The effect of groundwater availability on the release source term in a low hydraulic conductivity environment

Many nuclide release and migration models derive their analysis from a combination of assumed bulk-waste leach-rate and calculated groundwater velocity without accounting for volumetric flow rate through the repository. This paper describes a simple model which derives a source term from calculated volumetric flow rates in granitic rocks, combined with laboratory experimental data on near-field waste-rock-groundwater interactions. The results show that for realistic hydrogeological and geochemical conditions simple velocity/leach rate models, whilst being assumed pessimistic, can in fact overestimate release rate and peak dose values by several orders of magnitude. Calculations are presented of proposed source terms for Tc99 and I129 in a reference case crystalline rock environment, their likely effect on subsequent migration behaviour discussed, and the implications for canister lifetime and backfill integrity outlined