State of the in situ Febex test (GTS, Switzerland) after 18 years: a heterogeneous bentonite barrier

The in situ Full-scale Engineered Barriers Experiment or Febex was a full-scale test reproducing the near field of a nuclear waste repository. It was performed in a gallery excavated in granite at the Grimsel Test Site in Switzerland, with a heater whose surface temperature was set to 100°C, simulating the waste canister and a bentonite barrier composed of highly compacted blocks. The test was completely dismantled after 18 years of operation. Numerous samples of bentonite were taken for the on-site determination of dry density and water content. The on-site measurements showed that the physical state of the barrier was very much affected by the processes to which it had been subjected, namely, hydration with the granite groundwater and/or thermal gradient. Although the degree of saturation of the bentonite was overall quite high, there were important water content and dry density gradients everywhere in the barrier, but the gradients were steeper around the heater. These gradients did not impair the performance of the barrier, but imply that the barrier can be irreversibly inhomogeneous.