Uncertainties In Performance Assessments For The Yucca Mountain Site And The 'Edge- of-Compliance'

摘要

Numerical performance assessments of deep geologic disposal systems of long-lived radioactive wastes serve as the primary tool for quantitatively assessing projected performance and for making regulatory compliance decisions, both in the U.S. and disposal programs abroad. However there is a significant body of opinion, expressed in the international literature, that the confidence that can be placed on such assessments decreases significantly as the analysis time frames increase into the many hundreds of thousands of years. With the inclusion of highly corrosion resistant metals in the planned waste package designs for the Yucca Mountain (YM) system commercial spent fuel and defense-generated high-level radioactive waste disposal, published projections for the site show doses to the receptor at times extending into the many hundreds of thousands of years. The question of relative confidence in dose projections becomes more important at very long time frames, particularly when the time frame for significant doses increases dramatically reflecting the effects of using highly corrosion resistant materials.To examine uncertainties in performance assessments of the YM disposal system over very long time frames, a site model developed by the U.S. Department of Energy to assess sensitivities in peak dose performance was modified and used to examine the propagation of uncertainties for a hypothetical disposal system. These analyses start with a hypothetical disposal system at the "edge-of-compliance" at 10,000 years, reflecting the generic repository standard in 40 CFR Part 191. The hypothetical system was poised to give a mean dose of 0.15 millisievert/year (15 mrem/yr.) at 10,000 years, by allowing a fixed number of waste packages to "fail" within the first 5,000 years after closure. By maintaining the number failed waste packages constant over time, the spread in dose estimates over the time to peak dose was calculated for this system perched at the "edge-of-compliance" at 10,000 years. This hypothetical construct removes waste package performance from the analyses and allows the site model to explore the effects of uncertainties in the natural barrier and site conditions on the dose projections out to the time of peak dose. The sensitivities of these projections to various "driver" parameters were examined, including infiltration rates, solubility constraints, water chemistry and roof collapse assumptions.Overall, the initial construct showed a one and one-half order of magnitude spread between the 5th and 95th percentiles at 10,000 years, the initial state for the analyses. This spread increased to approximately three and one-half orders of magnitude at peak dose, reflecting the effects of transport and retardation mechanisms on the fixed source term. The model eliminated portions of the transport path from the repository where relatively little retardation would be expected.Travel times were therefore significantly reduced thereby eliminating radioactive decay as an important mechanism in reducing projected doses. Solubility controls appear to be a major source of uncertainties in the dose projections for the model used. These results support the general international consensus that confidence in dose projections over very long time frames does decrease, and illustrates that this conclusion also applies to assessments of the YM projected performance as well.