Reconstruction of past U migration in a sedimentary deposit (Coutras, France): Implications for a radwaste repository

This study is focused on ancient U mobility in unconsolidated kaolinite-bearing sediments from the outcrop area of the Coutras U deposit (Gironde, France). The analysis of radiation-induced defects by electron paramagnetic resonance spectroscopy shows significant variations among the investigated samples, and provides the paleodoses due to U. At Coutras, the highest paleodose does not correspond to the highest U concentrations, indicative of a past concentration of uranium different from the present one. The paleodose gives an estimate of the equivalent U concentrations (eU) responsible for the defect concentration, assuming an age for the kaolinites and considering extreme degrees of disequilibrium among U decay chain as uncertainties of the method. The comparison between the equivalent (eU) and normalized present-day U concentrations ([U]norm) reveals past migration of U when these two concentrations are different. Past occurrence of U (and daughters) is revealed even when this radioelement has been subsequently leached. Indeed, surface sandy beds show a slight, barely significant leaching evidence of U (eU > [U]norm). In contrast, several samples show [U]norm ≫ eU, which suggests past accumulation of U younger than the sediment deposition, particularly in organic-rich clay layers. These late enrichments may reach several hundreds ppm U. Isotopic constraints from the literature imply that accumulation is older than 1 Ma in the organic clayey beds. Although the time of transfer or spatial dispersion of U cannot be specified from the methodology and the limited set of samples, the quantitative analysis of radiation-induced defects in clays yields intensity of past radioisotope transfers at the geological timescale. It is relevant for the analysis of U mobility in sedimentary natural analogues of high-level nuclear waste repositories. It is also complementary to classical isotopic analysis of U and Th series.