<i>In-situ</i> diffusion of HTO, ²²Na⁺, Cs⁺ and I^- in Opalinus Clay at the Mont Terri underground rock laboratory

Summary The diffusion properties of the Opalinus Clay were studied in the underground research laboratory at Mont Terri (Canton Jura, Switzerland) and the results were compared with diffusion data measured in the laboratory on small-scale samples. The diffusion of HTO, 22 Na + , Cs + and I - were investigated for a period of 10 months. The diffusion equipment used in the field experiment was designed in such a way that a solution of tracers was circulated through a sintered metal screen placed at the end of a borehole drilled in the formation. The concentration decrease caused by the diffusion of tracers into the rock could be followed with time and allowed first estimations of the effective diffusion coefficient. After 10 months, the diffusion zone was overcored and the tracer profiles measured. From these profiles, effective diffusion coefficients and rock capacity factors could be extracted by applying a two-dimensional transport model including diffusion and sorption. The simulations were done with the reactive transport code CRUNCH. In addition, results obtained from through-diffusion experiments on small-sized samples with HTO, 36 Cl - and 22 Na + are presented and compared with the in situ data. In all cases, excellent agreement between the two data sets exists. Results for Cs + indicated five times higher diffusion rates relative to HTO. Corresponding laboratory diffusion measurements are still lacking. However, our Cs + data are in qualitative agreement with through-diffusion data for Callovo–Oxfordian argillite rock samples, which also indicate significantly higher effective diffusivities for Cs + relative to HTO.