UNDERSTANDING THE BEHAVIOR OF ENGINEERED AND NATURAL BARRIERS FOR GEOLOGIC DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE: THE ROLE OF FULL-SCALE HEATER EXPERIMENTS

This presentation gives on overview of the complex coupled processes occurring upon the disposal of heat-producing high-level radioactive waste in geologic repositories located several hundreds of meters below the surface. These processes are often being tested at (almost) full-scale in underground research laboratories situated in rock formations that are similar to the potential host rocks for geologic disposal. The United States research program has recently advanced collaboration with several institutions or initiatives in Europe and Asia that provide access to international underground research laboratories and allows for sharing the significant investment needs for large-scale field experiment. Over the past five years, a comprehensive research portfolio has been built with such experiments at the center. We start in this presentation with a brief review of selected international cooperation initiatives and then describe a few examples of full-scale heater experiments that have advanced our knowledge of the short-term and long-term behavior of engineered and natural barriers exposed to strong hydrological, thermal, mechanical, and chemical perturbations. A specific focus will be on the role of compacted bentonite which is commonly used as an engineered backfill material for emplacement tunnels because of its low permeability, high swelling pressure, and retardation capacity of radionuclide. We will present data, and related modeling analysis, from a large heater experiments conducted for over 18 years in the Grimsel Test Site in Switzerland, will discuss the main lessons learned, and will briefly touch on a new experiment that will evaluate the impact of raising the temperature limits for bentonite exposure from roughly 100^oC up to almost 200^oC.