Comparison between<i>in situ</i>and laboratory diffusion studies of HTO and halides in Opalinus Clay from the Mont Terri

Abstract

Summary A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide ( 127 I - ) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficients D L and accessible porosity for HTO, 36 Cl - and 125 I - were also measured on centimetric Opalinus clay samples using the through diffusion technique. The evolution of tritium and iodide concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation. A detailed analysis of the results pointed out the effect of a disturbed zone around the borehole with higher diffusion coefficients. The best estimate values for HTO and iodide in the undisturbed rock are D L = 5×10 -11 m 2 /s and D L = 1.5×10 -11 m 2 /s respectively. For the laboratory tests, D L values for HTO are in the range of 5×10 -11 m 2 /s to 8.5×10 -11 m 2 /s. For 125 I - and 36 Cl - the measured values are D L =νmber1.4×10 -11 and D L =1.6×10 -11 m 2 /s respectively. All HTO results obtained with a through diffusion technique are within the same range as those obtained in the in situ tests. The D L values obtained in diffusion cells with 125 I - and 36 Cl - and the value drawn from the interpretation of stable 127 I - concentration profiles from the in situ tests are very close. In fact, some significant uncertainties could be identified ( i.e. a likely chemical retention of iodide on argillites, effect of the disturbed zone).