AbstractThe disposal of high-level radioactive wastes in a potential geologic repository at Yucca Mountain, Nevada is governed by the U.S. Environmental Protection Agency (EPA) standards and U.S. Nuclear Regulatory Commission (NRC) regulations. The EPA has the responsibility for setting public health and safety and environmental standards for radioactive waste disposal at Yucca Mountain and it is the NRC's responsibility to implement those standards in its regulations to ensure public health and safety and the environment are protected. The U.S. Department of Energy (DOE), as the developer of the potential repository, must submit a license application to the NRC to seek approval to construct the repository. DOE must comply with NRC's regulations for NRC to authorize construction and license operation of a potential repository at Yucca Mountain. In 2005, EPA issued proposed revised standards and NRC issued proposed revised regulations for conducting performance assessment beyond 10,000 years up to 1 million years. The challenge for the EPA and NRC is to develop standards and regulations that provide an appropriate method for evaluating the safety of the potential repository given the unprecedented time period to be analyzed and the inherent uncertainties in estimating the future evolution of the Yucca Mountain site and the containment of the waste in the waste packages. A fundamental aspect of the proposed EPA standard is the specified approach for limiting undue speculation on future behavior of the site by constraining the features, events, and processes that need to be considered in the performance assessment. EPA proposed to limit the assessment of specific features, events, and processes in the period after 10,000 years to effects on the repository system that are most relevant (i.e., ignoring lesser or secondary effects that may add to speculation and uncertainties but would not be expected to have a significant effect on peak dose over a 1 million year period). For example, DOE's performance assessment may (1) limit the analysis of seismic activity to the effects caused by damage to the drifts and the waste package; (2) limit analysis of igneous activity to effects on the waste package that result in release of radionuclides to the atmosphere or ground water; (3) require DOE to include general corrosion in its analysis of engineered barrier performance, and (4) limit the effect of climate variation to those resulting from increased water flowing to the repository. NRC has been reviewing its performance assessment models and techniques to assure they are consistent with EPA's proposed requirements for the period after 10,000 years and adequate to assist the review of a potential license application from the DOE. Currently, only slight modifications to the models and approaches used in the performance assessment for the initial 10,000 years are expected to be needed to accommodate calculations for longer times (e.g., modifications to implement a steady-state value to represent the proposed climate change). NRC staff will be able to use the modified performance assessment to identify additional sensitivities associated with estimating doses over very long time periods and improve its understanding of the performance of a potential repository at Yucca Mountain.
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