NATURAL ANALOGS: INDEPENDENT REASONING FOR BUILDING CONFIDENCE IN DEEP GEOLOGIC DISPOSAL OF NUCLEAR WASTE

Natural analogs provide an independent line of reasoning that supports the understanding of how natural and engineered processes would occur over time frames of thousands of years and km-size spatial scales at a potential nuclear waste repository at Yucca Mountain, Nevada. Analogs also serve an important function in illustrating technical concepts related to waste isolation to the public. Care is exercised in the selection of appropriate analogs to exclude those for which initial or boundary conditions are poorly known and where important data, such as the source term, are poorly constrained or may not be attainable. Although typically focus is placed on settings having analogous processes rather than on total system analogs, the attempt is made to match as many features and processes as possible when identifying suitable analogs. Negative analogs are also instructive in demonstrating why certain processes are not expected to occur.

Although analogs are used in a quantitative or qualitative sense to support every process and sub-process model (e.g., waste package degradation, waste form dissolution, seepage into drifts, colloidal transport of radionuclides), emphasized here are examples of analogs to flow and transport under ambient and thermally-driven conditions in the unsaturated zone. For instance, around the ~8 Ma uranium ore deposit at Peña Blanca, Mexico, uranium transport in unsaturated, welded ash flow tuff has been almost entirely along mesofractures (~0.5 cm aperture) with almost no matrix diffusion. The majority of our uranium-series data indicate a system closed to uranium migration for the past 300 k.y. Second, analyses of selected Yellowstone core horizons within conductive and convective regimes demonstrate episodic and lithology-controlled permeability changes. Third, at Paiute Ridge, Nevada, where a basaltic sill intruded rhyolitic ash flow tuff, alteration-accompanied permeability changes diminish rapidly with distance from the sill-tuff contact. The implications of observations and modeling results from these and additional studies are discussed.