| Abstract: | The diffusion of tritiated water (HTO) in Opalinus clay (OPA) samples from bore cores from the Benken area (Northern Switzerland) was studied using the radial through- and out-diffusion technique. Results from inverse modelling of out-diffusion data for HTO indicated the presence of two preferential diffusion pathways: a fast and a slow one. Analysing through-diffusion data, however, provides hardly any information concerning a second transport-relevant porosity. Only by also analysing the out-diffusion phase can finer details of the diffusion process and information on sample heterogeneity be recognised. The extracted values for the effective diffusion coefficient are in the order of 3 × 10–11 m2 s–1 for the faster transport porosity and roughly an order of magnitude smaller for the slower type of porosity. We had to account for tritium sorption on the clay minerals by a small but non-zero Kd-value in the order of 10–5 m3 kg–1 in order to reproduce the data with acceptable precision. In the model applied both porosities are considered as being independent from each other. Roughly 30% of the tracer diffused through the second, slower porosity; such a fact might be interesting for future performance assessments for radioactive waste repositories hosted by clay formations. Based on our present picture from water-saturated OPA, on a microscopic scale three different kind of waters can be discriminated: free water, double layer water and interlayer water. However, using HTO as tracer only, it could not be deduced which type of water-filled pores finally account for the transport-relevant porosity.Author for correspondence: Tel.: +41-56-3102257; Fax: +41-56-3104438; E-mail: luc.vanloon@psi.ch |
