Long term diffusion experiment (LTD); Analyses of H, Na and Cs diffusion in crystalline rock under real deep geological repository conditions

Havlov, V.*; Martin, A. J.*; Siitari-Kauppi, M.*; Tachi, Yukio  

The Long Term Diffusion Project (LTD) is an international project consisting of a series of experiments that aim was to obtain quantitative information on matrix diffusion under in situ conditions. In the 1st in-situ diffusion experiment, radionuclides (H, Na, Cs) were injected into the borehole in undisturbed rock matrix in Grimsel test site. The activity decrease was observed by regular sampling of the experimental solution in the system. After 800 days, the system was closed and the borehole was overcored. The active rock, surrounding the experimental interval, was then sawed and the tracer profiles were measured in the rock samples in order to determine the diffusion extent. Non sorbing H migrated up to 17 cm into the rock matrix. Na as slightly sorbing tracer migrated up to 7 cm. Finally, sorbing Cs migrated surprisingly up to 1.5 cm, instead expected several mms. The analyses results were afterwards used as a base for post mortem modelling.