RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

For long-term safety analysis of a potential radioactive waste disposal site it is, amongst others, a prerequisite to understand and characterize transport and retardation processes of relevant radionuclides in order to be able to adequately describe hypothetical release scenarios and assess the barrier capacity of the contaminant providing rock zone but also of the geological formation and the overburden. To mechanistically simulate migration processes of radionuclides or chemical homologues a sound understanding of solid-solution interface reactions is necessary to determine the influence of the geochemical environment on sorption (adsorption, absorption, cation exchange), precipitation, speciation, and dissolution processes. For long-term safety assessments of nuclear waste disposal sites the behaviour of activation and fission products as well as radionuclides from decay chains are of major interest. The trivalent lanthanide europium(III) is a chemical homologue for trivalent actinides such as curium(III) and americium(III) which are products of neutron capture reactions in nuclear reactors.

In the field of safety assessments for a potential radioactive waste repository there is still a need for sound data concerning the interaction of minerals with the surrounding solution even for presumingly well known surfaces such as quartz, muscovite, and orthoclase. Up to now only some studies have taken the comprehensive approach to study the interaction and interrelation of surface charge, surface complexation, and transport processes for trivalent lanthanides and actinides for orthoclase, muscovite, and quartz; and so far only very few studies tried to describe all processes under varying geochemical conditions with one set of mineral-specific parameters. The advantage of one parameter set for a range of geochemical conditions in contrast to one set for each individual geochemical environment is the reduction of the amount of mineral-specific parameter sets necessary to predict transport processes of potentially hazardous pollutants under varying geochemical conditions.

A vast amount of titration, batch, and column experiments were carried out and evaluated with mechanistic thermodynamic sorption models to derive so-called surface complexation parameters of Eu that were subsequently used to simulate Eu reactive transport under varying experimental boundary conditions with reactive transport models. It could be shown that the chosen approach to simulate different geochemical conditions with one surface complexation parameter set (derived from batch experiments) yielded adequate predictions of Eu transport under laboratory and close to nature conditions for the quartz systems; for orthoclase Eu transport under the influence of complexing ligands was satisfyingly represented. Generally, the simulation of column experiments involving orthoclase and muscovite was more challenging and revealed that more precise data but also knowledge are needed to adequately describe retardation and transport processes of the trivalent lanthanide.