The performance assessment program at NTS has been transitioning from one of performance assessment development to performance assessment maintenance and monitoring. Performance assessments have not been completed for Areas 3 and 5 and disposal authorization statements have been received or are expected. Both sites easily pass all performance assessment objectives. To facilitate transition to the next stage, NTS is moving from deterministic performance assessments to a probabilistic analysis of sensitivity. DOE Order 435.1, Radioactive Waste Management, tends to favor a deterministic modeling approach in conducting performance assessments.
Deterministic models provide a bounding analysis that tends to overestimate potential releases.
However, this kind of analysis introduces nonsystematic conservatism where uncertainty is not defined. Probabilistic analysis, on the other hand, propagates parameters defined as probability distributions using Monte Carlo simulations. Once performance assessments are switched from deterministic to probabilistic, uncertainty analyses can be performed by changing the parameter sheets that run the probabilistic models and by reviewing the changes in releases. Identification of key aspects of system uncertainty is critical in this process. NTS is moving forward with a vadose zone strategy that incorporates monitoring methods and data acquisition capabilities, a cost-effective regulatory strategy, and a focus on performance assessment sensitivity.