State of the bentonite barrier after five years operation of an in situ test simulating a high level radioactive waste repository

Abstract

The FEBEX is a research project for the study of the near field of a high level radioactive waste (HLW) repository. A part of this project is a large-scale in situ experiment performed at the Grimsel Test Site (GTS, Switzerland) that reproduces a gallery for HLW disposal according to the Spanish concept, with two heaters simulating the containers and a bentonite barrier around them. The system was thoroughly instrumented to follow its evolution, particularly with respect to humidity and temperature changes. After five years of operation, the heater 1 was switched off in February 2002. Following cooling of the system during four months, the bentonite barrier was dismantled and the heater extracted. During dismantling many bentonite samples were taken. Several determinations have been carried out in these samples with the aim of: (1) characterising the actual state of the bentonite and (2) determining the possible changes in its thermo-hydro-mechanical and geochemical properties occurred during the experiment, due to the combined effect of temperature, water content, joints and solutes. The results of the characterisation performed concerning physical properties are reported and analysed in this paper. The swelling of the bentonite has induced the sealing of all the construction gaps of the barrier. An overall degree of saturation of 85% has been reached in the bentonite. The distribution of water content and dry density of the bentonite in vertical sections presents an axial symmetry, with water content increasing from the gallery axis to the gallery wall and dry density following an inverse pattern. Despite the fact that the geological characteristics of the gallery were not homogeneous, there are no differences in average water content among the different sections around the heater. These facts are a consequence of the predominant control of the bentonite upon the hydration kinetics – as a result of the large difference between the bentonite and granite permeabilities – provided that the rock supplies enough water. The agreement between the gravimetric water contents measured on dismantling (by oven drying) and the measurements of relative humidity recorded in place by the instrumentation installed in the bentonite at Grimsel was very good.