Model for Understanding Flow Processes and Distribution in Rock Rubble

Recent studies of the potential high-level nuclear waste repository at Yucca Mountain, Nevada, suggest that degradation of emplacement drifts may be caused by either persistent stresses induced by thermal decay of the spent nuclear fuel disposed in the drifts or seismic ground motion. Of significant interest to the performance of the repository is how seepage of water onto the engineered barriers in degraded emplacement drifts would be altered by rubble. Difficulty arises because of the uncertainty associated with the heterogeneity of the natural system complicated by the unknown fragment size and distribution of the rock rubble. A prototype experiment was designed to understand the processes that govern the convergence and divergence of flow in the rubble. This effort is expected to provide additional realism in the corresponding process models and performance assessment of the repository, and to help evaluate the chemistry of water contacting the waste as well as conditions affecting waste package corrosion in the presence of rubble. The rubble sample for the experiment was collected from the lower lithophysal unit of the Topopah Spring (Tptpll) unit in the Enhanced Characterization of the Repository Block Cross Drift and is used as an approximate analog. Most of the potential repository is planned to be built in the Tptpll unit. Sample fragment size varied from 1.0 mm [0.04 in] to 15 cm [6 in]. Ongoing experiments use either a single or multiple sources of infiltration at the top to simulate conditions that could exist in a degraded drift. Seepage is evaluated for variable infiltration rates, rubble particle size distribution, and rubble layering. Comparison of test results with previous bench-scale tests performed on smaller-sized fragments and different geological media will be presented. This paper is an independent product of CNWRA and does not necessarily reflect the view or regulatory position of NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain.