Evaluation of repository safety

The United States high-level waste program requires evaluation of radiological safety during two distinct time intervals. The first interval, commonly referred to as the preclosure period, deals with receipt of waste at the site, transfer into disposal containers, if needed, emplacement in the underground openings, monitoring and maintenance activities, backfill and closure of the underground openings, and decontamination and decommissioning of the surface facilities of the geologic repository. The preclosure period may extend from a few tens of years to as long as a few hundred of years, depending on repository design and societal norms regarding a final decision to permanently seal the repository. During the preclosure or operational period, performance confirmation studies are conducted to provide a basis for updating and reevaluating estimates of postclosure performance and, finally, to provide a basis for a closure decision. The postclosure period during which expected repository performance must meet certain standards may range from ten thousands years, as it does in the United States, to millions of years, as it does in some European nations. Waste handling operations in the preclosure period are to be evaluated in relation to their potential effect on workers, members of general public, and the general environment. During this period, releases of radioactivity are to be monitored and appropriate actions taken whenever established limits are approached or exceeded. Preclosure safety is highly dependent on facility design, operational hardware and automated systems, operational sequences, and reliability of humans involved in operations. Preclosure safety analyses conducted before operations begin play a major role in the design process, selection of equipment, and development of operational procedures. Because of the complexity, duration, and spatial scales of the operations, analyses are conducted using mathematical models implemented in computer codes. Mathematical models play an even larger role in evaluating repository safety in the much longer postclosure period. During this period, repository performance is evaluated considering gradual degradation of engineered barriers, together with possible slow changes in the natural system (e.g., climate) and under conditions of potential discrete and sudden disruptive events (e.g., volcanic eruption, seismic ground motion, and direct fault movement). The general aim of postclosure performance assessment models is to simulate the future behavior of the repository in a manner that is sufficiently simplified to be tractable, yet sufficiently realistic to give reasonable (or bounded) estimates of risk to future generations. The simplifications are based on information gained by using process level models, natural analog studies, and laboratory and fieldwork. Because of large uncertainties inherent in characterizing a large and complex system for such long periods, probabilistic simulations are generally preferred. In this paper, we briefly describe the preclosure and postclosure safety evaluation models developed jointly by the Center for Nuclear Waste Regulatory Analyses and the U.S. Nuclear Regulatory Commission. We give examples illustrating how we intend to use these models within the regulatory framework to evaluate the required