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Introduction

In the ongoing national debate on nuclear power as a source of electricity, a key issue is the disposition of the high level radioactive wastes produced in the process. At an earlier stage of this debate, Congress, aware of the importance of the waste issue, passed the Nuclear Waste Policy Act of 1982. This legislation required that the federal government develop a geologic repository for the permanent disposal of the high level radioactive wastes from civilian nuclear power plants. This waste consists primarily of spent nuclear fuel. Congress designated the Department of Energy (DOE) to implement the provisions of the act.

The Department of Energy established the Office of Civilian Radioactive Waste Management (OCRWM) in 1983 in response to the legislation and set about to identify potential sites. When OCRWM had selected three potential sites to study, Congress enacted the Nuclear Waste Policy Amendments Act of 1987, which directed the DOE to characterize only one of those sites, Yucca Mountain, in southern Nevada.

To characterize the site, the DOE must study in detail the natural environment and the various natural processes to which a proposed deep geologic repository might be subject. For a site to be acceptable, these studies must demonstrate that the site could comply with regulations and guidelines established by the federal agencies that will be responsible for licensing, regulating, and managing the waste facility. The regulations, which were promulgated to ensure the safety of the public, require that radiation will not be released above some



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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? 1 Introduction In the ongoing national debate on nuclear power as a source of electricity, a key issue is the disposition of the high level radioactive wastes produced in the process. At an earlier stage of this debate, Congress, aware of the importance of the waste issue, passed the Nuclear Waste Policy Act of 1982. This legislation required that the federal government develop a geologic repository for the permanent disposal of the high level radioactive wastes from civilian nuclear power plants. This waste consists primarily of spent nuclear fuel. Congress designated the Department of Energy (DOE) to implement the provisions of the act. The Department of Energy established the Office of Civilian Radioactive Waste Management (OCRWM) in 1983 in response to the legislation and set about to identify potential sites. When OCRWM had selected three potential sites to study, Congress enacted the Nuclear Waste Policy Amendments Act of 1987, which directed the DOE to characterize only one of those sites, Yucca Mountain, in southern Nevada. To characterize the site, the DOE must study in detail the natural environment and the various natural processes to which a proposed deep geologic repository might be subject. For a site to be acceptable, these studies must demonstrate that the site could comply with regulations and guidelines established by the federal agencies that will be responsible for licensing, regulating, and managing the waste facility. The regulations, which were promulgated to ensure the safety of the public, require that radiation will not be released above some

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? established safe limit, determined by the Environmental Protection Agency (EPA), for at least 10,000 years after the repository is permanently sealed. Earth scientists recognized early in the development of the concept of the mined geologic disposal system (MGDS) that radioactivity from spent nuclear fuel rods was most likely to be released to the outside environment through water entering the repository and transporting the radionuclides into the ground-water system. Thus, the design for a potential MGDS at Yucca Mountain, in the southern Nevada desert, calls for a location in the bedrock 200-400 m above the water table, in the unsaturated zone. The elevation of the water table there is 740 m above mean sea level (AMSL), and about 400 m below the land surface. During development of the site characterization plan (DOE, 1988), preliminary field studies into the geology and ground-water flow system were conducted to determine the scope of the investigations needed to understand the natural systems that may be active in the region. Based on his interpretations of field observations, a DOE staff scientist suggested that the ground water had risen well above the level proposed for the MGDS more than once in the geologically recent past and that such an event could happen again, flooding the mined geologic repository. This concern developed into a report by the staff scientist (Szymanski, 1989) detailing his views of the evidence and processes that caused the postulated events. As an internal peer review group failed to be convinced by the arguments and interpretations presented in that report, and an External Review Panel selected by both DOE and the staff scientist could not agree on the validity of the hypothesis (Powers et al., 1991; Archambeau and Price, 1991), the concern remained unresolved. In an effort to resolve these concerns, the DOE asked the National Academy of Sciences' National Research Council (NAS/NRC) to evaluate the hypothesis and assess the likelihood that the process described by the DOE staff scientist could result in raising the water table to the level selected for the MGDS. The NAS/NRC established the Panel on Coupled Hydrologic/Tectonic/Hydrothermal Systems at Yucca Mountain, Nevada, under the auspices of the Board on Radioactive Waste Management in April 1990. The panel was asked to evaluate (1) if the water table had been raised in the geologically recent past to the level of the proposed repository, or MGDS, through the action of coupled processes involving tectonic, hydrothermal, volcanic or climatic events; and (2) if it is likely that it will happen in the manner described in the DOE staff report (Szymanski, 1989) within the 10,000-year period covered

