Performance Assessment for Waste Disposal and Decommissioning

In the context of disposal of radioactive waste, a performance assessment is a quantitative evaluation of potential releases of radioactivity from a disposal facility into the environment, and assessment of the resultant radiological doses. The term performance assessment can refer to the process, model, or collection of models used to estimate future doses to human receptors. Typically, a performance assessment is conducted to demonstrate whether a disposal facility has met its performance objectives. In general, a performance assessment considers the following factors:

• Selected scenario (specific features and processes at the disposal facility and in the surrounding area, such as the location of the potential release, location and general characteristics of the receptors, and applicable transport pathways through which radionuclides might reach the environment and pose a threat to the selected receptor groups)
• Performance of the cask or other engineered barrier system used to store low-level waste, limit the influx of water, and reduce the release of radionuclides
• Release and migration of radionuclides through the engineered barrier system and geosphere (those deep-underground portions of the disposal facility where human contact is generally not assumed to occur)
• Radiological dose(s) to the selected receptor group(s)

Because it is not possible for computer models to precisely replicate all conditions of a realistic disposal facility, the staff of the U.S. Nuclear Regulatory Commission (NRC) uses abstraction to simplify the information to be considered in a performance assessment. The degree of abstraction normally reflects the need to improve reliability and reduce uncertainty, balanced with other practical considerations (such as making the model and its results easy for people to understand). Nonetheless, it is important for the model to be sufficiently detailed to ensure that it yields valid results for the performance assessment.

Also, while traditional deterministic methods have been sufficient to ensure adequate safety, performance assessments can be more explicitly quantified through probabilistic approaches. In particular, a probabilistic performance assessment considers the risk triplet: "What can go wrong?" "How likely is it?" and "What are the consequences?" Use of performance assessment tools and methodologies aids the NRC in applying a risk-informed and performance-based approach to regulatory decision-making.