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

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.