Effects of cementitious leachates on the EBS

This report describes the method using which the effects of cement based materials on the engineering barrier system in the repository for spent nuclear fuel planned to Olkiluoto has been assessed. The phenomena considered are degradation of cement based materials due to interaction with groundwater, release of degradation products to groundwater, migration of degradation products in fracture network, interaction of degradation products with rock, and interaction of degradation products with buffer and backfill. Assessment of degradation of cement based materials is based on theories presented in the literature, measurements, and on numerical simulations. Migration of degradation products from the porewater for cementitious materials to the groundwater passing by gives rise to formation of a plume downstream of the material. This plume is comprised of degradation products. The ability of the plume to transport degradation products is assessed using the concept of concentration boundary layers. In the assessment of plume migration in hydraulically active fracture network it has been assumed that the plume does not get mixed with the surrounding groundwater such that the concentrations of the degradation products in the plume remain unaltered. In addition to this, only the transmissivities of various parts of the fracture network limit the migration of the plume. The migration of the plumes is assessed by using the values of known transmissivities in the potential flowpaths. The reactions of the plume with fracture minerals has been assessed, but due to the uncertainties in the parameters used in these assessments the mechanisms potentially decreasing the amounts of degradation products and their concentrations in the plume are not accounted for. Interactions with engineering barrier system are limited to the interactions of the plume with buffers and deposition tunnel backfills. These reactions are not considered in details due to insufficient understanding, but on mass balance basis. The parameter values used are argumented partly using measurements committed at Olkiluoto and partly by assumptions argued conservative. As results it is presented amongst others that degradation of cement based materials is so slow in Olkiluoto groundwater that only a fraction of the cement based materials degrade during the period relevant from the point of view of this assessment. Moreover, the velocities of groundwaters in contact with cement based materials are so low that the release rates of degradation products to groundwater remain low as well. This way the amounts of degradation products migrated by the plumes remain small. The factor dominating the migration of the plumes in fracture network is the ratio of transmissivities of the largest transmissivity upsteam from any location to the smallest transmissivity downstream from the same location; the larger the ratio, the smaller the fraction of the plume in question gets migrated to the less transmissive fractures at downstream locations. As a summary it is concluded that the effects of the cement based materials on engineered barrier system remain inconsequential. The biggest contributions are due to the so called higher pH cementitious material used at locations more distant with respect to the repository and due to the so called low-pH cementitious materials used in the repository. (orig.)