ISOTOPIC COMPOSITION AND NEUTRONICS OF THE OKÉLOBONDO NATURAL NUCLEAR REACTOR

The Oklo-Okélobondo and Bangombé uranium deposits, in Gabon, Africa, host Earth’s only known natural nuclear fission reactors, which operated ~2 Ga. The uraninite (UO₂) in the Okélobondo reactor zone (500 m depth) has been studied as a means by which to constrain the source term of fission product and actinides elements produced during reactor operation. The source term depends on the neutronic parameters of the reactor, which include the neutron energy spectrum, reactor flux and reactor operation duration. Reactor operation has been modeled using a point-source computer simulation (Oak Ridge Isotope Generation and Depletion, ORIGEN, code) for a light water reactor. The model results have been constrained using secondary ionization mass spectroscopy (SIMS) measurements of fission product isotopes of Nd (¹⁴³Nd, ¹⁴⁴Nd, ¹⁴⁵Nd and ¹⁴⁶Nd) and Te (¹²⁵Te, ¹²⁶Te, ¹²⁸Te and ¹³⁰Te), as well as U (²³⁵U and ²³⁸U) in uraninite from 18 samples distributed across a two-dimensional slice of the reactor zone. The SIMS data were corrected for instrumental mass fractionation using uraninite samples from reactor zone 10 that had been previously characterized by TIMS or ICP-MS. The isotopic compositions of fission product (e.g., ¹⁴³Nd/¹⁴⁴Nd_measured=0.859 to 0.978 and ¹²⁵Te/¹²⁸Te_measured=0.107 to 0.160) and actinide elements in the reactor zone vary significantly from terrestrial values due to fission, spontaneous fission, decay and neutron capture events that occurred during and after reactor operation. Using the constraints placed on the operating conditions by the ²³⁵U/²³⁸U ratio, the pre-reactor concentrations of Nd (150 ppm ± 75 ppm) and Te (<1 ppm) in uraninite were determined. The final fission product inventories of Nd (90 to 1200 ppm) and Te (10 to 110 ppm) were then calculated as a function of reactor burnup (0.7 to 13.8 GWd/MTU, which corresponds to a range of ²³⁵U/²³⁸U of 0.0072 to 0.0054). Similarly, the ranges of all other fission products and actinides produced during reactor operation were calculated as a function of burnup and time. These results provide a source term against which the present elemental and decay abundances at the fission reactor can be compared.  Furthermore, they provide new constraints on the extent to which a “fossil” nuclear reactor can be characterized on the basis of its isotopic signature.