A methodology was developed to determine the effectiveness of different components/barriers in isolating waste from the accessible environment for the potential nuclear waste repository at Yucca Mountain, Nevada [1]. Quantifying the performance offered by different barriers aids in prioritizing the testing and analysis activities aimed at resolving uncertainty in processes and parameters of greatest significance to long-term performance. If a substantial fraction of the overall performance of a repository system may be attributed to the performance of a particular barrier, then it is prudent to devote the necessary resources toward enhancing the confidence in our predictive ability regarding that barrier (through more in-depth characterization and testing).