Challenge. Provide performance assessment (PA) modeling and analyses to support Viability Assessment, Environmental Impact Statement (EIS), Site Recommendation, and License Application for the U.S.’s first HLW repository

Solution. For over 18 years, INTERA provided PA and environmental systems modeling services for DOE’s Yucca Mountain Project (YMP). Prior to the project’s termination in 2009, Yucca Mountain was the proposed site for the nation’s first long-term geologic repository for spent nuclear fuel and high-level radioactive waste. To obtain a license for constructing the repository, U.S. Nuclear Regulatory Commission and EPA regulations require that the safety of the entire repository system be evaluated via a probabilistic PA. The tool for accomplishing this was a model called Total System Performance Assessment (TSPA). As the lead TSPA contractor from 1991 through 2001 and a key subcontractor from 2001 to 2009, INTERA personnel contributed to a number of critical DOE milestones including: completion of the Viability Assessment and Final EIS in 1998, completion of the Site Recommendation in 2001, and the completion of the License Application and supplemental EIS in 2008. All of these efforts required the updating and submittal of a supporting TSPA report. Uncertainty analysis was a key component of the TSPA. As the lead PA contractor, INTERA applied uncertainty analysis methodologies to compute the probabilistic human health risk at receptor sites down gradient from the Yucca Mountain facility. The three components of INTERA’s approach included: uncertainty characterization (i.e., fitting/assigning marginal and joint distributions to model inputs), uncertainty propagation (i.e., translating the uncertainty in model inputs into uncertainty in model outputs), and importance analysis (i.e., determining the key drivers of output uncertainty). We not only computed distributions around model predictions as a product of the uncertainty analysis, but also identified the key input parameters contributing most to the model’s predictive uncertainty. The total system model included the effects of: climate change; water infiltration into the mountain; water movement above the repository level; water seepage in the repository tunnels; waste degradation; radionuclide transport through the tunnel floor, rock below the repository, and in the groundwater; releases to the environment; and the dose to a human receptor down gradient. Once the TSPA was completed, our team conducted an importance analysis using sensitivity techniques to identify the input parameters that had the greatest impact on the risk assessment results. We also developed visualization and animation tools to communicate PA model results. Because of the high-profile nature of the YMP, we presented the results of the PA activities to a wide range of technical and non-technical audiences including external scientific and regulatory review panels.