Geochemical Characteristics of an Ancient Nuclear Reactor "Oklo"

Abstract

The Oklo uranium deposit at the Republic of Gabon, central Africa, had partly functioned as natural fission reactors. Large-scale fission chain reactions spontaneously occurred at 16 separate areas in the Oklo deposit, so-called “reactor zones (hereafter RZs)”, two billion years ago, and sustained for 24,000 to 200,000 years. Two more RZs have been identified at the Okelobondo and Bangombé uranium deposits close to Oklo. Figure 1(a) shows a map of the Oklo and Okélobondo uranium deposits. Many elements of the Oklo RZs and the related samples show the variations in the isotopic composition caused by a combination of nuclear fission, neutron capture, and radioactive decay. Isotopic studies provide useful information to estimate reactor conditions and to understand behavior of radionuclides in geological media. The data are also of value in particle physics to examine time-variations of fundamental physical constants. Most of previous isotopic analyses for the Oklo studies were based on bulk analysis of the rock samples with chemical separation for individual elements. As the first stage of the Oklo study, retentivities of the most of the fissiogenic isotopes such as rare earth elements (REE), platinoid elements, and alkaline elements in the Oklo reactors were quantitatively determined from the isotopic data of the bulk analyses using thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS). Although the bulk analyses have provided precise isotopic data, the results indicate average information in the samples. It is known that some kinds of fission products are heterogeneously distributed in uranium matrices, even if they have Geochemical Characteristics of an Ancient Nuclear Reactor “Oklo”