Abstract A UK repository concept currently under consideration for the disposal of intermediate-level radioactive waste and some low-level waste not suitable for surface disposal involves using large quantities of cementitious materials for construction, grouting, waste containers, waste isolation matrix and buffer/backfill. CO 2 generated from the degradation of organic material in the waste will result in cement carbonation and associated mineralogical changes. Hydraulic and gas permeability tests were performed on Nirex Reference Vault Backfill (NRVB) cement at 40 °C and either 4 or 8 MPa. Carbonation reactions using CO 2 gas halved the permeability of the NRVB under simulated repository conditions. A greater decrease in permeability (by three orders of magnitude) was found during carbonation using dissolved CO 2 . Mineralogical changes were found to occur throughout the cement as a result of the reaction with CO 2 . However, a narrow zone along the leading edge of a migrating reaction front was associated with the greatest decrease in porosity. Fluid pressures increased slightly due to permeability reductions but fluid flow still continued (albeit at a lower rate) preventing the build-up of overly high pressures. Overall, the observed reductions in permeability could be beneficial in that they may help reduce the potential for fluid flow and radionuclide migration. However, continued carbonation could lead to potential issues with regards to gas pressure build-up.

Abstract The sequestration of CO 2 in the deep geosphere is one potential method for reducing anthropogenic emissions to the atmosphere without a drastic change in our energy-producing technologies. Immediately after injection, the CO 2 will be stored as a free phase within the host rock. Over time it will dissolve into the local formation water and initiate a variety of geochemical reactions. Some of these reactions could be beneficial, helping to chemically contain or ‘trap’ the CO 2 as dissolved species and by the formation of new carbonate minerals; others may be deleterious, and actually aid the migration of CO 2 . It will be important to understand the overall impact of these competing processes. However, these processes will also be dependent upon the structure, mineralogy and hydrogeology of the specific lithologies concerned and the chemical stability of the engineered features (principally, the cement and steel components in the well completions). Therefore, individual storage operations will have to take account of local geological, fluid chemical and hydrogeological conditions. The aim of this paper is to review some of the possible chemical reactions that might occur once CO 2 is injected underground, and to highlight their possible impacts on long-term CO 2 storage.