[en] Both the US Environmental Protection Agency (EPA) and the US Nuclear Regulatory Commission (NRC) have promulgated regulations regarding the performance of geologic repositories for the disposal of high-level nuclear waste. Specifically, the EPA has promulgated three quantitative, postclosure requirements that apply to the entire disposal system, while the NRC's three quantitative, postclosure requirements apply only to particular subsystems of the repository. To assess compliance with all six of these quantitative requirements, the phenomena that can affect the performance of the repository, the processes by which these phenomena are produced, and the parameters associated with these processes will have to be identified and quantified. In addition, the analyses performed to assess compliance will have to be conducted in accordance with a performance assessment methodology to ensure that all regulatory criteria are addressed. A performance assessment methodology proposed by Sandia National Laboratories is composed of scenario development and screening, consequence analysis, uncertainty analysis, and sensitivity analysis. This methodology can be used to assess compliance with the EPA's and NRC's requirements. 57 refs., 18 figs