Geochemical characterization of uranium mill tailings (Bois Noirs Limouzat, France) highlighting the U and 226Ra retention

As with other metals, the management of tailings from former uranium (U) mines requires a good knowledge of the geochemical mechanisms governing the retention of radioelements of interest: U and Ra-226. This article presents the results of the study of the bearing phases featuring these two radioelements within the Bois Noirs Limouzat tailings storage facility (Loire), the only site in France where the tailings (a sandy silt facies and a clayey silt facies) are currently stored only under water. The aim is to gain a better understanding of their respective mobility under current storage conditions. For this purpose, a multi-scale approach was adopted combining historical research and airborne image analysis to select the core location, chemical and radiological analyses, mineralogical characterizations supplemented by sequential extractions (two specifically developed protocols). The results show that U and Ra-226 are mainly found in the clayey silt facies with an average U concentration of 243.3 ppm (132.3 ppm in the sandy silt facies) and an average Ra-226 mass activity of 64.7Bq/g (18.0Bq/g in the sandy silt facies). These results are in accordance with the initial U grade of the ore (2 parts per thousand), the extraction efficiency of the ore processing plant (95%) and the age of mineralization (305 Ma). The approach adopted made it possible to highlight several mineralogical traps available for each radioelement, regardless of the facies type. Thus, a significant part of the U is still trapped within the primary phases, resistant to treatment and therefore relatively immobile under current storage conditions (49.6%-77.8% for the sandy silt facies and 27.2%-36% for the clayey silt facies). Most of the leached U is mainly associated with weakly crystalised iron oxyhydroxides (8.7%-42.4% for the sandy silt facies and 50.9%-71.8% for the clayey silt facies) and to a lesser extent with clay minerals (5%-12.3% for the sandy silt facies and 0.8%-11.5% for the clayey silt facies). For the Ra-226, irrespective of the facies type, a significant part remains trapped within phosphate phases, resistant to the leaching process and therefore also relatively immobile under storage conditions (24.4%-38.9% for the silty sandy silt facies and 39.9%-98.9% for the clayey silt facies). Sequential extractions revealed a different geochemistry of Ra-226 depending on the facies. For the silty sandy silt facies, most of the Ra-226 is mainly associated with the clay minerals (6.4%-69.2%) and to a lesser extent with iron oxyhydroxides, barite or aluminum phosphate sulphate minerals (APS) (6.4%-33.9%). For the clayey silt facies, most of the Ra-226 is mainly associated with iron oxyhydroxides, barite or APS (6.4%-53.3%) and to lesser extent clay minerals (0.4%-6.8%). The leaching process did not allow the differentiation between the contributions of each of these phases to the retention of Ra-226. At last, all the identified bearing phases demonstrate that the U is relatively immobile under the current storage conditions, irrespective of the facies. For the Ra-226, the bearing phases differ according to the facies. Within the sandy silt facies, the Ra-226 is mainly borne by clay minerals and can be mobilised more easily. However, the sandy silt facies represents only one third of the tailings currently.