TWENTY-FIVE YEARS OF SITE CHARACTERIZATION AT YUCCA MOUNTAIN

In 1987, Congress legislated that Yucca Mountain would be the only site characterized for a geologic repository for high-level nuclear waste. Until that time, the U.S. had studied a variety of sites and areas to a limited extent, including those in bedded salt, salt domes, crystalline rocks, volcanic tuff and basalt. Yucca Mountain is now perhaps the most thoroughly studied site on Earth. The key question that defined the site characterization program was whether Yucca Mountain would safely isolate spent nuclear fuel and high-level waste for the regulatory period of 10,000 years. Congress' decision to concentrate study at Yucca Mountain presented unique challenges because of its hydrogeologic setting in a deep unsaturated environment and its location in the tectonically active Basin and Range physiographic province. Specifically, the amount and timing of infiltration and flow in the unsaturated zone (UZ) and the location of flow pathways required investigation. Whether flow was steady or transient, what portion of flow occurs in fractures vs. the rock matrix, the relative proportion of flowing fractures, and the existence of fast pathways were some of the key hydrologic questions. Transport properties of the volcanic and alluvial media encountered along portions of the flow paths were also important. The flow regimes required consideration in the context of future climate predictions as well as present conditions. The future potential for volcanism and seismicity to disturb Yucca Mountain was also of high priority for investigation. The studies presented in this session cover the breadth of site characterization activities over 25 years. Some of the work has been published and some is presented here for the first time. Collectively, this body of work forms the basis for a conceptual understanding of the processes anticipated to occur at Yucca Mountain in the future and will provide the foundation for the U.S. Department of Energy's License Application.