Modeling of an in-situ diffusion experiment in granite at the Grimsel Test Site

Abstract

Matrix diffusion is a key process for radionuclide retention in crystalline rocks. Within the LTD project (Long-Term Diffusion), an in-situ diffusion experiment in unaltered non-fractured granite was performed at the Grimsel Test Site (www.grimsel.com, Switzerland). The tracers included 3H as HTO, 22Na+, 134Cs+ and 131I- with stable I- as carrier.The dataset (except for 131I- because of complete decay) was analyzed with different diffusion-sorption models by different teams (NAGRA / IDAEA-CSIC, UJV-Rez, JAEA, Univ. Poitiers) using different codes, with the goal of obtaining effective diffusion coefficients (De) and porosity (ϕ) or rock capacity (α) values. A Borehole Disturbed Zone (BDZ), which was observed in the rock profile data for 22Na+ and 134Cs+, had to be taken into account to fit the experimental observations. The extension of the BDZ (1-2 mm) was about the same magnitude as the mean grain size of the quartz and feldspar grains.De and α values for the different tracers in the BDZ are larger than the respective values in the bulk rock. Capacity factors in the bulk rock are largest for Cs+ (strong sorption) and smallest for 3H (no sorption). However, 3H seems to display large α values in the BDZ. This phenomenon will be investigated in more detail in a second test starting in 2013.

The tritiated water and iodine migration in situ in Grimsel granodiorite. Part I: determination of the diffusion profiles

“…The HTO activities against the rock mass are in good agreement with the activities measured at the National Research Institute in Czech Republic from cores NRI1 and NRI2 (shown in Fig. 3) which varied from 0.1 to 10 Bq g -1 [29,32]. I in situ diffusion profiles versus distance from the circulation borehole wall are shown in Fig.…”

“…A more detailed description of the experiment and hydraulic properties of the host rock can be found in Moeri et al [28] and Soler et al [29].…”

Matrix diffusion and sorption of Cs+, Na+, I– and HTO in granodiorite: Laboratory-scale results and their extrapolation to the in situ condition

“…This implies that additional effects on in situ diffusion and sorption of HTO or additional processes other than diffusion and sorption (unknown at this moment) must be present in order to fill the gaps in the HTO behaviour observed in the in situ condition. Although, a larger α and K d for HTO can lead to better fit for both the tracer depletion and depth profile for HTO (as shown in Soler et al, 2014), this issue will be further investigated in more detail in a second in situ diffusion experiment (Soler et al, 2014).…”

“…To set reliable parameters of matrix diffusion and sorption relevant to performance assessment, it is necessary to understand as much as possible the processes of diffusion and sorption both in the laboratory and in situ experiments, and to develop a way to extrapolate from the laboratory to in situ conditions. This was the aim of the work presented in this paper as part of the Long Term Diffusion (LTD) project (Martin and Blechschmidt, 2009;Soler et al, 2014) at the Grimsel Test Site (GTS), Switzerland. Diffusion and sorption mechanisms for cationic Cs + and Na + , anionic I -and neutral HTO tracers were interpreted from through-diffusion tests using core samples cut into discs and batch sorption tests using crushed rock samples.…”

Modeling Transport of Cesium in Grimsel Granodiorite With Micrometer Scale Heterogeneities and Dynamic Update of Kd

“…The TDRW method has been used to study the effect of centimeter scale structural heterogeneities on diffusion in various cases (Robinet et al, ; Sardini et al, ; Voutilainen et al, ). The TDRW method has also been found to be a powerful tool for analyzing results from both in situ and laboratory experiments especially when the initial or boundary conditions are complicated (Ikonen et al, ; Soler et al, ). Previously Dentz et al () have modified the TDRW method by including sorption of diffusing elements in heterogeneous media.…”