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

Crystalline rocks such as granites have been investigated as potential host rocks for the geological disposal of radioactive waste in many countries (e.g., EUR, 2005; NEA, 2012; NASEM, 2015). Radionuclide (RN) transport in fractured crystalline rocks can be conceptualized using a dual-porosity model where RNs are transported by advective water flow through a fracture and are retarded by diffusion and sorption into the surrounding rock matrix (Neretnieks, 1980). RNs can then diffuse from the fracture surfaces into the pore network of the fracture fillings and rock matrix and sorb onto pore walls. RN sorption on fracture surfaces and diffusion into the adjacent rock matrix are a key process influencing the safety of the geological disposal. To develop a realistic model and reliable parameters for long-term safety assessments, it is necessary to understand and quantify RN diffusion and sorption processes in the heterogeneous rock matrix and fracture systems.The different types and scales of heterogeneities that must also be considered for RN transport in fractured crystalline rocks include (a) heterogeneous distribution of minerals and pores in the rock matrix, (b) heterogeneity in mineral and pore distribution near fracture surfaces (e.g., fracture fillings and altered zones near fracture surfaces), and (c) heterogeneous flow distribution in the complex channel structures along fracture openings. As discussed in detail in Tachi et al. (2018), the first heterogeneity (a) in a rock matrix affects sorption and diffusion at a relatively smaller scale. The domain pore spaces between mineral grains and intragranular secondary pores form water-filled, heterogeneous networks with varying diffusion-related geometric factors, such as tortuosity and constrictivity, and accessible reactive surfaces related to sorption (e.

Modelling of the LTDE-SD radionuclide diffusion experiment in crystalline rock at the Äspö Hard Rock Laboratory (Sweden)

“…Modelling was performed considering 1D-linear (A cores) and 1D-radial (D cores) models using GoldSim, as reported by Tachi et al (2017Tachi et al ( , 2021. The diffusionsorption equations were analogous to those given by equations 1 and 2.…”

“…The study of matrix diffusion started as early as 1975, when Foster (1975 looked at the issue of tritium in chalk and its diffusion into the porous rock matrix. Since then, many studies have been devoted to the importance of this mechanism for radionuclide retention (Bibby, 1981;Carrera et al, 1998;Glueckauf, 1980;Pickens, 1980, 1981;Guimerà and Carrera, 2000;Haggerty et al, 2000;Maloszewski and Huber, 1990;Neretnieks, 1980Neretnieks, , 2002Ota et al, 2003;Polak et al, 2003;Shapiro, 2001;Skagius and Neretnieks, 1986;Wood et al, 1990), including also field experiments (Aalto et al, 2009;Birgersson and Neretnieks, 1990;Cramer et al, 1997;Cvetkovic, 2010;Hartikainen et al, 1996;Hodgkinson et al, 2009;Ohlsson et al, 2001;Soler et al, 2015Soler et al, , 2022Vilks et al, 2003;Waber et al, 2011;Widestrand et al, 2010b;Zhou et al, 2007).…”