Comparison betweenin situand laboratory diffusion studies of HTO and halides in Opalinus Clay from the Mont Terri

Summary A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide (127I-) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficientsDLand accessible porosity for HTO,36Cl-and125I-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 andin situprofiles 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 areDL= 5×10-11m2/s andDL= 1.5×10-11m2/s respectively. For the laboratory tests,DLvalues for HTO are in the range of 5×10-11m2/s to 8.5×10-11m2/s. For125I-and36Cl-the measured values areDL=νmber1.4×10-11andDL=1.6×10-11m2/s respectively. All HTO results obtained with a through diffusion technique are within the same range as those obtained in thein situtests. TheDLvalues obtained in diffusion cells with125I-and36Cl-and the value drawn from the interpretation of stable127I-concentration profiles from thein situtests 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).