Visual Investigation of Flow Through a Capillary Barrier

The use of a capillary barrier as an engineered backfill system to divert water away from radioactive waste potentially stored in a Yucca Mountain emplacement drift is investigated. A capillary barrier consists of a fine-grained material overlying a coarser-grained material, the resultant capillary discontinuity at the interface inhibits downward migration of water. A two-dimensional test replicated the physical system to one-quarter scale (1.4 m diameter) and included the simulated emplacement drift wall, waste canister, pedestal, capillary barrier backfill, and host-rock fracture system. Water was supplied at the top of the simulated drift and allowed to discharge by way of wicks (simulated fractures) located along the left wall of the cell or by a gravity drain at the bottom of the right side. Photographs captured the migration of water within the system, scales measured the mass balance of water, and tensiometers measured the capillary tension at numerous locations. Although results indicate that the capillary barrier diverted the majority of flow toward the edges of the system and away from the simulated waste canister, wetting of the entire system was evident by the end of the test (112 days). We also found that the configuration of the drains influenced the performance of the capillary barrier, as evidenced by differences in the wetting and drainage of the two sides of the chamber. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.