Uranium-series radionuclide and element migration around the Sanerliu granite-hosted uranium deposit in southern China as a natural analogue for high-level radwaste repositories

Abstract

The Sanerliu uranium deposit located in Hunan Province, South China, has a number of features analogous to aspects considered in the performance assessment of concepts for the disposal of high-level radwaste deep in granite. The deposit is hosted by medium- to coarse-grained porphyritic granite with a Rb–Sr isochron age of 215.2±6.3 Ma. The orebodies have a burial depth of more than 160–400 m under the surface of the ground. The major uranium mineralizations have U–Pb ages (pitchblende) of 103.2±0.6, 88.3±0.5 and 51.2±0.4 Ma and a hydrothermal origin. The isotopic data show that the Rb–Sr systems in the whole-rock samples of less- and un-fractured host granite around the orebodies have been little disturbed since the altered granite formed (132 Ma ago). Such samples have close to secular equilibrium activity ratios for 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th, indicating that obvious water–rock interactions have not taken place in them within the interval of at least the last 1 Ma. The migration of uranium, thorium and most trace elements in the ores and the host granite are limited, often less than 30–35 m over 51 Ma. The alpha activity ratios 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th for the samples from the fracture zones show obvious deviations from secular equilibrium, indicating the deposition of uranium during the last 1 Ma and of radium during the last 10 3 yr. The fracture zones are channels for groundwater flow. However, the migration of uranium-series radionuclides along the fracture zones is limited by adsorption onto the clays. This is confirmed by the absence of any hydrogeochemical indication on the ground surface of the orebodies. The ores have been well preserved in a natural aquifer system without any specially designed artificial barriers. The migration behavior of radionuclides in the ores of the Sanerliu deposit may be termed a worse-case scenario for the safety assessment of a high-level radwaste disposal repository hosted by granitic rocks.