Dismantling of the first section of the FEBEX in situ test: THM laboratory tests on the bentonite blocks retrieved

Abstract

The FEBEX in situ experiment at the Grimsel Test Site (Switzerland) reproduces a gallery for high-level radioactive waste disposal according to the Spanish concept, with two heaters simulating the waste containers and a bentonite barrier around them. After five years of operation, heater 1 was switched off in February 2002. Following cooling of the system, the bentonite barrier was dismantled and the heater extracted. During dismantling many bentonite samples were taken to determine in several positions water content, dry density, thermal conductivity, permeability, swelling capacity, water retention capacity and consolidation behaviour, among others. The aim of these determinations was: (1) characterise the actual state of the bentonite and (2) determine the possible changes in its properties occurred during the experiment. The results of the thermo-hydro-mechanical (THM) characterisation are reported and analysed herein. The distribution of water content and dry density of the bentonite in vertical sections presents axial symmetry. All the THM parameters determined present values correlated to the dry density and water content of the bentonite. Accordingly, the thermal conductivity is higher for the bentonite blocks of the external ring of the barrier, whereas their swelling capacity is lower. The water retention capacity, the hydraulic conductivity and the swelling capacity of the samples from Grimsel have not irreversibly changed after operation. The preconsolidation pressure of the Grimsel samples has decreased due to the microstructural changes associated to the volume increase experienced during hydration and to the swelling stress applied. This expansion of the bentonite has caused the filling of all the construction gaps of the barrier.