Postclosure environmental conditions and waste form stability in a geologic repository for high-level nuclear waste

Abstract

Following the closure of a geologic repository for nuclear waste, the solid wasteform will be subjected to demanding physical and chemical conditions that reflect both geologic and engineered factors. Significant among these conditions are an effective confining pressure of about 10 MPs (about 1500 psi), possible differential pressure up to 35 M Pa (about 5000 psi), possible temperatures locally up to 400 °C (about 750 °F), and the likely presence of water in a reducing environment. Behavior of the waste form in the selected geologic environment must be predicted over unprecedented periods of time. From the standpoint of both the waste isolation system and of the waste form in that system, the thermal period of a geologic repository will be the most critical. The repository contents and surroundings are likely to be subjected to the greatest thermal, mechanical, and chemical stress during the first several hundred years after closure of the repository when heat production is greatest and dominated by the decay of fission products. Thus, most physical and chemical changes within the waste form or host rock that would affect the relationships among the waste, the host rock, and contained fluids would be expected to proceed at the highest rate during this period. Although thermal loading can be controlled, a maximum thermal loading that will not compromise the isolation system is desirable because this would minimize waste form and repository volumes. The higher temperatures that will result can be expected to contribute to increased rates of waste/rock/fluid interactions within the repository, higher stress levels, and other effects. The waste form must therefore be thoughtfully tailored to the host medium, contained fluids, and other characteristics of the specific geologic system selected for waste isolation.