The Swedish Radiation Safety Authority (SSM) reviews and follows associated research related to the Swedish Nuclear Fuel Company’s (SKB) work with establishing a repository for spent nuclear fuel and an encapsulation facility.
The objective of this study is to understand how data for groundwater compositions were used in producing the Site Descriptive Model for Forsmark and as input data for the safety assessment, SRSite. Improving this understanding sheds further light on the significance of data uncertainties, data processing and interpretation on the degree of uniqueness in the descriptive model, parameterisation of processes, and long-term safety assessment.
The first part of the report (Sections 2 to 4) summarizes the geochemical parameters that need to be measured on groundwater samples and their relevance for site characterization and long-term safety assessment. The chemistry parameters that are considered to be most significant for performance of the engineered and geosphere barriers are identified as ‘safety functions’ with specifications (as requirements or preferences) that need to be assured for the repository system and the associated ‘safety function indicator criteria’ that are used in scrutiny of data from the site investigation. There needs to be sufficient understanding of how groundwater compositions might evolve to allow geochemical parameter values that are measured for present-day groundwaters to be modelled forwards in time for the duration of the safety assessment period. Geochemical measurements have two major uses in the SDM and in safety assessment: (i) interpretations of hydrochemical and isotopic data that test conceptual and numerical models of groundwater movement and solute transport, and (ii) the hydrogeochemical model of the bedrock that describes the evolution of groundwater chemistry around a repository and between repository and biosphere in the long-term future so that the potential influences on the engineered barrier system (EBS) and on radionuclide mobility and retention can be forecast. Hydrochemical data are processed by statistical analysis to support and calibrate the groundwater model. The analysis quantifies the proportions of end-member or reference waters that represent groundwater sources with distinct compositions and inferred ages. The development and application of this approach to describing the palaeohydrogeology of the system, i.e. how the system evolved from post-glacial conditions to its present state, for the SDM is explained. Some of the aspects that introduce uncertainty into the interpretations are discussed. The site-scale long-term groundwater flow and solute transport model has been calibrated by comparing palaeohydrogeological simulations using initial and boundary conditions defined in terms of reference waters with proportions of reference waters calculated for sampled groundwaters. This method has been further developed to calibrate matrix diffusion in the transport model by comparing simulated and measured pore water salinities and stable isotopes. Specific comparisons for model calibration have been made using individual borehole depth profiles and could also be done at site-scale in 2D or 3D representations. Heterogeneity and sparse data constrain the rigour of this approach. The reported comparisons for calibration show quite large discordances and it is unclear how these have been used to calibrate model parameters. The site descriptive model of hydrogeochemistry provides a conceptual model of the processes controlling long-term evolution of groundwater compositions, especially in relation to the repository
SSM 2018:15 volume. It has been tested by showing that the processes account for the observed compositions at the present-day. There are many hydrogeochemical processes that could influence the chemical environment for the EBS to function. The conceptual hydrogeochemical model tends to be rather simplified, for example for pH and redox buffering, sulphide production, water-rock reactions controlling cation concentrations in dilute groundwaters. The modelling method for describing hydrogeochemical evolution of groundwater compositions over time is based on hydrodynamic transport and mixing of individual reference water or end-member components. Model validity in forecasting how specific solutes evolve over time can be tested by
comparing palaeohydrogeological reactive transport simulations with measured data. Results suggest that the hydrogeochemical model requires further development and detailed parameterisation of both transport and geochemical processes. The present model has been used for forecasting evolution of groundwater compositions to 10,000 years in the future. Other approaches have been used for longer timescales and variable boundary conditions that have to be considered in the safety assessment. Measured analytical data for certain chemical entities, mostly at trace concentrations that have specific significance in safety assessment, have been processed and interpreted in SDM-Site and SR-Site to derive best estimates and ranges of uncertainty for their present-day concentrations in the groundwater system. These data and the resulting interpretations of geochemical (and biogeochemical) processes are the basis for describing initial state and for hydrogeochemical modelling of future evolution of these entities in the safety assessment. The second part of the report reviews the use and requirements of geochemical data, including many of the trace entities reviewed in the previous section, for assessment of two important processes of the EBS: canister corrosion, and buffer alteration and erosion. It concludes that the processing and evaluation of data against safety function indicator criteria is reasonable. The third part of the report compares the hydrochemical properties of deep groundwaters at the Finnish site at Olkiluoto with those for groundwaters at Forsmark. There are various substantial differences, e.g. in concentrations of dissolved methane, in contrast to many similarities. The reasons for the differences are not yet fully explained and might be relevant to understanding potential future evolution of Forsmark groundwaters. The final section of the report (Section 8) summarises some recommendations and suggested objectives for future geochemical data acquisition at Forsmark, especially exploiting the opportunities that become available for underground measurements during construction operations.
The challenge for both processes and their relevant geochemical data is to increase confidence that causes of variability are understood and adequately incorporated into the hydrogeochemical models. This requires further data and research of observed systems, perhaps in future underground investigations at Forsmark or at analogue localities. Results would be used to further develop the coupled reactive transport to a level of complexity that provides adequate simulations of the heterogeneity observed now and that could potentially occur in the future.