Hydrologic issues associated with nuclear waste repositories

Significant progress in hydrology, especially in subsurface flow and solute transport, has been made over the last 35 years because of sustained interest in underground nuclear waste repositories. The present paper provides an overview of the key hydrologic issues involved, and to highlight advances in their understanding and treatment because of these efforts. The focus is not on the development of radioactive waste repositories and their safety assessment, but instead on the advances in hydrologic science that have emerged from such studies. Work and results associated with three rock types, which are being considered to host the repositories, are reviewed, with a different emphasis for each rock type. The first rock type is fractured crystalline rock, for which the discussion will be mainly on flow and transport in saturated fractured rock. The second rock type is unsaturated tuff, for which the emphasis will be on flow from the shallow subsurface through the unsaturated zone to the repository. The third rock type is clay-rich formations, whose permeability is very low in an undisturbed state. In this case, the emphasis will be on hydrologic issues that arise from mechanical and thermal disturbances; i.e., on the relevant coupled thermo-hydromechanical processes. The extensive research results, especially those from multiyear large-scale underground research laboratory investigations, represent a rich body of information and data that can form the basis for further development in the related areas of hydrologic research. 1. General Introduction Significant progress in hydrology, especially in subsurface flow and solute transport, has been made over the last 35 years because of sustained interest in the performance assessment of proposed underground nuclear waste repositories. The present paper attempts to provide an overview of the key hydrologic issues involved and to highlight the advances in their understanding and treatment because of these efforts. This paper is not focused on the development of high-level nuclear waste repositories and their safety assessment; it focuses instead on the advances in hydrologic science which have emerged from such studies. These advances may find applications in other hydrology-related problems of current importance at the national and international levels, such as environmental contamination remediation, carbon dioxide geosequestration, and natural gas production through fracking. In the next section, we present, in a general way, the context for the hydrologic framework related to performance and safety assessment of underground nuclear waste repositories that are proposed to be constructed in three particular rock formations—crystalline rock, unsaturated tuffs, and clays. Then, we discuss some particularly challenging factors concerning the research common to these three formations. This discussion will be followed by three sections devoted to each of these three rock types, respectively, to present a review of hydrologic research in these areas.