TESTING THE CONCEPT OF DRIFT SHADOW WITH X-RAY ABSORPTION IMAGING EXPERIMENTS

The proposed High-Level Nuclear Waste (HLNW) Repository at Yucca Mountain, NV is located within the fractured Topopah Springs welded tuff (TSw). A main concern with HLNW storage is groundwater seepage into constructed drifts, or storage cavities. An analytical model proposed by Philips, et al. (1989) predicts flow diversion around drifts due to the presence of a capillary barrier resulting in a drift shadow beneath the drift. The amount of drift diversion and the outflow location are controlled by the drift diameter, capillary pressure, and fracture aperture. To verify the presence of the capillary barrier and its resulting drift shadow and to validate Philips's model, two centimeter-scale test cells with different, continuous fracture apertures were constructed from TSw rock samples. Fractures were located throughout the middle of the rock samples, parallel to the rock faces and normal to the drift axes. A potassium iodide (KI) solution was introduced into the test cells and outflow collected at different lateral locations at the base of the cells and inside the drift cavities. X-ray absorption imaging visualized and captured pictures of KI solution movement through the fracture. While the preliminary results from three tests show the difficulties in comparing flow behavior between heterogeneous media and the predicted behavior in a homogenous media, they also demonstrated the presence of the drift shadow and flow diversion away from the drift.