Mobility of uranium in tertiary argillaceous sediments: a natural analogue study

The aim of the Natural Analogue study at Ruprechtov site (CZ) is to investigate the potential mobility of uranium in tertiary argillaceous sediments under representative long-term conditions. Such sediments often build upthe overburden of host rocks for deep geological waste repositories. The Ruprechtov site represents a tertiary basin with argillized volcano-detritic sediments, which is surrounded by granite and underlain by kaolin and granite. U-enrichment mainly occurs in distinct layers of limited thickness on top of the kaolin close to aquiferous horizons and the clay-lignite seams. After a general survey of the site a new set of drillings has been sunk in 2002 in order to investigate the hydraulic, geological and chemical conditions of the area, including detailed investigations of kaolin and granite. Based on actual results new conceptual models for geological evolution and uranium transport pathways at Ruprechtov site have been developed. There is now strong evidence that U-migration at the site is mainly determined by three different processes: a) diffusion through kaolin, b) advection via fault zones across the kaolin and c) advection through aquiferous layers at the interface kaolin/Tertiary. Uranium source is the so-called Erzgebirgs- (Krusne hory) granite, underlying the Tertiary and kaolin and outcropping at the margins of the Tertiary basin.