K_d setting approach through semi-quantitative estimation procedures and thermodynamic sorption models: A case study for Horonobe URL conditions

Abstract

The use of generic sorption data in PA requires the transfer of the data to the PA-specific conditions. A site-specific K_d setting approach for PA calculations was tested, comparing two data transfer procedures. First transfer of sorption data can be done through semi-quantitative estimation procedures, by considering differences between experimental and PA geochemical conditions (sorption capacity, radionuclide speciation, competitive reactions, etc.). On the other hand, thermodynamic sorption models allow to estimate K_d variations directly, based on quasi-mechanistic understanding. The present paper focuses on illustrating example calculations regarding the derivation of K_d values, and their uncertainties, of Cs, Ni, Am and Th, for the mineralogical and geochemical conditions of the mudstone system at the Horonobe URL. Clay minerals (illite and smectite) were considered as sorption-relevant minerals in all cases. The K_d setting results were compared with K_d measured for Horonobe mudstone by batch experiments. The results indicate that K_d can be quantitatively evaluated from generic sorption data when adequate data and models are available. The careful evaluation and conjunctive use of calculated and measured K_d values can enhance the reliability of K_d setting and uncertainty assessments.