A model approach to radioactive waste disposal at Sellafield

Sellafield in West Cumbria is the potential site of a repository for radioactive, Intermediate Level Waste (ILW). The proposed repository lies at 650 m beneath the ground surface to the west of the 1000 m uplands of the Lake District. The fractured Borrowdale Volcanic Group (BVG) host rock is overlain by a sequence of Carboniferous and Permo-Triassic sediments. Fresh, saline and brine groundwaters exist in the subsurface. Upward trending fluid pressure gradients have been measured in the area of the potential repository site. Steady-state, 2-D simulations of fluid flow were undertaken with the OILGEN code. Topographically driven flow dominates the regional hydrogeology. Subsurface fluid flow trended persistently upwards through the potential repository site. The dense brines to the west of the site promoted upward deflection of groundwaters. The groundwater flow rate through the potential repository site was dependent upon the hydraulic conductivity of the BVG. Calibration of the model was achieved by matching simulated subsurface pressures to those measured in-situ. Emergent repository fluids could reach the surface in 15,000 years. The measured BVG hydraulic conductivity is up to 1000 times too high to be simply declared safe. Geochemical simulations, with Geochemist's Workbench?, showed natural BVG groundwaters display redox disequilibrium. The in-situ Eh is most probably +66 mV. Pyrite, absent from rock fractures, would not enforce a reducing -250 mV Eh. Steel barrels and alkaline cement are intended to geochemically retain 2.5x106 kg of uranium. Simulations of repository cement/BVG groundwater interactions produced pH 10 at 80°C but no change in the +66 mV Eh. Steel barrel interactions produced an alkaline fluid with Eh -500 mV. Uranium solubility in the high pH repository near field was as high as 10-2.7 M, regardless of steel interactions. Uranium solubility adjacent to the repository (pseudo near field) was controlled by Eh; ranging from 10-13 M in the presence of steel, to 10-2.7 M with no steel. Uranium retention is controlled only by steel barrel durability. Oxidising, natural BVG groundwater will enhance steel barrel destruction. Distant from repository (far field) uranium solubility was 10-5.4 M if Eh was as measured in-situ. Thermodynamic data variations affect the calculation of uranium solubility; uranium near field solubility can be as high as 10-1.4 M. Uranium solubilities in near-field high pH groundwater could be more than 600 times greater than the 10-5.5 M used by the UK Nirex Ltd. in their safety case simulations.