Occurrence of Pu and fissiogenic REE in hydrothermal apatites om the fossil nuclear reactor 16 at Oklo (Gabon)

Neutron-fission induced nuclear reactions took place 2 Ga ago in the Oklo uranium deposit which is located in the Franceville sedimentary basin (Gabon). The heat released during the running time of the natural reactors led to extensive alteration and desilicification of the sandstones enclosing the reactors and subsequent new formation of clays and apatites, among other phases. The purpose of this study is to determine the capacity of hydrothermally formed apatite to retain actinides and fission products. Combined optical microscopy, scanning electron microscopy and electron microprobe observations revealed the occurrence of fluoroapatite in reaction zone 16. Isotopic and elemental analysis of U, Sm-Nd, Rb-Sr, Bi and Th were performed by ICP-MS and MS-thermoionization on separated apatite grains of various sizes devoid of uraninite inclusions. Most fractions show a slight depletion in 235U. However, one sample shows a significant 235U enrichment (235U/238U = 0.00804 instead of 0.007254 for the natural U) together with high contributions of fissiogenic Nd and Sm. This result shows that 239Pu which is produced by 238U epithermal neutron capture was trapped by the apatite structure during the nuclear reactions or soon after the end of the reactions. It is suggested that combined in situ epithermal neutron irradiation and chemical Pu/U fractionation during the high temperature leaching had contributed to the occurrence of the high 235U/238U ratio in the sample. (orig.)