Release consequence methodology developed under the Assessment of Effectiveness of Geologic Isolation Systems (AEGIS) program has now been applied to four hypothetical repository sites. This paper summarizes the results of these four studies in order to demonstrate that the far-field methodology developed under the AEGIS program offers a practical approach to the post-closure safety assessment of nuclear waste repositories sited in deep continental geologic formations. Because of the generic

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... hypothetical nature of the various studies reported, along with the different levels of complexity of the models used, one should not attempt to use the information to judge the merits of one site or medium as against another. Instead it can be used to draw some general conclusions regarding release consequence methodology and other important areas related to nuclear waste repositories elucidated by these studies. The four studies are briefly described and compared according to the following general categories: • physical description of the repository (size, inventory, emplacement depth) • geologic and hydrologic description of the site and the conceptual hydrologic model for the site • description of release scenario • hydrologic model implementation and results • engineered barriers and leach rate modeling • transport model impl ementati on and resul ts • dose model implementation and results. These studies indicate the following: • Numerical modeling is a practical approach to post-closure safety assessment analysis for nuclear waste repositories. • Near-field modeling capability needs improvement to permit assessment of the consequences of human intrusion and pumping well scenarios. iii • Engineered barrier systems can be useful in mitigating consequences for postulated release scenarios that short-circuit the geohydrologic system. • Geohydrologic systems separating a repository from the natural biosphere discharge sites act to mitigate the consequences of postulated breaches in containment. • Engineered barriers of types other than the containment or absorptive type may be useful. iv CONTENTS