TECHNICAL ISSUES RELEVANT TO HYDROLOGICAL PERFORMANCE ASSESSMENT AT YUCCA MOUNTAIN - A HIERARCHICAL PERSPECTIVE

Hydrological Performance Assessment (PA) is an important component of Yucca Mountain Site Characterization Project (YMP). The goal of PA is to assess, as far quantitatively as possible, whether radioactive cont aminants will be released to the groundwater and transported to the ac cessible environment in adverse quantities should a potential reposito ry for high level radioactive wastes (HLW) be constructed and operated at Yucca Mountain. Ideally, it would be desirable to demonstrate with certitude, using scientific data, that the wastes could be safely con tained at the site over a 10,000-year time frame. Nevertheless, the sc ientific issues involved are many and the necessary data cannot be gat hered and interpreted over the next several years to answer all the qu estions. Yet, available evidence suggests that the site has many attri butes favorable for long-term containment of high level radioactive wa stes. Under the circumstances, a phased, long-term approach to site ch aracterization appears prudent. Integral to such an approach will be l ong-term data acquisition and monitoring, should construction be permi tted. Considering the complex, difficult-to-access nature of earth sys tems, the long-term approach should provide for an open-endedness to a ccommodate unforeseen contingencies which may come to light during lon g-term monitoring. To guide data acquisition and monitoring, it is nec essary to identify the various technical issues relevant to repository performance. We present these issues in a systematic, hierarchial fra me-work