Vault-Scale Modelling of pH Buffering Capacity in Crushed Granite Backfills

Some engineered barrier designs for geological repositories for radioactive wastes rely upon the use of cement as chemical conditioning agents for the wastes. Although the hyper alkaline pore fluids characteristic of cements may have a positive effect upon near-field performance, their migration into the geosphere poses problems regarding potentially deleterious interactions along groundwater flow paths, e.g. change of sorption or matrix diffusion properties of the host rock. To counteract these potential effects, SKB has developed the concept of a gravel backfill for its SFL 3-5 repository, where it is anticipated that the gravel would act as a 'sacrificial' reactive barrier between cement conditioned wastes and the geosphere. This role is dependent upon the reaction of silicate and aluminosilicate minerals in the gravel through hydroxyl ion-catalysed mineral dissolution reactions and the associated precipitation of hydroxyl ion bearing solids, such as calcium silicate hydrates. This barrier concept has been evaluated by the simulation of groundwater flow and chemical reaction of cement pore fluid through a realistic backfill geometry and various potential groundwater flow scenarios. Potential groundwater flow conditions through gravel backfill for the SFL 3-5 repository concept were modelled in 3D using MODFLOW. The regional groundwater flow field was assumed to be horizontal and its interaction with a homogeneous backfill of dimensions in accordance with published designs for the SKB SFL 3-5 repository, with uniform physical and hydraulic properties was investigated. The host rock also had uniform properties, except where simulations explicitly represented transmissive features. Calculations of cement pore fluid migration and reaction with backfill were carried out using Raiden2, a fully-coupled reaction-transport simulator. Flow fields generated in 3D with MODFLOW were converted to 2D 'slices' for reaction-transport calculations. The backfill was assumed to consist of grains of quartz of uniform size of either 4 or 32 mm diameter. The cement pore fluid diffusing from the waste package was assumed to be pure water saturated with portlandite [Ca(OH)2 ] at 25 deg C. The pore fluid saturating the backfill was assumed to be pure water equilibrated with quartz at 25 deg C. A number of different simulations were carried out for both 4 and 32 mm diameter gravel backfill grains: 1. A 2D horizontal slice through the vault length with a transmissive feature in the host rock parallel to the vault length, but positioned roughly two vault widths from the vault. 2. As (1) above, but considering a 2D vertical slice through the vault. 3. A 2D horizontal slice through the vault width with a transmissive feature in the host rock normal to the vault length. 4. A 2D horizontal slice through the vault length with a transmissive feature in the host rock intersecting the vault at 45 deg to the vault. An additional simulation was also carried out for the geometry considered in model variant (4) above where reaction of quartz was excluded so that only the reaction of Ca(OH)2-saturated fluid with ambient groundwater was simulated. The results of the model variants incorporating reaction of quartz were broadly similar, with few differences apparent for the different orientations of the transmissive feature in the host rock. Most simulations showed that the gravel backfill was capable of maintaining pH < 11 in the backfill adjacent to the waste package and pH < 9.5 adjacent to the host rock, at timescales up to 10 000 years. However, some model variants showed pH ∼11 in the backfill adjacent to the host rock at certain locations (beneath the waste package in some simulations, or at outflow regions in others). In all simulations, gradients of pH across the backfill were marked. Appreciable amounts of quartz (in the order of 104 moles after 10,000 years in a representative 1 m thick slice through the backfill) were dissolved in the gravel adjacent to the waste package to achieve pH buffering. All of the dissolved quartz was converted to CSH solids (tobermorite and gyrolite). Reactions in simulations with the 4 mm diameter backfill grains were slightly faster, but the results were otherwise identical to those for the 32 mm diameter grains. The model variant with no chemical reaction of the backfill particles was considerably different from the other simulations with pH > 11 throughout the backfill volume after 3,000 years of simulated time. Only trace amounts of a CSH solid (tobermorite) precipitated in this simulation, reflecting the minor amount of silica in the backfill pore fluid. This simulation highlighted the role of quartz/granite reactivity in retarding the migration of the hyper alkaline pore fluid. The results of the modelling reported here broadly show that a gravel backfill (here represented by quartz) is capable of retarding the migration of high pH pore fluids derived from hydrolysis of a cement/concrete waste package. However, some model variants investigated here suggest that pore fluid pH at the backfill-host rock boundary may increase to pH ∼11, particularly beneath waste packages or at outflow regions of the EBS. These pore fluids have a pH considerably above that expected for groundwaters in Swedish granites and may thus be reactive with respect to minerals lining groundwater pathways. Dissolution of fracture-lining silicates and carbonates may be expected, with concomitant precipitation of solids such as zeolites. These reactions may serve to increase radionuclide retardation, but may need to be considered explicitly in safety assessment