EXECUTIVE SUMMARY

The Committee for Yucca Mountain Peer Review: Surface Characteristics, Preclosure Hydrology, and Erosion was formed by the National Research Council to provide an independent, expert peer review of the U.S. Department of Energy's (DOE's) Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion (DOE Report YMP/TBR-001, Rev. 0, April 1995), referred to as the TBR. The committee addressed its review to the scientific and technical aspects of the TBR as prescribed by its statement of task (Appendix C). The review focused on several issues, including the adequacy of the scientific methodologies employed in data collection, synthesis, and analysis; the validity of data and interpretations; the adequacy of the treatment of uncertainties in describing the current state of understanding; and the effectiveness of the presentation of data, analyses, and interpretations.

The committee based its evaluation of the TBR entirely on scientific judgment. The committee made no attempt to evaluate the science in terms of management decisions related to the suitability of Yucca Mountain as a high-level waste repository. The committee is not properly constituted to make such judgments, and it is specifically prohibited from doing so by the statement of task. The committee attempted to identify weaknesses in data, methodologies, interpretations, and conclusions in the TBR. In some cases, the committee recommended that additional work be done to significantly improve scientific understanding. The com-



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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion EXECUTIVE SUMMARY The Committee for Yucca Mountain Peer Review: Surface Characteristics, Preclosure Hydrology, and Erosion was formed by the National Research Council to provide an independent, expert peer review of the U.S. Department of Energy's (DOE's) Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion (DOE Report YMP/TBR-001, Rev. 0, April 1995), referred to as the TBR. The committee addressed its review to the scientific and technical aspects of the TBR as prescribed by its statement of task (Appendix C). The review focused on several issues, including the adequacy of the scientific methodologies employed in data collection, synthesis, and analysis; the validity of data and interpretations; the adequacy of the treatment of uncertainties in describing the current state of understanding; and the effectiveness of the presentation of data, analyses, and interpretations. The committee based its evaluation of the TBR entirely on scientific judgment. The committee made no attempt to evaluate the science in terms of management decisions related to the suitability of Yucca Mountain as a high-level waste repository. The committee is not properly constituted to make such judgments, and it is specifically prohibited from doing so by the statement of task. The committee attempted to identify weaknesses in data, methodologies, interpretations, and conclusions in the TBR. In some cases, the committee recommended that additional work be done to significantly improve scientific understanding. The com-

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion mittee made no attempt to determine whether the identified weaknesses would have a significant impact on the management decision to site a facility at Yucca Mountain. Nor has the committee determined whether the recommended work would change the management decision to site such a facility. In the committee's view, such judgments are best left to DOE scientist-managers and their oversight bodies. The committee 's goal is to help the DOE improve the scientific quality of its TBR. The primary source of material for the committee's review was the TBR itself and many of the references cited therein. The committee also held two information-gathering meetings to obtain additional information from DOE and other federal agencies, the State of Nevada, and other interested organizations and individuals. These meetings were held in Las Vegas and Beatty, Nevada; the second meeting included a three-day field excursion to Yucca Mountain. Both meetings were open to the public. The committee's review of the TBR is organized into four chapters. Chapter 1 provides a brief discussion of the purpose and scope of the TBR and an explanation of the procedures used by the committee in its review. Chapter 2 provides the committee's review of the surficial geology and erosion sections of the TBR. Chapter 3 provides the committee's review of the preclosure hydrology sections of the TBR. Chapter 4 addresses the overall effectiveness of the TBR as a synthesis of currently available site characterization data, analyses, and technical interpretations. The main scientific and technical issues addressed by the TBR are summarized below (in italics) and are followed by the committee' s significant conclusions and recommendations. Additional details are provided in Chapter 2 and Chapter 3 of this report.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Distributions and Relative Ages of Surficial Deposits. The identification of surficial deposits as presented in the TBR is based on traditional and accepted techniques of analysis. Better age determinations on these surficial deposits are needed, however, in order to estimate erosion rates. Additionally, surficial data from Crater Flat (the drainage to the west of Yucca Mountain), which have been published by State of Nevada scientists, should be referenced, discussed, and integrated into the TBR. The results from surficial mapping need to be integrated better with efforts to evaluate hillslope and stream erosion processes at Yucca Mountain. This integration of mapped surficial units, their ages, and erosion mechanisms should form the basis for evaluation of erosion potential and should be reported in the TBR. Potential for Stream and Debris Flow Erosion. Fluvial erosion rate estimates reported in the TBR are based on dynamic equilibrium assumptions and may underestimate the fluvial erosion potential of the region. Fluvial erosion rate estimates should be based on bounding calculations, and the possible effects of climate change on fluvial erosion potential should be addressed. The effectiveness of debris flows and landslides as erosive agents of the landscape under current and possible future climatic conditions should also be addressed. Ages of Hillslope Deposits. The analysis of hillslope ages presented in the TBR is inadequate because it is based on the use of a single geochronological method (cation ratio dating), which is not well understood or calibrated, that is applied to only one type of deposit (heavily varnished hillslope boulder deposits). Different dating techniques (e.g., 3He and 10Be) should be applied to check the cation ratio results, and different types of geomorphic surfaces should be dated by using a variety of techniques in order

