Probability density functions for radionuclide migration properties of cementitious materials and their application in safety assessment

Post-closure safety assessment for repositories of radioactive waste includes long-term radionuclide release modelling from a multi-barrier surface or deep repository to the geosphere and biosphere. Modelling of the complex and interacting physical and chemical processes governing radionuclide migration in the near field of a repository requires adequate models and model parameters. This paper discusses a data collection and analysis procedure that is implemented in the safety assessment methodology developed at SCK•CEN in collaboration with ONDRAF/NIRAS. The emphasis here is on flow and transport parameters characteristic of cementitious materials such as structural concrete, concrete waste containers, and cementitious buffer material. The methodology for deriving best estimate parameters together with their probability density functions (pdfs) will be illustrated for the following parameters: K (saturated hydraulic conductivity), Dp (pore-water diffusion coefficient), Kd (distribution coefficient), and Cs (solubility). Best estimate values for parameters Dp, Kd, and Cs and their pdfs are discussed for sixteen radionuclides typical of low- and intermediate level short-lived radioactive waste. Data for this analysis was compiled from a literature review. Applications address radionuclide migration calculations from a surface repository to groundwater by means of a state-of-the-art numerical code. The results discussed include stochastic uncertainty and sensitivity analysis using the derived parameter pdfs. Results from sensitivity analysis show that, among the parameters tested, the variation in distribution coefficient Kd has the largest effect on radionuclide fluxes from the repository.