The UK contribution to the CEC PACOMA Project: far-field modelling of radioactive waste disposal in clay

PACOMA (Performance Assessment of Confinement for Medium-active and Alpha-bearing wastes) is a multinational project supported as part of the Commission of the European Community's R and D programme on radioactive waste management and storage. The aim of the project is to assess the radiological impact of deep geological disposal of intermediate level waste in three different types of geological formation: clay, granite and salt. The contribution of AEA Technology is a study of the groundwater flow and radionuclide migration in the far field. This report describes the far-field modelling. The three-stratum model used in previous hydrogeological studies of the Harwell region is used as a basis for the far-field two-dimensional section through the chalk, clay and Corallian strata underlying Harwell. Each of the three layers is represented as a continuous porous medium with an assumed best-estimate value of permeability, and the groundwater flow is predicted by solving the Darcy equations over the complete section using the NAMMU finite-element code. Two-dimensional transport equations describing the migration of radionuclides in the groundwater are solved, also using NAMMU. The fluxes of radionuclides normal to the surface of the water table are calculated as a function of space and time. The most significant result of the calculations of radionuclide transport through the geosphere is the prediction of multiple pathways for radionuclide movement in the geological strata, and hence multiple release points into the biosphere. Particular attention is paid in the study to the performance and appropriation of the numerical methods and the physical models used for the far-field calculations. (author)