As mandated in the Nuclear Waste Policy Act of 1982, the U.S. Department of Energy (DOE) has been investigating a candidate site at Yucca Mountain, Nevada, to determine whether it is suitable for development of the nation's first repository for permanent geologic disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW). The Nuclear Waste Policy Amendments Act of 1987 directed that only Yucca Mountain be characterized to evaluate the site's suitability. Three main components of the DOE site characterization program are testing, design, and performance assessment. These program components consist of: Investigation of natural features and processes by analyzing data collected from field tests conducted above and below ground and from laboratory tests of rock, gas, and water samples Design of a repository and waste packages tailored to the site features, supported by laboratory testing of candidate materials for waste packages and design related testing in the underground tunnels where waste would be emplaced Quantitative estimates of the performance of the total repository system, over a range of possible conditions and for different repository configurations, by means of computer modeling techniques that are based on site and materials testing data and accepted principles of physics and chemistry. To date, DOE has completed and documented four major iterations of total system performance assessment (TSPA) for the Yucca Mountain site: TSPA-91 (Barnard et al. 1992), TSPA-93 (Wilson et al. 1994; CRWMS M and O 1994), TSPA-95 (CRWMS M and O 1995), and the Total System Performance Assessment-Viability Assessment (TSPA-VA) (DOE 1998a, Volume 3). Each successive TSPA iteration has advanced the technical understanding of the performance attributes of the natural features and processes and enhanced engineering designs. The next major iteration of TSPA is to be conducted in support of the next major programmatic milestone for the DOE, namely the Site Recommendation (SR). The Total System Performance Assessment-Site Recommendation (TSPA-SR) will present a compliance evaluation of overall system performance against the guidelines and requirements in the revision of the DOE siting guidelines to be promulgated at 10 CFR 963, U.S. Nuclear Regulatory Commission (NRC) regulation for HLW disposal at proposed 10 CFR 63 (the proposed rule has been published at 64 FR 8640), and U.S. Environmental Protection Agency (EPA) environmental radiation protection standard to be promulgated at 40 CFR 197. At present, the NRC has issued the proposed 10 CFR 63 (64 FR 8640) for public comment whereas the EPA standard and the revised DOE siting guidelines are currently being developed. EPA has announced the release of 40 CFR 197 proposed rule on its website (www.epa.gov/radiation/yucca/rule.qui.htm) and the Federal Register announcement (which initiates the 90 day public comment period) is expected by the end of August, 1999. The purpose of this document is to present the overall goals, objectives, scope, methods, approach, and assumptions to be used in the development of the TSPA-SR. This document will serve as a communication tool for coordinating the DOE TSPA activities and for keeping the NRC staff informed of the TSPA activities for the SR.

Total System Performance Assessment (TSPA) is used as a key decision-making tool for the potential geologic repository for high level radioactive waste at Yucca Mountain, Nevada, USA. Because of the complexity and uncertainty involved in a post-closure performance assessment, an important goal is to produce a transparent document describing the assumptions, the intermediate steps, the results, and the conclusions of the analyses. An important objective for a TSPA analysis is to illustrate confidence in performance projections of the potential repository given a complex system of interconnected process models, data, and abstractions. The methods and techniques used for the recent TSPA analyses demonstrate an effective process to portray complex models and results with transparency and credibility.