Nuclear Energy and Waste Program

radioactive waste is currently the preferred means of disposal for many countries worldwide. The role of ESD's Nuclear Waste Program (NWP) is to assist the U.S. Department of Energy, the United States, and other countries in achieving the safe disposal of high-level radioactive waste—by means of high-quality scientific analyses that encompass modeling, laboratory and field experiments, and technology development. Research within NWP focuses on flow and transport, as thermally driven hydrological, chemical, and mechanical coupled processes. Many of the studies within NWP relate to Yucca Mountain, the proposed site for the permanent storage of highlevel nuclear waste in the USA; although NWP has also collaborated on nuclear-waste disposal issues with countries such as Japan, Finland, Switzerland, Spain, Sweden, and China. The geologic repository program in the United States is at a point where the Department of Energy (DOE) is close to completing the license application for repository construction at Yucca Mountain to the Nuclear Regulatory Commission (scheduled for June 2008. The safe performance of a high-level nuclear waste repository hinges on the multiple-barrier concept—namely, that the natural system and the engineered system would each contribute significantly to prevent radionuclides from leaving the repository and entering the biosphere. The proposed repository at Yucca Mountain, consisting primarily of fractured volcanic tuffs that vary in degree of welding, will be located about 350 m below ground surface within a thick unsaturated zone (UZ) above the water table. Over the last decade, NWP's work at Yucca Mountain consists of site characterization studies aimed at understanding the barrier function of the UZ, through field testing in an underground facility, an 8 km long underground tunnel known as the Exploratory Studies Facility (ESF). Complex numerical models have also been developed to simulate and understand the relevant processes related to multiphase, nonisothermal flow and transport through the UZ. Some of the key questions addressed by NWP scientists include: • How much water percolates through the UZ to the repository at Yucca Mountain? • What fraction of the water flows in fractures and what fraction flows through the rock matrix blocks? • How much of this water will seep into the emplacement drifts (tunnels)? • How will the radionuclide migration from the repository to the water table be retarded? • How will coupled TH (thermal-hydrological), THC (thermal-hydrological-chemical), and THM (thermal-hydrological-mechanical) processes affect flow and transport? Earth Sciences Division Berkeley Lab Research Summaries 2006-2007