Characteristics of uranium uptake of Boda Claystone Formation as the candidate host rock of high level radioactive waste repository in Hungary

Abstract

In Hungary two geological sites of the Boda Claystone Formation (BCF) located in the W-Mecsek mountains (South Hungary) have been selected for the study of potential host rocks for high level radioactive waste, Gorica (G) Block and W-Mecsek Anticline (WMA) Block. The aim of the study was to obtain information on the uranium uptake characteristics of both sites. Our results revealed that in the sample, taken from WMA Block, where dolomites have ankerite rims and U-bearing rings, newly formed FeOOH precipitations were observed, which partly replaced the ankerite at near neutral pH (6.8). UO2 2+ ions were adsorbed easily due to the enhanced specific surface area and high adsorption capacity of FeOOH formed from the dissolution of ankerite. The oxidation of Fe2+ leads to the formation of FeOOH parallel to partial reduction of U6+ to U4+. 75 % of uranium was taken up by clay minerals and 25 % by FeOOH although the ankerite concentration is as low as 6 % in WMA Block of BCF. In G Block, the argillaceous matrix was found to be mainly responsible for the UO2 2+ uptake. The study demonstrated that the U retention capacity of different BCF locations was significantly influenced by the mineralogy of these sites.