U.S. studies of the Oklo phenomenon

Analyses of samples from the Oklo site are in agreement with previous reports by Naudet et al concerning the occurrence of a natural chain reaction in Precambrian times. A preliminary modeling of the reactor suggests that the event took place 1.7 to 1.9 b.y. ago, and that the period of criticality lasted at least 2 x 105 years. Uranium may have accreted and, to a lesser extent, migrated during the period of criticality or subsequent to it. (U.S.)