Guidelines for thermodynamic sorption modelling in the context of radioactive waste disposal

Payne, T. E.*; Brendler, V.*; Ochs, M.*; Baeyens, B.*; Brown, P. L.*; Davis, J. A.*; Ekberg, C.*; Kulik, D.*; Lutzenkirchen, J.*; Missana, T.*; Tachi, Yukio  ; Van Loon, L.*; Altmann, S.*

Thermodynamic sorption models (TSMs) can be utilised to provide a scientific basis for Kd setting in the safety case, and for assessing the response of Kd to changes in chemical conditions. The TSM development involves a series of decisions on model features such as surface sites, sorption reactions and electrostatic corrections. There is a lack of consensus on the best ways to develop TSMs, and the NEA has therefore co ordinated an international project to assess the strategies and processes for building a TSM. This paper presents recommendations from the project on a number of aspects of TSM development in the context of radioactive waste disposal. Key recommendations include: definition of modelling objectives, identification of major decision points, a clear decision making rationale with reference to experimental or theoretical evidence, a suitable consultative and iterative model development process, testing to the maximum practicable extent, and documentation of key decisions.