Two methods for the structured assessment of model uncertainty by experts in performance assessments of radioactive waste repositories

The assessment of the performance of high-level radioactive waste repositories is based on the use of models for predicting system behaviour. The complexity of the system together with the large spatial and temporal scales imposed by the regulations introduce large uncertainties in the analysis. The difficulty of validating the relevant models creates the need of assessing their validity by means of expert judgments. This paper addresses the problem of model uncertainty both from a theoretical and a practical point of view and presents two mathematical approaches to the treatment of model uncertainty that can assist the experts in the formulation of their judgments. The formal elicitation of expert judgments is investigated within the Technical Facilitator/Integrator (TFI) framework that has been proposed by the Senior Seismic Hazard Analysis Committee. Within this framework, the mathematical formulations for the treatment of model uncertainty are regarded as tools for sensitivity analyses that give insights into the model characteristics and are helpful in structuring the expert opinion elicitation process itself. The first approach, referred to as the alternate-hypotheses formulation, amounts to constructing a suitable set of plausible hypotheses and evaluating their validity. The second approach to model uncertainty is referred to as the adjustment-factor formulation and it requires that a reference model be identified and its predictions be directly modified through an adjustment factor that accounts for the uncertainty in the models. Furthermore, both approaches require a clear understanding of the distinction between aleatory and epistemic uncertainties. The implications that these two formulations have on, and the issues that they raise in, the elicitation of expert opinions are explored. A case study of model uncertainty regarding alternative models for the description of groundwater flow and contaminant transport in unsaturated, fractured tuff is presented.