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion to obtain estimates of the spatial variability of hillslope ages at Yucca Mountain. The maps of surficial geology referenced in the TBR identified a variety of hillslope units that may have significantly different ages than the heavily varnished hillslope boulder deposits. Long-Term Rates of Erosion. The analysis of erosion rates presented in the TBR is too narrowly focused on estimating a temporally and spatially averaged rate that can be compared to a regulatory standard. The analysis in the TBR should be expanded to assess the spatial variability of erosion in the region. The TBR should consider the range of erosional processes operating at the site, and identify those portions of the landscape that may be eroding much faster than average. It should also consider the temporal variability of erosion, particularly as it might be affected by climate change. Potential for Surface Flooding of the Proposed Repository. The application of probable maximum flood (PMF) procedures to estimate flood events as outlined in the TBR is consistent with practices used to design civil structures such as bridges and dams. The values of parameters used in the flood routing calculations (Manning's n and the bulking factor), and the assumption of critical flow velocity as the maximum flow velocity at all locations in the channels, provide conservative estimates of flow depths (i.e., overestimates of flow depths). However, the parameter estimates are not supported by data or documentation in the TBR. More work should be done to assess the sensitivity of PMF estimates to these values and assumptions. Potential for Subsurface Flooding of the Proposed Repository. The discussion of subsurface flooding potential in the TBR is overly brief and devoid of analysis. Two potential sources of subsurface flooding, deep seepage of surface infiltration and

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion rising water tables, are not addressed. Perched water is discussed briefly in the TBR, but its distribution, volume, and age are not adequately discussed or illustrated with graphics. It does not appear to the committee that perched water will pose problems during construction and operation that cannot be handled by reasonably available technology (i.e., technology used in other applications such as mining), but the TBR does not make effective use of existing data to make this point. Availability of a Water Supply in the Yucca Mountain Region for Construction and Operation of the Proposed Repository. The TBR lacks a clear statement of the technical questions that must be addressed to establish the sufficiency of the water supply for repository construction and operation. The TBR discussion emphasizes historic pumping rates and water level declines predicted by using a numerical model that is based on one conceptual model of the flow system. It provides few data to support technical interpretations and does not consider alternative conceptual models of the ground water system. It is likely that water supply availability can be established by means of bounding calculations, although such a calculation is not provided in the TBR. OVERALL EFFECTIVENESS OF THE TBR Chapter 4 of this report addresses the effectiveness of the TBR as a synthesis of currently available data, analyses, and interpretations. In addressing the question of effectiveness, the committee considered the context for the TBR. This TBR is the product of an effort of great national importance: to assess the suitability of Yucca Mountain as a facility for the safe permanent disposal of

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion high-level radioactive waste. For this effort to succeed, the committee believes that DOE will need to demonstrate that it has a good understanding of the scientific and technical issues that would affect the construction and long-term performance of such a facility. Given the importance of this undertaking to the health and safety of present and future generations of this nation's citizens, it is this committee's opinion that the scientific and technical analyses presented in the TBR should meet the highest standards of scientific quality. Considered in this context, it is the committee 's judgment that this TBR is not an effective synthesis of data, analyses, and interpretations related to surface characteristics, erosion, and preclosure hydrology. The concluding chapter of this report provides substantiation for this conclusion and—in the spirit of peer review as a method to improve scientific quality—offers eight recommendations for ways to increase the effectiveness of this and future TBRs. These recommendations are summarized below: The audiences for the TBR should be stated explicitly in the report, and the TBR should be written to be comprehensible by these groups. The designation of target audiences is a DOE policy issue. The committee recommends that DOE policymakers carefully consider the advantages of writing for a broad audience to build scientific credibility and public acceptance for its site characterization program. The TBR should contain a clear statement of the questions to be addressed and hypotheses to be tested for each technical topic. These questions will serve to focus the supporting technical analyses and make it easier for the reader to assess their adequacy.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion All available scientific and technical information related to the issues addressed in the TBR should be cited and discussed. In addition, essential information (e.g., data, equations, and model parameters) used in analyses should be provided in the TBR, and primary sources should be referenced. The TBR should provide a complete discussion of analyses supporting the technical interpretations; a discussion of alternative hypotheses and the methods used to test them; and a discussion of remaining uncertainties and additional data needed to address them. The TBR should be prepared with the direct involvement of the scientists who did the site characterization studies, and these scientists should be identified in the report. The TBR should also provide a discussion of how data and analyses were selected and integrated. Multiple methods of analysis should be employed in the TBR to improve understanding, reduce uncertainties, and thereby build confidence in the interpretations and conclusions. Bounding calculations are particularly well suited to many of the issues addressed in the TBR and should be applied as part of a multiple-methodology approach to place minimum or maximum bounds on processes and phenomena. The TBR should be illustrated with informative graphics to orient the reader to the region and to illustrate spatial relationships among the various elements of the site. Of particular importance are geologic and topographic maps and cross sections that illustrate the locations of land surfaces, drainages, rock units, surficial deposits, perched and ground water, the proposed repository, and selected data collection sites.

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Review of U.S. Department of Energy Technical Basis Report for Surface Characteristics, Preclosure Hydrology, and Erosion Preparation of the TBR should include provision for a rigorous program of peer review by scientists whose work is used in the report. This should occur in addition to external peer review, which serves as an independent check on scientific and technical quality. The peer review process should also include provisions to ensure that the results of internal and external peer review effectively feed back into this and future TBRs and, when appropriate, into the associated scientific and technical programs.