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Abstract

The U.S. Department of Energy (DOE) has brought data, analyses, and conclusions on landscape stability and denudation rates from the Yucca Mountain site characterization program to the U.S. Nuclear Regulatory Commission (NRC) for evaluation. In the United States the NRC must license a geologic repository to dispose of spent nuclear reactor fuel and high-level radioactive wastes from military reprocessing through phases of construction, operation, and closure. The DOE reported rates of hillslope denudation at Yucca Mountain, valley incision in bedrock over the repository block, and alluvial stream incision on the major drainage system in the area. These data were presented in combination with a regulatory compliance argument that concluded extreme erosion had not occurred during the Quaternary Period and that erosion would not jeopardize performance of a geologic repository system over the next 10,000 years, if located there.

The NRC defined “extreme erosion” and the duration of the Quaternary Period in a qualitative regulatory context as one of 24 “potentially adverse conditions” (NRC, 1991) to be evaluated during site characterization. The DOE made a compliance argument using the NRC’s regulatory requirement, supplemented by qualitative definitions from internal NRC guidance. The NRC was critical of the DOE’s technical documentation and compliance argument because (1) the erosion assessment used time-averaged denudation rates, (2) erosion rates for the most recent 10,000 to 100,000 years of the Quaternary Period were not provided, and (3) multiple dating methods were not used to establish age of the surficial landforms examined.

The DOE encountered several difficulties interpreting the regulation and its administrative record, framing a compliance argument, and documenting the data, analyses, and conclusions from a field study program. Among these, (1) the DOE misinterpreted the regulation and its administrative record by primarily relying on extant written guidance to explain how the NRC staff interpreted their regulation; (2) the DOE had no knowledge of how the NRC would evaluate a limited and specific compliance argument against their regulation; (3) the DOE’s technical documentation, in retrospect, was not robust enough to meet the NRC’s expectations of high confidence in conclusions accompanied by low uncertainty; and (4) the NRC’s comments implied expectations for geochronologic accuracy and degree of resolution that may be considered beyond the state of the practice.

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