YUCCA MOUNTAIN OVERSIGHT

In 2002 the President and Congress approved Yucca Mountain, a ridge of welded and nonwelded volcanic tuffs in the Great Basin, as the potential site for a geological repository to dispose of commercial spent nuclear fuel and defense high-level radioactive waste. The Department of Energy (DOE) must now show that this first-of-a-kind facility can safely isolate these nuclear wastes for 10,000 years. In 1987 the Congress created the Nuclear Waste Technical Review Board (NWTRB) to independently review the technical and scientific aspects of DOE's work at Yucca Mountain. The NWTRB pursues that mission through regular public meetings with project and concerned scientists, review of Yucca Mountain documents, and field excursions. As part of its efforts, the NWTRB has focused on a range of hydrogeologic issues including: site characterization, fluid inclusions, geochemistry, climate change, infiltration, deep percolation and seepage into drifts, thermohydrology, and characterization of basin sediments.

The DOE's task of predicting water flow and radionuclide transport at Yucca Mountain for 10,000 years is unprecedented. Hydrogeologic factors that may be significant to that prediction include: precipitation; percolation through the unsaturated zone and seepage into repository tunnels; presence or absence of fractures and faults; thermal perturbation of natural hydrologic processes from emplacement of hot wastes; matrix diffusion; sorption and retardation; colloid-facilitated transport; secondary mineralization; anisotropy of sedimentary and volcanic rocks and faults; and climate change.

Substantial progress has been made in creating regional and local hydrostratigraphic and hydrogeologic models, as well as in analyzing paleoclimate signals at Devil's Hole and in packrat middens. However, a vigorous effort is still required to reduce uncertainty and increase confidence in models and predictions of repository performance and to improve fundamental scientific understanding.