Some aspects of release and transport of gases in deep granitic rocks: possible implications for nuclear waste repositories

Radon, radium and helium data from three sites in granitic rock in Sweden (Forsmark and Laxemar) and Finland (Olkiluoto) from depths greater than 1,000 m were interpreted by a model that describes how daughter nuclides, including helium from uranium and thorium decay, escape from minerals, enter the pore water in the rock matrix and diffuse to the seeping water in the fractures in the rock. The radium concentrations are on the order of < 30 Bq/l of water that has recently infiltrated and then emerged from the rock. Radon concentrations are mostly several orders of magnitude larger. The model predicts values in the same range. The fair agreement between model results, field data and laboratory data over a scale spanning micrometres over meters to kilometres, and time scales of days to millions of years, shows that the micropores of the rock matrix are connected even at depths down to and beyond a kilometre. Molecular diffusion in the matrix pore water is a key migration mechanism. Laboratory-derived sorption coefficients for radium are of the same magnitude as those needed in the modelling of the in situ data to give good agreement.