Analytical results, recommended nuclear constants and suggested correlations for the evaluation of Oklo fission-product data

Several samples from the Oklo site have been analysed in varying degrees of detail to provide information relative to defining the operating history of this reactor and to determine selected fission-product mobility within and around the reactor site. The isotopic composition and the absolute atom abundances of the fission-product component and natural element component were measured for the isotopes of Rb, Sr, Zr, Mo, Ru, Ba, Ce, Nd, Sm and U. The measurement technique was isotope dilution mass spectrometry. Using these absolute data, concordant diagrams were constructed involving the measured quantities; fission density, isotopic abundance of 235U, and the neodymium isotopic ratios 144/143 and 146/145. From these diagrams, the point of concordance provides an estimate of the fluence and the fraction resonance flux, and the degree of concordance provides a means for determining the reliability and consistency or abnormality of the data used in the evaluation. Of particular concern in attempting to reconstruct the Oklo phenomenon is the use of internally consistent nuclear data such as fission-product isotopic data and cross sections. The evaluation is based on data obtained from the analysis of many specially prepared capsules irradiated in a well-characterized and well-moderated thermal reactor. The nuclear constants used for all of the calculations are given. Redistribution and migration data based on absolute concentration measurements are given for the alkali metals, alkaline earths, zirconium, molybdenum, ruthenium, cerium and samarium relative to neodymium. (author)