Analogues to features and processes of a high-level radioactive waste repository proposed for Yucca Mountain, Nevada

Natural analogues are defined for this report as naturally occurring or anthropogenic systems in which processes similar to those expected to occur in a nuclear waste repository are thought to have taken place over time periods of decades to millennia and on spatial scales as much as tens of kilometers. Analogues provide an important temporal and spatial dimension that cannot be tested by laboratory or field-scale experiments. Analogues can be used in a qualitative mode to provide a means of understanding complex or abstract processes or in a quantitative mode to test models. Analogues provide one of the multiple lines of evidence intended to increase confidence in the safe geologic disposal of high-level radioactive waste. Although the work in this report was completed specifically for Yucca Mountain, Nevada, as the proposed geologic repository for high-level radioactive waste under the U.S. Nuclear Waste Policy Act (and it is referred to as such throughout this report), the U.S. Geological Survey believes that the applicability of the science, analyses, and interpretations is not limited to a specific site. The work is important as a contribution not only to investigations of future waste-disposal options, such as the assessment of alternative sites or solutions, but also as a contribution to scientific investigations unrelated to waste disposal. Isolation of radioactive waste at a mined geologic repository would be through a combination of natural features and engineered barriers. In this report we examine analogues to many of the various components of the Yucca Mountain system, including the preservation of materials in unsaturated environments, flow of water through unsaturated volcanic tuff, seepage into repository drifts, repository drift stability, stability and alteration of waste forms and components of the engineered barrier system, and transport of radionuclides through unsaturated and saturated rock zones. Hundreds of delicate and easily destroyed artifacts and biological materials have been well preserved both in natural (for example, caves and rock shelters) and in manmade (for example, tombs and mines) underground openings. The maintenance of a stable microclimate is a critical feature in making caves suitable for long-term preservation. The survival of metal artifacts over prolonged periods of time is related to the corrosion-resistant properties of metals and metal alloys, the development of protective passive film coatings with the onset of corrosion, and sequestering artifacts with water, which is enhanced by the location of artifacts in arid to semiarid environments. Numerous examples demonstrate that both natural and manmade underground openings can exist for thousands of years in a wide variety of geologic settings, even with minimal or no engineered supports. Examination of these openings also leads to the conclusion that seismic events at or near a mined repository are not likely to cause significant damage to the emplacement tunnels. As a consequence, the tunnels should be expected to be a long-term hydrologic feature. Analogues add valuable insight to understanding long-term waste-form degradation processes through the record left behind in secondary minerals and groundwater chemistry. In addition, measurement of the concentration of fission products as tracers in rock and groundwater surrounding uraninite provides a satisfactory approach to estimating natural dissolution rates, as was tested at a number of sites that demonstrated a more rapid dissolution rate under oxidizing conditions. Models predict that much of the water percolating through the unsaturated zone will be held in the wall rock of tunnels by capillary forces rather than entering the tunnels as seepage. Analogues in natural and manmade underground openings demonstrate the tendency for water that does become seepage to run down the walls of underground openings rather than drip from the ceiling; thus, not all seepage would affect stored waste. In the event of waste mobilization and migration away from the emplacement drifts, the rate of radionuclide transport through the unsaturated zone is determined by the percolation flux and by the hydrologic properties and sorptive properties of the tuff units. Fractures act both as transport pathways and as places of retardation at a number of unsaturated analogue sites, including the Idaho National Laboratory, near Idaho Falls; Pena Blanca, Mexico; Akrotiri, Greece; and volcanic tuff-hosted uranium deposits in northern Nevada. In the saturated zone, advective transport along fractures has been identified as a more significant transport mechanism than matrix diffusion in all the analogue sites studied, although matrix diffusion may account for loss of lead in uraninites at Oklo, in Gabon. The Pocos de Caldas site in Brazil highlighted the importance of amorphous phases in suspension or as coatings on rock as the principal sorptive surfaces for many trace elements in solution. Some of the fixing processes appeared to be irreversible over long time scales. Sorption onto fracture coatings, particularly calcite, also efficiently retards uranium transport in fractures at Palmottu, Finland, and El Berrocal, Spain. Matrix diffusion in crystalline rock is generally limited to only a small volume of rock close to fractures, but even a small volume can make a significant difference in radionuclide retardation. In most studies of natural systems, a proportion of the total uranium, thorium, and rare earth elements (REE) in the groundwater was associated with colloids. Colloid transport appears to be an important factor for migration of thorium in one open unsaturated system, Steenkampskraal, in South Africa, but not in another, Nopal I, in Mexico. Colloidal transport of uranium was shown to be minimal at the analogue site in Koongarra, Australia, where filtration of colloids appears to be effective. Observations from the Nevada Test Site lend support to the concept that radionuclide transport in the saturated zone can be facilitated by colloids; but so far, no natural analogue studies have quantified the importance of this process. The emplacement of heat-generating waste in a geologic repository located in the unsaturated zone will cause perturbations to the natural environment through heat transfer, as well as by associated geochemical and geomechanical changes taking place in the repository near-field and altered-rock zones. The unsaturated conditions, lower temperatures, and much lower fluid-flow rates predicted for the Yucca Mountain system should result in less extensive water/rock interaction than is observed in geothermal systems. Evidence from fossil hydrothermal systems indicates that mineral alteration resulting from flow of hot fluids through fractures extends only a few centimeters from the fracture wall into the matrix. Simulations indicate that only small reductions in fracture porosity (4-7 percent) and permeability (less than 1 order of magnitude) will occur in the near field as a result of amorphous silica and calcite precipitation. Changes in permeability, porosity, and sorptive capacity are expected to be relatively minor at the mountain scale, where thermal perturbations will be reduced. The Yucca Mountain Project has applied analogues for testing and building confidence in conceptual and numerical process models and has less frequently used analogues to provide specific parameters in total system performance assessment (TSPA) models. Analogues have been widely used as model validation of aspects of Yucca Mountain characterization. In conclusion, natural and anthropogenic analogues have provided and can continue to provide value in understanding features and processes of importance across a wide variety of topics in addressing the challenges of geologic isolation of radioactive waste.