A Scaling Approach for Retention Properties of Crystalline Rock: Case Study of the In‐Situ Long‐Term Sorption and Diffusion Experiment (LTDE‐SD) at the Äspö Hard Rock Laboratory in Sweden

Abstract

AbstractThe in‐situ long‐term sorption and diffusion (LTDE‐SD) experiment performed at the Äspö Hard Rock Laboratory in Sweden aims at increasing knowledge of sorption and diffusion processes of radionuclides in natural fractures and granodiorite matrix under in‐situ conditions. This paper presents a comprehensive modeling exercise focusing on the scaling approach for sorption and diffusion parameters from laboratory to in‐situ conditions using the LTDE‐SD data set for six representative radionuclides. The near‐surface heterogeneities at both the fracture surface and rock matrix were evaluated by the conceptual model with high porosity, diffusivity, and sorption capacity, as well as their gradual changes at the near‐surface zones. The modeling results for non‐sorbing Cl‐36 and weak‐sorbing Na‐22 validated the model concept and parameter estimation of porosity and diffusivity by considering a 5‐mm‐thick disturbed zone with gradual parameter changes and an undisturbed matrix characterized by constant parameters. The effective diffusivities (De) of these cationic and anionic tracers showed typical cation excess and anion exclusion effects; however, the cation excess diffusion was more pronounced due to the heterogeneous pore networks in this granodiorite. The modeling results for high‐sorbing tracers (Cs‐137, Ra‐226, Ni‐63, and Np‐237) with different sorption mechanisms validated the scaling approaches of distribution coefficients (Kd) as a function of particle size and their relation to near‐surface disturbances. Sorption dependence on particle size could be understood in relation to different sorption mechanisms and mineralogical features. This modeling exercise provided key recommendations toward better scaling approaches for estimating reliable De and Kd parameters under in‐situ conditions.