Modeling of an in-situ diffusion experiment in granite at the Grimsel Test Site
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Modeling of an in-situ diffusion experiment in granite at the Grimsel Test Site Josep M. Soler1, Jiri Landa2, Vaclava Havlova2, Yukio Tachi3, Takanori Ebina3, Paul Sardini4, Marja Siitari-Kauppi5 and Andrew J. Martin6 1
IDAEA-CSIC, Barcelona, Catalonia, Spain. UJV-Rez, Husinec-Rez, Czech Republic. JAEA, Tokai, Japan. 4 University of Poitiers, France. 5 University of Helsinki, Finland. 6 NAGRA, Wettingen, Switzerland. 2 3
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 nonfractured 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 (I) or rock capacity (D) 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 D 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 D values in the BDZ. This phenomenon will be investigated in more detail in a second test starting in 2013. EXPERIMENTAL SETUP Several tracers (3H as HTO, t1/2 = 12.33 a; 22Na+, t1/2 = 2.602 a; 134Cs+, t1/2 = 2.065 a; I , t1/2 = 8.021 d, with stable I- as carrier) were continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of about 2.5 years. The experimental setup is shown in Fig. 1.1. The length and radius of the injection interval were 70 cm and 2.8 cm, respectively. The total volume of solution in the circulation system was 8 L [1]. At the end of the experiment, the borehole section was overcored and the tracer profiles in the rock analyzed. Transport distances in the rock were about 20 cm for 3H, 10 cm for 22Na+ and 1 cm for 134Cs+. Recovery of activities was not complete. Experimental results included the evolution of tracer concentrations (activities of 3H, 22Na+, 134Cs+) in the circulation system and tracer distribution profiles in the rock around the injection interval. 3H and 22Na+ showed a similar decrease in activity in the circulation system (slightly larger drop for 3H). The drop in activity for 134Cs+ is much more pronounced. Transport distances in the rock are about 20 cm for 131 -
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H, 10 cm for 22Na+ and 1 cm for 134Cs+. I- diffusion was only evaluated from results of outleaching tests performed on
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