US Department of Energy Approach to Probabilistic Evaluation of Long-Term Safety for a Potential Yucca Mountain Repository

Download or read book US Department of Energy Approach to Probabilistic Evaluation of Long-Term Safety for a Potential Yucca Mountain Repository written by and published by . This book was released on 2005 with total page 1 pages. Available in PDF, EPUB and Kindle. Book excerpt: Regulatory requirements being addressed in the US geological repository program for spent nuclear fuel and high-level waste disposal specify probabilistically defined mean-value dose limits. These dose limits reflect acceptable levels of risk. The probabilistic approach mandated by regulation calculates a ''risk of a dose, '' a risk of a potential given dose value at a specific time in the future to a hypothetical person. The mean value of the time-dependent performance measure needs to remain below an acceptable level defined by regulation. Because there are uncertain parameters that are important to system performance, the regulation mandates an analysis focused on the mean value of the performance measure, but that also explores the ''full range of defensible and reasonable parameter distributions'' ... System performance evaluations should not be unduly influenced by ... ''extreme physical situations and parameter values''. Challenges in this approach lie in defending the scientific basis for the models selected, and the data and distributions sampled. A significant challenge lies in showing that uncertainties are properly identified and evaluated. A single-value parameter has no uncertainty, and where used such values need to be supported by scientific information showing the selected value is appropriate. Uncertainties are inherent in data, but are also introduced by creating parameter distributions from data sets, selecting models from among alternative models, abstracting models for use in probabilistic analysis, and in selecting the range of initiating event probabilities for unlikely events. The goal of the assessment currently in progress is to evaluate the level of risk inherent in moving ahead to the next phase of repository development: construction. During the construction phase, more will be learned to inform a new long-term risk evaluation to support moving to the next phase: accepting waste. Therefore, though there was sufficient confidence of safety at the end of the site-characterization phase to warrant moving ahead to construction, the expectation is that still more confidence may be had in the next evaluation of risk for a repository at Yucca Mountain. More confidence does not always mean lower risk, just as less uncertainty does not necessarily mean lower risk. What needs to be shown is that there is a basis for confidence in the outcome of such evaluations, meaning that the potential repository promises to provide acceptable public safety, as defined by the regulation, at every phase in its long life.