Ventilation experiment in the Mont Terri underground laboratory

Abstract

The required ventilation of the underground drifts during the construction and operational phases of a radioactive waste repository could give rise to a process of desaturation of the rock around the drifts, changing its hydraulic and thermal properties. This change of rock properties may have an impact on the design of the repositories (drift spacing and repository size), which depends on the thermal load that the clay barrier and the rock can accept. The ventilation experiment (VE) has been carried out at the Mont Terri underground laboratory (Switzerland). Its main objective was to evaluate ”in situ” and better understand the desaturation process of a consolidated clay formation, when subjected to a flow of dry air during several months. The experiment has been performed in a 10 m long section of a non-lined horizontal microtunnel (diameter = 1.3 m), excavated in 1999 in the shaly facies of the Opalinus clay. In the Summer of 2002 (3.4 years after the microtunnel excavation), the test section was sealed-off, and monitored with a total of 86 sensors (rock water potential, water content, temperature and displacement, and condition of the air in the test section). Specifically, in a rock thickness of approximately 2 m, 24 piezometers, 32 hygrometers, 8 extensometers and 5 electrode chains (geoelectrical survey) were installed. The ventilation equipment generates a flow of air ( Q in) with specified values of the relative humidity (RH in) and temperature ( T in), which is sent to one of the ends of the test section through an inflow pipe, and then evacuated with an outflow pipe in the other end. Values of Q out, RH out and T out are also measured in the outflow pipe. The desaturation period ( Q in = 30 m 3/h) began in July 2003, with a phase (≈2 months) where RH in was set equal to 30% and another one (≈5 months) with air inflow almost dry (RH in = 1–3%). After this desaturation period, the test was finished with a 3-months resaturation period (target Q in = 20 m 3/h and RH in ≈ 100%). The most important general conclusion obtained from this experiment, from the rock monitoring data and also from the modelling results, is the following: it can be reasonably predicted that, under real repository ventilation conditions (relative humidity of the air much higher than in the desaturation period of the VE test), the desaturation of clayey rocks of low hydraulic conductivity ( K < 10 −12 m/s) will not be a relevant issue. The thermal and hydraulic rock bulk characteristics will not be practically affected by the ventilation, except in a narrow ring around the wall of the galleries (thickness less than 30 cm), where the rock degree of saturation can be lower than 95%.