CONSIDERATION OF UNCERTAINTY IN THE PERFORMANCE ASSESSMENT OF RADIOACTIVE-WASTE DISPOSAL FROM AN INTERNATIONAL REGULATORY PERSPECTIVE

Regulations governing the disposal of radioactive wastes require that the performance of disposal facilities be assessed quantitatively. Such assessments, commonly called either 'performance assessments' or 'probabilistic risk assessments', rely on the use of predictive models to arrive at a quantitative estimate of the potential impact of disposal on the environment and on the safety and health of the public. It has been recognized that a suite of uncertainties affect the results of a performance assessment (herein we use 'performance assessment' to be synonymous to probabilistic risk assessment). These uncertainties are conventionally categorized as (1) uncertainty in the future state of the disposal system (facility and surrounding medium), (2) uncertainty in models (including conceptual models, mathematical models, and computer codes), and (3) uncertainty in data and parameters. Decisions regarding the safety of a radioactive waste disposal facility must be made in the light of these uncertainties. Neglecting the uncertainties, in either the analysis or the presentation of results, would be a misrepresentation of one's knowledge (e.g. overconfidence) about the performance of the disposal facility. Hence, an approach is needed that would allow the explicit consideration of these uncertainties so that their impact on the estimated consequences of disposal can be evaluated. Regulations for radioactive waste disposal in many countries, explicitly or implicitly, require the consideration of uncertainties in order to arrive at a meaningful decision regarding the safety of a disposal facility. A probabilistic risk assessment approach provides the formalism for treating the uncertainties and the technical basis that the decision makers can use in discharging their duties.