Development of a Risk-Based Performance Assessment Method for Long-Term Cover Systems--Application to the Monticello Mill Tailings Repository

Abstract

A probabilistic, risk-based performance-assessment methodology is being developed to assist designers, regulators, and involved stakeholders in the selection, design, and monitoring of longterm covers for contaminated subsurface sites. This report presents an example of the risk-based performance-assessment method using a repository site in Monticello, Utah. At the Monticello site, a long-term cover system is being used to isolate long-lived uranium mill tailings from the biosphere. Computer models were developed to simulate relevant features, events, and processes that include water flux through the cover, source-term release, vadose-zone transport, saturatedzone transport, gas transport, and exposure pathways. The component models were then integrated into a total-system performance-assessment model, and uncertainty distributions of important input parameters were constructed and sampled in a stochastic Monte Carlo analysis. Multiple realizations were simulated using the integrated model to produce cumulative distribution functions of the performance metrics, which were used to assess cover performance for both present-and long-term future conditions. Performance metrics for this study included the water percolation reaching the uranium mill tailings, radon flux at the surface, groundwater concentrations, and dose. Results of this study can be used to identify engineering and environmental parameters (e.g., liner properties, long-term precipitation, distribution coefficients) that require additional data to reduce uncertainty in the calculations and improve confidence in the model predictions. These results can also be used to evaluate alternative engineering designs and to identify parameters most important to long-term performance.

Near‐Surface Disposal Performance Assessment: Modeling Monthly Precipitation and Temperature in Various Climate Environments

“…With the abundance of sites across the United States and the variability in operational management at each site, the US DOE introduced DOE Order 435.1, Radioactive Waste Management, in 1999 to assess the performance of these facilities. While the order requires uncertainty analyses, it may be unclear to users with respect to whether these requirements address important long-term features associated with climate (Ho et al, 2001 entire United States may be impacted by a changing climate, the extent to which certain effects are prevalent should be determined on a regional basis. Therefore, any approach to understanding how long-term features will affect environmental performance must be performed at a regional level using numerical models that assess the design integrity and performance of disposal facilities.…”

“…Since these models require temperature and precipitation inputs, they are directly impacted by climate change. Traditional approaches to evaluating near surface facility performance neglect the effects of increases in average temperatures or the occurrence of more frequent and extreme weather conditions (Ho et al, 2001). Typically, records are examined from earliest records to the present.…”