Performance assessment model development and parameter acquisition for analysis of the transport of natural radionuclides in a Mediterranean watershed

This paper describes the methodology developed to construct a model for predicting the behaviour of the natural radioisotopes of U, Th and Ra in a Mediterranean watershed. The methodology includes the development of the performance assessment model, obtaining water flow and radiological parameters based on experimental data and analysis of results. The model, which accounts for both water flows and mass balances of the radionuclides in a semi-natural environment, provides assessments of radionuclide behaviour in grassland and agricultural soils, rivers and reservoirs, including the processes of radionuclide migration through land and water and interactions between both. From field and laboratory data, it has been possible to obtain parameters for the driving processes considered in the model, water fluxes, source term definition, soil to plant transfer factors and distribution coefficient values. Ranges of parameter values obtained have shown good agreement with published literature data. This general methodological approach was developed to be extended to other radionuclides for the modelling of a biosphere watershed in the context of performance assessment of a High Level Waste (HLW) repository under Mediterranean climate conditions, as well as for forecasting radionuclide transport under similar Mediterranean conditions that will occur in the future in other areas. The application of sensitivity and uncertainty analysis was intended to identify key uncertainties with the aim of setting priorities for future research. The model results for the activity concentration in the reservoir indicate that for 238U and 230Th the most relevant parameter is the initial concentrations of the radionuclides in the reservoir sediments. However, for 226Ra the most important parameter is the precipitation rate over the whole watershed.