to a neutron
• Slows down neutrons so fission can occur
• Prevent meltdown
• At Oklo water was the moderator

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Implications

• Geological information pertaining to containment
• Implications for improvement of repositories like
  Yucca Mountain

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Nature trumps man 100 of the time