Criticality of Oklo Natural Reactors: Realistic Model of Reaction Zone 9

Oklo Natural Nuclear Reactors

“…At Oklo, the neutron moderator was primarily water present in the porous and fractured rocks which host the reactors. However, it has been suggested that carbon associated with organic matter (described below) may have also played a role as a neutron moderator/reflector in some reactors (Naudet 1991;Bentridi et al 2013). A final requirement is that there is a minimum of neutron poisons in the reactor zones.…”

“…To date, 17 natural nuclear reactors have been discovered in the Oklo and the associated Okelobondo and Bangombe uranium ore deposits in the Republic of Gabon (Bentridi et al 2013). The reactors occur as small bodies in the uranium ores.…”

“…During natural reactor operation at Oklo, temperatures in the reactors reached as high as 400 C (Gauthier-Lafaye 2002). As a result of heat and circulating fluids, the reactors experienced the progressive dissolution of the quartz sandstone host rock and the concomitant formation of a clay envelope around the reactor cores that was impervious to fluid flow (Bentridi et al 2013). The cores finally collapsed with the progressive loss of quartz in and around the reactors leading to a significant reduction of porosity and water in the core.…”

“…Their eventual collapse together with the consumption of significant quantities of the 235 U fuel led to the reactor cores becoming subcritical and finally shutting down. In the organic matter-rich reactors where this material appears to have also contributed to neutron moderation, the reactor shutdown may have been due in part to the redistribution and migration of the organic matter as viscous bitumen away from the reactor cores (Bentridi et al 2013).…”