Implementation of full thermodynamic models in performance assessment (PA) calculations (large domain and very large timescales) is practically unfeasible due to excessive computational times. The complex competitive sorption processes of radionuclides are often simplified.

In this paper, such simple sorption models (i.e. linear Kd and Langmuir isotherms) were compared with more complex thermodynamic models for a reduced geometry and relatively short timescales (compared to traditional PA calculations).

Within the context of a safety case the value of this study is twofold. Primarily, it provides support information on how to choose adequate parameter values in a consistent simplified analysis for compliance with safety criteria. For the cases studied in this paper it became clear that a linear model is sufficient to represent sorption, but a proper choice of the Kd values is critical. Secondly, the comparison of results from these compliance calculations with more realistic analyses demonstrates quantitatively the safety margin implicitly present in PA calculations through model abstraction.