Evaluating the effectiveness of liquid diversion around an underground opening when evaporation is non-negligible
Interpretation of observations of seepage into under-ground openings is often complicated by evaporative potentials created by the necessity to ventilate the openings. Evaporation removes liquid from the seepage face. By doing so, it reduces both the likelihood of seepage onset and the seepage flux, and thus can enhance the perceived effectiveness of the capillary barrier. We modeled liquid-release tests at the proposed high-level waste repository in Yucca Mountain, using an enhanced version of the EOS9 module of iTOUGH2, which incorporates evaporation as Fickian diffusion. The evaporation boundary layer thickness (BLT) over which diffusion occurs was estimated using free-water evaporation experiments conducted at Yucca Mountain under known relative humidity, temperature, and ventilation conditions. The BLT, which represents the thickness of the laminar flow regime, is inversely related to the ambient airflow velocity. At Yucca Mountain, the estimated values of BLT were 5-7 mm for open underground tunnels and 20 mm for closed niches. Compared to previous models that neglected the effect of evaporation, this new approach shows significant improvement in capturing seepage fluctuations into the open tunnels. Increased confidence is gained for the use of the estimated effectiveness of the capillary barrier formed by the tunnel for simulations of future seep-age under differing evaporative potentials.