Abundance of fissiongenic and pre-reactor natural rare-earth elements in a uranium ore sample from Oklo.

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 pat tern 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.

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

“…On the basis of these data, the proportions of fissiogenic nuclides to total abun dances of nuclides could be approximately evaluated. Details of this evaluation were reported previously (Hidaka et al, 1988). These evaluated values are compiled in Table 4.…”

“…In practice, abundance (ppm) of each of fissiogenic nuclides is divided by its mass and the obtained value is designated as ppm /mass. Therefore, plotting ppm/mass values against mass number corresponds to relative fission pro duct yield curve (Hidaka et al, 1988). The com parison of these curves and the reference curves makes it possible to estimate the retentivity of fissiogenic nuclide in Oklo samples (Hagemann and Roth, 1978).…”

“…The official nomenclatures of these four samples are unknown. One is "sample A" of our previous REE study (Hidaka et al, 1988). This sample comes from reactor Zone 7, 8, 9 and it is uraninite.…”

“…Three samples from Zone 10 are not high-grade U ores, but it is easily recognized that 235U is depleted and the fission products are contained to some extent in each sample. The neutron fluence evaluated from Gd isotopes is also listed in Table 1 (Hidaka and Masuda, 1988). diluted HCI. Fractions for each element of REE were separated by cation exchange resin columns with a-hydroxyisobutyric acid (a-HIBA) (Hidaka et al, 1988).…”

“…The neutron fluence evaluated from Gd isotopes is also listed in Table 1 (Hidaka and Masuda, 1988). diluted HCI. Fractions for each element of REE were separated by cation exchange resin columns with a-hydroxyisobutyric acid (a-HIBA) (Hidaka et al, 1988). Separation procedures and mass spectrometric techniques of other elements were conventional: Rb and Sr (Faure and Powell, 1972), Zr (Shima, 1978), Mo (Tromp et al, 1981, Kawashima, 1990, Ru (Terada et al, 1975), Pd, Ag and Te (Loss et al, 1983), Ba (Eugster et al, 1969), U (Chen and Wasserburg, 1981).…”