REFLECTIONS ON YUCCA MOUNTAIN: WHAT WERE THE KEY TECHNICAL AND SCIENTIFIC ISSUES?

Over the more than twenty-year history of scientific investigations at Yucca Mountain, there were a number of important geoscience issues raised – upwelling of water from below, rapid movement of water from above, the seismicity and volcanism of the Basin and Range province, the effect of oxidizing conditions on the UO₂ of spent fuel, thermally driven hydrology in the near-field, colloid-facilitated transport in the far-field – to name just a few. The resolution, or not, of each issue had finally to be evaluated in the context of the standard and regulations, which developed and changed over the course of the project. Because the standard and regulations, as well as the regulatory process, were key to the evaluation of “safety”, they were also an inherent part of interactions with the public, but there was never a clear connection between the concept of “safety” and public acceptance.

This presentation summarizes U.S. experience in developing a standard and regulations for the geologic disposal of spent nuclear fuel and high-level radioactive waste. The standard and its implementing regulations are designed to protect human health and the environment, but the structure of the standard and regulations, as well as the standard-of-proof for compliance, should not extend beyond what is scientifically possible and reasonable. The demonstration of compliance must not only be compelling, but it must also be able to sustain scientific and public scrutiny. This was the major challenge of the Yucca Mountain project.

Finally, there is a special role for earth scientists in defining the methodology by which the “safety” of a geologic repository is judged. Earth scientists have been humbled in their effort to understand how complex geologic systems actually work over large spatial and long temporal scales, which for a geologic repository stretch to tens of kilometers and a million years. At the same time, during the past twenty years, earth scientists have had considerable success in understanding and modeling these systems. The standard and regulations should be designed to insure that the latest scientific advances are part of the safety assessment.