ABSTRACT

The behavior of rare-earth elements (REE) in the Oklo natural nuclear reactors was studied. Isotopic ratios of all REE in the Oklo uranium ore samples were measured with a thermal ionization mass spectrometer, and the REE abundances were determined by the stable isotope dilution method. On the basis of these data, the behavior of REE in the process of formation of ore body and during and/or after the operation of the natural nuclear reactor was investigated. The total amount for each of the REE is allotted to natural and fissiongenic fractions by a calculation based on the nuclear data. The pattern for pre-reactor natural REE explicitly endorses the participation of water in formation of this ore deposit. Meanwhile, a fission yield curve involving Nd, Sm and Gd is found to be generally smooth. But deviations in La and Ce, and shifts among the fissiongenic segments for Nd, Sm and Gd are observed. These features can be accounted for as reflecting the systematic differential partial loss of fissiongenic light REE. The partial loss of progenitors of ¹⁴⁰Ce and ¹⁴²Ce is found to be unlikely.