Conceptual and numerical modelling of radionuclide transport in near-surface systems at Forsmark. SR-Site Biosphere

In the framework of the SR-Site safety assessment, a conceptual and numerical modelling of radionuclide reactive transport in near-surface systems (including till and clay systems) at Forsmark has been carried out. The objective was to evaluate the retention capacity of the near-surface systems, composed of Quaternary deposits, which would be the last natural barrier for an eventual radionuclide release from the deep repository prior to reaching the biosphere. The studied radionuclides are 14C, 129I, 36Cl, 94Nb, 59Ni, 93Mo, 79Se, 99Tc, 230Th, 90Sr, 226Ra, 135Cs and U. Conceptual description and numerical simulations of radionuclide reactive transport show that cation exchange and surface complexation on illite are active processes for the retention of several radionuclides (U, Th, Ni, Cs, Sr, Ra). Surface complexation on iron hydroxide is an active process in the till system, able to effectively retain U and Ni. Another retention process of importance is the incorporation of the radionuclides into mineral phases, either by the precipitation of pure phases or solid solutions. Quantitative modelling has been useful to illustrate the incorporation of C and Sr in the carbonate solid solution in the considered model domains (till and clay), as well as the precipitation of uraninite in the clay sediments and the precipitation of native selenium and radiobarite in the till. Other mineral phases that could, a priori, retain U, Se, Nb and Tc do not precipitate in the simulations, either due to the pH-Eh conditions and/or because the dissolved concentration of the element is not high enough under the considered simulation conditions. It is important to keep in mind that changes in these parameters and in the boundary conditions could modify the predicted behaviour of these elements. The radionuclides that are most significantly retarded are Th, Ni and Cs, mainly through sorption onto illite. Therefore, if the amount of illite (or available sorption sites) decreases, the retardation of these elements will also decrease accordingly, as illustrated by the sensitivity analyses performed. The high retardation predicted for these elements is in good agreement with reported Kd values for Forsmark till and lake sediments. According to the models, Cs, Th and Ni are highly retained, while C, U, Sr and Ra are more mobile. The simulations also show that Nb and Tc behave conservatively in both domains, as expected due to their anionic character under these conditions, and Se only in the clay domain. The reported Kd in Forsmark soils and sediments, although not directly comparable to the calculated effective Kds, show a similar general trend (i.e. the most strongly retained elements are Th, Cs and Ni, followed by U and Sr). The computed behaviour of Se and Nb are the two exceptions that do not agree with reported Kd values. It needs to be recalled also that not all the possible retention processes considered in the conceptual model were included in the simulation, due to either the lack of reliable knowledge and/or the scarcity of thermodynamic data. Besides the retention mechanisms, other processes that produce attenuation of the radionuclides are dilution of the radionuclide-bearing deep groundwater, which applies to all elements, and decay, as we are dealing with radionuclides. The radionuclides that will be more significantly reduced by decay are 226Ra and 90Sr, although 226Ra is a permanent product of the decay of 238U. In this sense, if one considers the possibility that carbonate will be dissolved in a future evolution of the simulated domains, the release into water of the previously retained Sr will not be of significance. Conversely, 14C has a longer half-life and could still be present in the system and contribute to increased radioactive doses.