Sorption Databases for Increasing Confidence in Performance Assessment

Abstract

World-wide activities focus on the remediation of radioactively contaminated sites. One common aim is to deliver a more profound chemical base for risk assessment, namely all those physico-chemical phenomena governing the contamination plume development in time and space. Coupled transport codes able to tackle this challenge have to simplify the resulting very complex reaction pattern. To do so in an adequate way requires extending the knowledge about retardation and mobilisation phenomena and the underlying basic processes and interactions (e.g. physisorption, chemisorption, surface precipitation). Interactions at the solid-liquid interface can be described by complementary approaches, the empirical Kd concept and the mechanistic Surface Complexation Models (SCM). Kd’s are used by most reactive transport and risk assessment codes due to the straightforward numerics involved. In addition, the Kd concept is often the only feasible option for complex solid phases. However, the Kd concept is a rather simplistic approach. Many very different basic physico-chemical phenomena are subsumed in just one conditional parameter. Therefore, extrapolating Kd values may yield very large uncertainties. SCM account adsorption of ions on surface sites as complexation reaction comparable to complexation in solution. The electrical charge at the surface is determined by the chemical reactions of the mineral functional groups, including acid-base reactions and formation of ion pairs and coordinative complexes. The required parameters are site-independent and applicable despite large variations in geochemical conditions. This presents a high potential to increase confidence in safety analysis and risk assessment studies (performance assessment). The mechanistic description of sorption processes with SCM allows a thermodynamically consistent calculation of the species distribution between liquid and solid phase combined with more reliable inter- and extrapolations. However, this requires that all mineral constituents of the solid phase are characterized. Another issue is the large number of required parameters combined with time-consuming iterations. Addressing both approaches, we present two sorption databases, developed mainly by or under participation of the Forschungszentrum Dresden-Rossendorf (FZD). Both databases are implemented as relational databases, assist identification of critical data gaps and the evaluation of existing parameter sets, provide web based data search and analyses and permit the comparison of SCM predictions with Kd values. RES3T (Rossendorf Expert System for Surface and Sorption Thermodynamics) is a digitized thermodynamic sorption database (see www.fzd.de/db/RES3T.login) and free of charge. It is mineral-specific and can therefore also be used for additive models of more complex solid phases. ISDA (Integrated Sorption Database System) connects SCM with the Kd concept but focuses on conventional Kd. The integrated datasets are accessible through a unified user interface. An application case, Kd values in Performance Assessment, is given.

From experimental variability to the sorption related retention parameters necessary for performance assessment models for nuclear waste disposal systems: The example of Pb adsorption on clay minerals

“…In principle, these models could be used in performance assessment (PA) with calculations carried out with reactive transport codes. In practice, this is seldom the case, but recently, a hybrid approach, named "Smart K D ", which takes advantage of SCM flexibility with regards to changes in environmental conditions together with simplified models more amenable to PA calculations, has been advanced to include more flexibility, predictability and transferability in these PA calculations (Richter et al 2009, Stockmann et al 2012, Druteikien et al 2017. Near-field conditions will change over time following local perturbations induced by the presence of the waste repository.…”