Reconstruction of cumulative fission yield curve and geochemical behaviors of fissiogenic nuclides in the Oklo natural reactors.

Abstract

Isotopic ratios and elemental abundances of rare earth elements (REE), Rb, Sr, Zr, Mo, Ru, Pd, Ag, Te, Ba and U in four samples from two Oklo Reactor Zones 7, 8, 9 and 10 were determined by thermal ionization mass spectrometry. These elements have unusual but reasonable isotopic anomalies due to the effect of fission and/or neutron capture reactions. In order to discuss the retentivity of fissiogenic nuclides, the fission product yield curves covering the wide mass range from 90 to 160 were reconstructed on the basis of our results. We have estimated the behavior of fissiogenic nuclides by means of the differences between the measured curves and empirical ones. From comparison of these two sets of curves, measured and empirical, the behaviors of fissiogenic nuclides were deduced. Our results are in agreement with previous Oklo work at the following points: (I) Ru, Pd, Te and most of a series of REE (except for La and/or Ce) have been well retained in the samples, (II) Rb, Sr and Ba have been lost to a great extent, and their isotopic ratios are nearly the same as those of terrestrial standard values, and (III) Zr, Mo and Ag have been partially removed from the reactors. The behaviors of Zr and Mo seem to have been highly affected by the chemical and geological environments in and around the reactors. The present work reveals that, among REE, La and Ce might have been partially removed in contrast to the good retention of other REE. Also, there is a possible remobilization of 90Sr during operation of the Oklo reactors.