Abstract: Two Dimensional Simulations of Regional Hydrogeology at Britain's First Nuclear Waste Repository

C. McKeown, R. S. Haszeldine, G. D. Couples

It is proposed that Sellafield in England is to be the site for Britain's first underground nuclear waste repository. The site is within the Borrowdale Volcanic Group (BVG); 6000 m of folded and metamorphosed Ordovician andesites and tuffs. These are unconformably onlapped by 400 m of Carboniferous limestones, Permian clastics and evaporites and more than 1500 m of Triassic sandstones. The company responsible for building and maintaining the repository, UK Nirex Ltd., indicate that groundwater velocities through the proposed repository location are negligible. We maintain that their simplistic numerical modelling did not adequately address the true geological setting and that the overall geometry of the rock units and groundwater salinities are major controls on the paths and rates of fluid flow. Our numerical modelling, performed using OILGEN (a 2-D finite element code with coupled thermal and hydraulic processes), indicates that the repository will be subject to major fluxes of topographically-driven water. Our results predict flows through the suggested site at rates of 1.1 to 19 m/yr., with return flow to the ground surface within 10,000 years in certain cases. We find that flow through the repository is directly controlled by the permeability of the BVG and that the incorporation of lateral changes in salinity is essential to obtain a realistic model. If the engineered containment system fails, then chemical processes in the BVG fractures may be the only remaining barrier to radionuclide release. This study has shown that the hydrogeological all-round suitability of this site is questionable.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994