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? by the regulations. The report claimed that such flooding had repeatedly occurred in the past and could be expected to happen again. This hypothesis was based primarily on interpretations of geological observations at Yucca Mountain and vicinity. The report ascribed near-surface and subsurface veins composed mainly of carbonate and silica, breccias cemented by carbonate and silica, and surficial, surface-parallel deposits of carbonate and silica to emanations of hydrothermal fluids driven to the surface by tectonic and thermal pressurization of ground water by earthquake or thermal processes. The thesis of the origin of the breccias and carbonate-silica deposits outlined in the report is controversial. Scientists involved in the project from the United States Geological Survey (USGS) and DOE, as well as independent researchers, generally believe that the nearsurface carbonate-silica veins and cementation formed by deposition from surface waters filtering downward through the desert alluvium and, in some cases, reaching greater depths in steep fractures associated with faults. The panel members have wide-ranging expertise in geology and hydrologic processes, some with special knowledge of the Basin and Range geologic province in which Yucca Mountain is located. The areas of specialization of the panel members include Quaternary geology, hydrology, geochemistry, geophysics, rock mechanics, numerical modeling of thermal/mechanical processes and its geological applications, paleoclimatology, tectonics and earthquake risk analysis. To conduct the study, the panel read the report (Szymanski, 1989) and other pertinent literature, and interviewed or consulted with many of the scientists involved in field and laboratory investigations of Yucca Mountain and the region, for the DOE, the State of Nevada, and independent scientists. Those interviewed included the author of the report, experts in the paleoclimatology and hydrology of the area, and all five members of the External Review Panel previously mentioned. A majority of the NAS panel members spent at least three days visiting sites considered to be critical by scientists on both sides of the controversy. The remaining panel members participated in a similar two-day trip. Five members returned for an additional three-day field trip led by J. Szymanski, author of the staff report, in April 1991, to allow the panel more complete exposure to observations and interpretations of those geologic features that formed the basis for his conclusion that the proposed repository level had been flooded repeatedly by ground water in the recent geological past. The charge set by the panel for itself in its first meeting was the following:

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? The panel will attempt to see whether there are plausible mechanisms, tectonic, thermal, or otherwise, by which the water level in the Yucca Mountain could rise to the level of the repository. We will attempt to look for evidence in the Yucca Mountain area as well as in neighboring parts of Nevada where data exist to see what previously has been the highest stand of the water level. In other words, what fluctuations have there been in the past that would lead one to believe that any of these mechanisms might actually work or might have happened before? The charge is general in nature rather than specific to the DOE report 's hypothesis. The panel, while acknowledging the importance of the stimulus provided by the author of the report, nevertheless took the position that any observations or theoretical mechanisms that might account for past or future flooding of the proposed repository level should be considered. Thus, while our report inevitably deals with details of Szymanski's interpretations, the conclusions and recommendations derive from broader considerations. The following chapters of this report provide details of the panel 's analysis of the field information and the published literature, the results of preliminary modeling studies done by panel members and others, and some direct analysis of raw data made available to the panel by field investigators of the region. Chapter 2 evaluates the evidence related to the level of the water table through time. It considers the field evidence, including that shown to panel members by proponents of the ground-water flooding hypothesis, and the geochemical and isotopic characteristics of the relevant features. Chapter 3 addresses the impact on the water table of potential climate changes by looking at evidence of past variability in rainfall and what is known of the present hydrologic regime. Chapter 4 evaluates the potential effects of an igneous intrusion near Yucca Mountain on the water level. Chapter 5 considers earthquakes as a potential driver of water from below to raise the water table. It looks at water table responses to historic earthquakes, modeling results, and the probabilities of occurrence of large earthquakes in the Yucca Mountain area. Chapter 6 summarizes the panel's conclusions and recommendations based on its experience and observations of the Yucca Mountain program and region. The panel has included some observations related to data acquisition practices and policies of the scientific program for site characterization of Yucca Mountain, and the need for coordination of scientific activities in resolving site characterization issues such as the one for which this panel was convened.

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Ground Water at Yucca Mountain: HOW HIGH CAN IT RISE? REFERENCES Archambeau, C. B., and N. J. Price. 1991. An Assessment of J. S. Szymanski's Conceptual Hydrotectonic Model, Minority Report of the Special DOE Review Panel. September. 144 pp. plus appendix. Powers, D. W., J. W. Rudnicki, and L. Smith. 1991. External Peer Review Panel Majority Report. August. 87 pp. including three appendices. Szymanski, J. S. 1989. Conceptual Considerations of the Yucca Mountain Ground Water System with Special Emphasis on the Adequacy of This System to Accommodate a High-Level Nuclear Waste Repository Unpublished DOE report. U. S. Department of Energy. 1988. Site Characterization Plan: Yucca Mountain Site, Nevada Research and Development Area, Nevada DOE/RW 0199 Office of Civilian Radioactive Waste Management Washington, D.C.