Uranium migration through the Swiss Opalinus Clay varies on the metre scale in response to differences of the stability constant of the aqueous, ternary uranyl complex Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>

Abstract. The simulation of uranium migration through the Swiss Opalinus Clay is used as an example to quantify influence varying values a stability constant in underlying thermodynamic database on lengths for repository scale. Values neutral, ternary uranyl complex Ca2UO2(CO3)3 differ literature by up one order magnitude. Within studied geochemical system, either neutral or anionic CaUO2(CO3)32- predominant one, depending chosen value complex. This leads changed interaction with diffuse double layers (DDL) enveloping clay minerals and thus can potentially diffusive transport uranium. Hence, two identical scenarios only differing were applied compare host rock scale (50 m) after time million years. We ran multi-component diffusion simulations shaly sandy facies Clay. A difference 1.33 log units changes 5 7 m facies, respectively. deviation caused anion exclusion effect. However, maximum distance 22 m, negligible