The Treatment of Uncertainty in Performance Assessment and Safety Case Development

This paper presents the results of a review of the treatment of uncertainty in performance assessment (PA) and safety-case development, carried out as part of the European Commission (EC) project PAMINA (Performance Assessment Methodologies IN Application to Guide the Development of the Safety Case). Information on the treatment of uncertainties was gathered from PAMINA participants and several other organisations using a questionnaire, and via a limited wider review of the literature. The questionnaire responses obtained represent 16 disposal programmes in 13 countries, including all of the countries with advanced programmes to implement deep geological disposal, allowing the review to give wide coverage of global activity. A two-day workshop was held in Brussels in March 2007, in which PAMINA participants reviewed an initial document which summarised the questionnaire results. Work plans were formulated for the remainder of the PAMINA programme. The 16 programmes represented are at diverse phases of maturity: four are at the conceptual development or feasibility stage, seven are at the site selection or site characterisation stage, two are at the licensing stage, one is at the construction stage, one has an operational repository, and one is at the decommissioning/closure stage. There is also wide variation in the development of regulations concerning the treatment of uncertainty for deep geological disposal of radioactive waste, with several countries having no specific regulations. The review indicates that there is a high level of consensus with respect to the nature of uncertainties in PAs and how they should be classified, although this is sometimes masked by variations in terminology and differences in the way uncertainties are treated in programmes. A system of classification is set out in this review, with reference to the nature of uncertainties. The review discusses how the principal classes of uncertainty are treated in PAs and safety cases. While nearly all programmes treat parameter and scenario uncertainties, some do not treat conceptual model uncertainties explicitly. Respondents expressed familiarity with sensitivity-analysis techniques, and clearly understand the difference between these and uncertainty analysis. It is less clear how widespread the use of sensitivity analysis is, especially formal mathematical schemes. Almost no organisations identified uncertainties that may challenge programmes, suggesting a high level of confidence in their ability to site and design deep geological disposal facilities so as to manage uncertainties effectively. However, respondents variously identified the engineered barrier system, the geosphere, the biosphere, and future human intrusion as key sources of uncertainty that require further investigation. Responses on the issue of communicating uncertainties were patchy: some respondents professed to have little experience in this area, whereas others chose not to answer the question. Some restricted themselves to discussing communication with regulators. Only a few programmes have gone as far as commissioning research into different approaches to communicating uncertainty to a variety of stakeholders.