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sulate the waste in a series of containers and barriers, emplace the waste, and close the repository are established or could be developed from established practices, and the associated cost is entirely compatible with the economics of energy production. A well-designed repository represents, after closure, a passive system containing a succession of robust safety barriers. These barriers are designed not solely to provide increasing levels of safety, but also to give increasing confidence that the overall system will remain safe even if individual barriers do not perform as well as they are designed to do. Our present civilization designs, builds, and lives with technological facilities of much greater complexity and higher hazard potential. In spite of these facts, there is a long-standing, intense debate on the feasibility of implementing “safe” repositories, that is, repositories that cause no harm to humans or to the environment.

The reason for much discussion is that extraordinarily long time scales must be considered explicitly in the analyses of repositories. There are inevitable uncertainties in the models and data used for these analyses and in the nature of events that might occur far into the future. There are particular uncertainties due to the role of the geological medium in isolating waste placed into a repository. Earth scientists are accustomed to descriptive, deductive reconstruction of the past, but for the purpose of a repository, they must develop quantified, inductive assessments of future system behavior. These factors make it a challenging task to analyze reliably the future evolution of the system. In practice, these difficulties are mitigated by two important facts. First, the engineered barriers can partially compensate for uncertainties in the understanding of the geological medium. Second, a single exact prediction is not needed; rather, understanding the range of potential future changes and assuring that these do not present unacceptable risks is a more correct description of the challenge. Long-term uncertainties also arise in analyses for the disposal of nonradioactive toxic wastes and for managing fossil fuel reserves. All such analyses require good science that will illuminate the physics, chemistry, and other mechanisms that will dominate repository behavior over the long time scales involved.

Nevertheless, the common perception is that for geological disposal specifically, one must be able to predict the future accurately—and it is beyond established engineering practices to predict accurately for many thousands of years how the waste and the repository will behave. It is also beyond established practice to predict accurately whether or not some of the radionuclides disposed in the repository may move through the geological formations and eventually come in contact with human beings and the environment in the future and cause them harm. As emphasized above, however, the challenge is not to accomplish these impossible tasks, but rather to assess the range of potential future behaviors with sufficient confidence to allow the appropriate societal decisions to be made.

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