Transient Behavior of a Clay Barrier Subjected to High Temperature Changes

The condition of a compacted expansive clay in an engineered barrier for nuclear waste disposal has been simulated in the mock-up FEBEX experiment. In this test two electrical heaters were placed in the center of a steel cylinder. The space between the heaters and the steel cylinder was filled with an engineered barrier made up of compacted bentonite. The barrier is hydrated uniformly from all around the cylinder. Simultaneously, the barrier is heated maintaining a constant temperature of 100 °C at the contact between heater and bentonite. The test is heavily instrumented with several sensors of temperature, relative humidity and stresses in different sections of the barrier. Approaching the fourth year of the experiment, an accidental overheating took place resulting in the bentonite achieving temperatures in excess of 200 °C. As a consequence of this overheating, a series of coupled Thermo-Hydro-Mechanical (THM) phenomena took place in the clay barrier. The transient THM behavior of the FEBEX bentonite during the overheating is analyzed in detail using a finite element program. This analysis has provided the opportunity to explore the behavior of the FEBEX bentonite at temperatures not previously reached in the past.