ISOTOPIC EVIDENCE FOR THE RETENTION OF SR-90 INFERRED FROM EXCESS ZR-90 IN THE OKLO NATURAL FISSION REACTORS - IMPLICATION FOR GEOCHEMICAL BEHAVIOR OF FISSIOGENIC RB, SR, CS AND BA

In order to investigate the mobility of fissiogenic Sr-90 in the geolo gical environment, the Zr isotopic compositions of seven samples from one of the newly formed Oklo natural reactor zones (i.e., reactor core and adjacent rocks (10, SF84)) in the Republic of Gabon were determin ed with an inductively coupled plasma mass spectrometer (ICP-MS). Zr i sotopes in uraninite grains from different reactor zones were also mea sured by secondary ion mass spectrometry (SIMS). Fissiogenic Zr isotop ic abundances of three samples from the reactor core have excess Zr-90 , which has never before been formed in previous Oklo samples. In this paper, the geochemical behaviour of Zr-90 is discussed by making use of the relative retentivity inferred from the isotopic abundance of Sr . The excess in Zr-90 suggests dependence on the degree of retention/m igration of Sr-90, the precursor of Zr-90 in the fission chain. In the aqueous phase, chemical fractionation between Sr and Zr could have oc curred before radioactive Sr-90 decayed. Considering the halflife of S r-90 (t1/2 = 29.1 y), considerable amounts of the latter have been pro duced during criticality. Sr and Zr (including Zr-90) could have been redistributed between the reactor core and its vicinity. The retentivi ty of fissiogenic Zr-90 in reactor core 10 is not homogeneous. In addi tion, the distributions of Rb, Cs and Ba is also heterogeneous.