Determination of recent 238U, 234U and 230Th mobility in granitic rocks: application of a natural analogue to the high-level waste repository environment

Abstract

The isotope activity ratios 234U/ 238U and 230Th/ 234U have been investigated in drill core samples up to 1300 m depth in three granitoids from Australia to identify the nature of possible recent (<1 Ma) radionuclide mobilisation. Most whole-rock samples from the Coles Bay Granite, Tasmania, display secular equilibrium between 234U and 238U ( 234U/ 238U∼ 1.0) and preferential 234U loss occurring in one highly fractured sample. Significant 230Th/ 234U disequilibrium throughout most of the drill core is attributed to a depletion in bulk U and production of excess 230Th. Radioactive disequilibrium which occurs between 238U, 234U and 230Th in drill core samples from the Kambalda Granodiorite, Western Australia indicates U gain as a result of interaction with ground waters enriched in 234U. Near-surface samples are weathered and display preferential loss of 234U through further surface water interaction. Significant radioactive disequilibrium in the Roxby Downs Granite, South Australia suggests that a number of interactive U accumulation and removal events have occurred. Initial U deposition from enriched ground water is indicated in samples to 530 m depth and has been followed by selective removal of 234U ( 238U/ 234U∼ 1.0 and 230Th/ 234U> 1.0) which has generated excess 230Th. These enriched ground waters do not appear to have penetrated below 530 m depth, and 230Th excess indicates bulk loss of U. The observed disequilibria indicate that significant radionuclide mobilisation has occurred in these rocks during the recent past (<1.2 Ma) as a result of ground water migration through the granitoids, principally via fracture flow and rock-matrix diffusion. Interaction between the migrating ground waters and the rock has been influenced by the presence of fracture-infilling and other secondary minerals. As an analogue for the behaviour of the actinides in solidified high-level waste forms (including spent fuel) in a granitic repository environment, the disequilibrium observed in these granitoids provides a qualitative identification of the migration processes and helps to characterise potential radionuclide behaviour which may occur within a crystalline high-level nuclear waste repository.