Performance assessments of nuclear waste repositories: A dialogue on theirvalue and limitations

Performance Assessment (PA) is the use of mathematical models to simulate t he long-term behavior of engineered and geologic barriers in a nuclear wast e repository; methods of uncertainty analysis are used to assess effects of parametric and conceptual uncertainties associated with the model system u pon the uncertainty in outcomes of the simulation. PA is required by the U. S. Environmental Protection Agency as part of its certification process for geologic repositories for nuclear waste. This paper is a dialogue to explo re the value and limitations of PA. Two "skeptics" acknowledge the utility of PA in organizing the scientific investigations that are necessary for co nfident siting and licensing of a repository; however, they maintain that t he PA process, at least as it is currently implemented, is an essentially u nscientific process with shortcomings that may provide results of limited u se in evaluating actual effects on public health and safety. Conceptual unc ertainties in a PA analysis can be so great that results can be confidently applied only over short time ranges, the antithesis of the purpose behind long-term, geologic disposal. Two "proponents" of PA agree that performance assessment is unscientific, but only in the sense that PA is an engineerin g analysis that uses existing scientific knowledge to support public policy decisions, rather than an investigation intended to increase fundamental k nowledge of nature; PA has different goals and constraints than a typical s cientific study. The "proponents" describe an ideal, six-step process for c onducting generalized PA, here called probabilistic systems analysis (PSA); they note that virtually all scientific content of a PA is introduced duri ng the model-building steps of a PSA; they contend that a PA based on simpl e but scientifically acceptable mathematical models can provide useful and objective input to regulatory decision makers. The value of the results of any PA must lie between these two views and will depend on the level of kno wledge of the site, the degree to which models capture actual physical and chemical processes, the time over which extrapolations are made, and the pr oper evaluation of health risks attending implementation of the repository. The challenge is in evaluating whether the quality of the PA matches the n eeds of decision makers charged with protecting the health and safety of th e public.