ABSTRACT The U.S. Department of Energy (DOE) is considering the possible recommendation of a site at Yucca Mountain, Nevada, for the potential development of a geologic repository for the disposal of high-level radioactive waste and spent nuclear fuel. To facilitate public review and comment, in May 2001 the DOE released the Yucca Mountain Science and Engineering Report (S&ER) (1), which presents technical information supporting the consideration of the possible site recommendation. The report summarizes the results of more than 20 years of scientific and engineering studies. Based on internal reviews of the S&ER and its key supporting references, the Total System Performance Assessment for the Site Recommendation (TSPA-SR) (2) and the Analysis Model Reports and Process Model Reports cited therein, the DOE has recently identified and performed several types of analyses to supplement the treatment of uncertainty in support of the consideration of a possible site recommendation. The results of these new analyses are summarized in the two-volume report entitled FY01 Supplemental Science and Performance Analysis (SSPA) (3,4). The information in this report is intended to supplement, not supplant, the information contained in the S&ER. The DOE recognizes that important uncertainties will always remain in any assessment of the performance of a potential repository over thousands of years (1). One part of the DOE approach to recognizing and managing these uncertainties is a commitment to continued testing and analysis and to the continued evaluation of the technical basis supporting the possible recommendation of the site, such as the analysis contained in the SSPA. The goals of the work described here are to provide insights into the implications of newly quantified uncertainties, updated science, and evaluations of lower operating temperatures on the performance of a potential Yucca Mountain repository and to increase confidence in the results of the TSPA described in the S&ER (1). The primary tool used to evaluate the implications of the three types of supplemental information described in the SSPA (3,4) is the Yucca Mountain integrated TSPA model. WM '02 Conference, February 24-28, 2002, Tucson, AZ-pg. 2 In the SSPA two types of analyses of the performance of the potential repository were conducted using the TSPA model. First, a set of "one-off" sensitivity analyses was conducted to evaluate the effects of incorporating the updated models and representations one at a time. Then, the updated models and representations were abstracted and aggregated to produce a modified TSPA model, referred to as the supplemental TSPA model, which captures the combined effects of those alternative representations. This supplemental TSPA model was used to evaluate system performance over a range of thermal operating modes. The supplemental TSPA model results were compared with results of the TSPA-SR to provide insights into the cumulative effects of all model changes on the system results and to demonstrate that the TSPA-SR analyses were conservative in nature, i.e., that a safety margin had been built into the suite of TSPA-SR models
