Natural decay series studies of the redox front system in the Poços de Caldas uranium mineralization

Abstract

As part of a major natural analogue study of relevance to radioactive waste disposal, natural decay series disequilibrium techniques were used to provide information on the mechanisms and rates of processes occurring at redox fronts in an extensive uranium deposit near Poços de Caldas, Brazil. This topic is of direct relevance to waste disposal safety assessment in view of the postulated development of radiolysis-induced oxidizing conditions in respositories. The study confirmed the downwards displacement of uranium by oxidizing groundwater interacting with the reduced rock, resulting in downwards movement of the redox front, with associated cyclical oxidation-induced dissolution and reduction-induced redeposition of uranium generating enhanced uranium concentrations at the redox fronts. Deposition of uranium occurs on both sides of the redox fronts, with reduction to insoluble U 4+ in the reduced rock and scavenging by secondary iron oxides being identified as the respective mechanisms of deposition. Radioactive disequilibrium considerations indicate that, while the uranium deposited in the oxidized rock is retained for times of at least 10 6 y, some slow redissolution does occur, possibly involving mainly preferential loss of 234U. Sequential leaching indicates that the redeposited uranium in both the oxidized and the reduced rock is more readily dissolved than the uranium in the unaltered reduced rock. It is concluded that the observed distributions of uranium were produced by diffusion in both directions away from the redox fronts in conjunction with groundwater flow downward along the direction of the fronts. Limited redistribution of thorium has occurred, but at a level at least two orders of magnitude less intense than that of uranium and there is a systematic separation of uranium from thorium with increasing depth in the rock. At two locations where the oxidized rock overlies the reduced rock, natural decay series disequilibrium indicated a rate of movement of the front of 2–20 m in 10 6 y, suggesting that the rate of movement of the redox front is controlled by the rate of regional erosion (about 12 m in 10 6 y). At a location where the reduced rock overlies the oxidized rock, the front was estimated to have moved less than 1 cm in 10 6 y. This location was also the site of the most intense mineralization of uranium in the samples studied. 226Ra mobility over distances of the order of 10 m was observed, along with preferential transfer of 226Ra from the reduced to the oxidized rock at the deeper redox fronts. The postulated maximum rate of far-field movement of a repository-related redox front of about 50 m in 10 6 y is concluded to be a realistic value for use in models. Scavenging of radionuclides by secondary minerals forming in oxidized rocks is identified as a potentially important retardation process which might not be apparent from purely thermodynamic considerations. The effect of a repository-related redox front would be initially to retard radionuclide movement by at least partial deposition at the front. If such a redox front were to penetrate a sufficient distance through the far field, it could, however, result in the breakthrough of a pulse of radionuclides to the near-surface zone.