An Integrated Systems Approach to Performance Assessment of Near Surface Disposal Facilities for Low Level Radioactive Waste Management
Rustick, Joseph Henry
An integrated systems approach framework was developed that defines the performance of a near surface low-level radioactive waste disposal facility as a system of three components (or subsystems): the engineered component (cover systems and bottom liners); the properties of the waste (composition, waste form and waste package); and the site-specific environmental features (climate, geology, hydrology). US government radioactive waste disposal facility design and management were examined and compared using this approach. The waste component, historically not considered when calculating waste movement within the facility, was evaluated in greater detail by looking at corrosion of carbon steel boxes filled with waste and buried in a humid environment. The time to hydraulic failure from initial burial to development of corroded holes was calculated for four corrosion scenarios under a constant and a slowing corrosion case. Corrosion rates were estimated from several historical studies and related to the corrosivity and aeration profile of the soil. The scenarios were chosen to represent a range of possible conditions at current and future U.S. Department of Energy disposal facilities. A leachate model was then created that could show the amount of liquid leachate present in each waste package at the time of failure. This model was applied to three different infiltration situations based off of past, current, and proposed future installations of operational and interim cover systems over recently buried waste packages. It was found that for past practices of a 25-year operational period, the estimated amount of leachate within the waste zone over current practices was greater than 300 percent. In order to reduce leachate for future planned disposal facilities, it would be useful to install interim cover systems immediately after waste burial, and fill the waste packages with grout before disposal. For all disposal facilities, leachate movement from the waste zone into the vadose zone would be a good target for performance monitoring.