A Demonstration of Dose Modeling at Yucca Mountain
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A DEMONSTRATION OF DOSE MODELING AT YUCCA MOUNTAIN TERRI B. MILEY AND PAUL W. ESLINGER Pacific Northwest Laboratorya, P.O. Box 999, Richland, WA 99352 ABSTRACT The U. S. Environmental Protection Agency is currently revising the regulatory guidance for high-level nuclear waste disposal. In its draft form, the guidelines contain dose limits. Since this is likely to be the case in the final regulations, it is essential that the U.S. Department of Energy be prepared to calculate site-specific doses for any potential repository location. This year, Pacific Northwest Laboratory (PNL) has made a first attempt to estimate doses for the potential geologic repository at Yucca Mountain, Nevada as part of a preliminary total-systems performance assessment. A set of transport scenarios was defined to assess the cumulative release of radionuclides over 10,000 years under undisturbed and disturbed conditions at Yucca Mountain. Dose estimates were provided for several of the transport scenarios modeled1 ' 2. The exposure scenarios used to estimate dose in this total-systems exercise should not, however, be considered a definitive set of scenarios for determining the risk of the potential repository. Exposure scenarios were defined for waterborne and surface contamination that result from both undisturbed and disturbed performance of the potential repository. The exposure scenarios used for this analysis were designed for the Hanford Site in Washington. The undisturbed performance scenarios for which exposures were modeled are gas-phase release of 14C to the surface and natural breakdown of the waste containers with waterborne release. The disturbed performance scenario for which doses were estimated is exploratory drilling. Both surface and waterborne contamination were considered for the drilling intrusion scenario. INTRODUCTION Regulatory changes to 40 CFR 1913 are pending that would impose limits on individual dose for undisturbed conditions, and population doses for both undisturbed and disturbed conditions on a global scale. In light of these changes, it is important to be able to estimate doses for scenarios being proposed at Yucca Mountain. Lengthening the regulatory time period from 1,000 to 10,000 years removes the possibility for the U. S. Department of Energy (DOE) to rely on waste container lifetimes to meet the individual dose limit requirement. The DOE's Yucca Mountain Site Characterization Project Office has conducted a series of preliminary total-systems performance assessments for the potential geologic repository at Yucca Mountain, Nevada. PNL produced dose estimates for several of the scenarios modeled using both PNL and Sandia National Laboratories (SNL) estimates of radionuclide release to the accessible environment. Table I summarizes the transport scenarios run and associated exposure scenarios. The scenarios modeled in this total-systems exercise were driven by transport considerations. The exposure scenarios run were designed for the Hanford Site and are not necessarily valid in the arid environment of Yuc
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