The pre-Fermi natural reactor: A note on the periodic mode of operation

Abstract

The isotopic compositions of xenon released from the Oklo reactor at temperatures below 1000°C are such that the abundances of¹³¹Xe,¹³²Xe and¹³⁴Xe relative to¹³⁶Xe are markedly enhanced when compared to the relative fission yields from the thermal neutron-induced fission of²³⁵U. These anomalies can be attributed to the fact that¹³¹Xe,¹³²Xe and¹³⁴Xe have fairly long-lived precursors: 8.04-day¹³¹I, 78.2-hour¹³²Te and 42-minute¹³⁴Te, respectively. It is possible to determine the duration of the time when the reactor was turned off from the ratios of excess¹³²Xe to excess¹³⁴Xe in these anomalous xenon fractions released from the Oklo reactor. Calculations based on the available xenon isotope data that the time period during which the reactor was turned off was approximately 2 to 3 hours.