Arbeitsbericht NAB 22-23

Diffusion Experiments on Clayey Rock Samples from Nagra's Deep Drilling Campaign as Part of a Benchmarking Study


This study involved diffusion experiments on a set of clay samples obtained from the ongoing deep borehole drilling campaign of Nagra as part of their site selection process. It is part of a benchmark study where SCK CEN was asked to perform diffusion studies on five core samples in collaboration with the main contractor PSI-LES in order to confirm previous findings that through-diffusion experimental measurement errors are acceptable.

Five clay plugs from the Trüllikon-1 borehole were subjected to diffusion of HTO, Cl-36 and Na‑22 in synthetic pore water. The samples were taken between 870 and 940 m depth. The independently determined diffusion parameters from SCK CEN and PSI-LES confirm previously found uncertainties. The variability of the effective diffusion coefficients estimated independently by both institutes was less than a factor 2 and often even much lower. Also, the parameter estimations of the capacity factor (or porosity in case of HTO and Cl-36) agreed well. As an additional benchmark modelling exercise, the experimental datasets of HTO and Cl-36 were also fitted by the model approach of the other institute. The variability among the obtained diffusion parameters was in the same order as the variability of the independently obtained parameters (effective diffusion coefficients and capacity factors).