Assessing the impact of waste-generated gas from the degradation of transuranic waste at the Waste Isolation Pilot Plant (WIPP): An overview of strongly coupled chemical, hydrologic, and structural processes

The Waste Isolation Pilot Plant (WIPP) is a US Department of Energy repository for the permanent disposal of approximately 180,000 cubic meters of transuranic waste. The repository is located approximately 650 meters below land surface in a bedded salt formation. Microbial degradation of cellulosics in the waste and anoxic corrosion of steel containers and iron-bearing materials in the waste may generate significant quantities of gas. Experimental and analytic studies are currently under way to evaluate the physical and chemical processes that control gas generation and repository response to gas pressurization. These studies indicate that the impact of waste-generated gas in the repository environment is likely to be characterized by strong coupling of chemical, hydrologic, and structural processes. For example, preliminary laboratory experiments suggest that gas generation rates depend on the availability of brine, which is controlled not only by hydrologic conditions surrounding the repository, but also by the rate at which gas pressure increases within the repository. Also, gas pressure in the repository is influenced by creep closure and consolidation of the room contents, which reduces the void volume available to store gas within a disposal room. However, the backstress of elevated gas pressure on the room walls may be capable of reversing room closure. Rigorous analysis of the coupled processes in the repository-rock system requires iterative development of experiments and models that characterize individual processes and system-scale models that characterize process coupling. Rigorous system-scale models are then used to test more streamlined performance assessment models that contain process and/or geometry simplifications in order to carry out probabilistic compliance calculations. 35 refs., 17 figs