This session will focus on unique simulation challenges posed by efforts to understand and manage the fate and transport of radionuclides in the environment, as specifically related to (1) high-level nuclear waste repository design and (2) the evaluation of long-term impacts of nuclear testing on water resource systems. As dissimilar as they may seem, both topics involve (a) consideration of large, multiscale hydrologic regimes (e.g., as influenced by geologic heterogeneity), (b) problem-specific hydrothermal or other multiphase flow issues, (c) model and parametric uncertainty concerns, especially in locally altered (engineered or otherwise) environments, (d) reactive transport of multiple radionuclide species in such systems, and/or (e) strict regulatory or design requirements, all of which serve to increase the complexity, data requirements, and validation needs of the simulation process. In turn, increased computational complexity will drive needs for fundamental improvements in algorithm design, sensitivity, and optimization strategies, and the kinds and ways in which field data are acquired, interpreted, and otherwise used to support these efforts. Thus we seek contributions that address one or more of these complexities in the context of the focal session topics.