An Evaluation of Long-Term Performance of Liner Systems for Low-Level Waste Disposal Facilities

Traditional liner systems consisting of a geosynthetic membrane underlying a waste disposal facility coupled with a leachate collection system have been proposed as a means of containing releases of low-level radioactive waste within the confines of the disposal facility and thereby eliminating migration of radionuclides into the vadose zone and groundwater. However, this type of hydraulic containment liner system is only effective as long as the leachate collection system remains functional or an overlying cover limits the total infiltration to the volumetric pore space of the disposal system. If either the leachate collection system fails, or the overlying cover becomes less effective during the 1,000's of years of facility lifetime, the liner may fill with water and release contaminated water in a preferential or focused manner. If the height of the liner extends above the waste, the waste will become submerged which could increase the release rate and concentration of the leachate. If the liner extends near land surface, there is the potential for contamination reaching land surface creating a direct exposure pathway. Alternative protective liner systems can be engineered that eliminate radionuclide releases to the vadose zone during operations and minimizing long term migration of radionuclides from the disposal facility into the vadose zone and aquifer. Non-traditional systems include waste containerization in steel or composite materials. This type of system would promote drainage of clean infiltrating water through the facility without contacting the waste. Other alternatives include geochemical barriers designed to transmit water while adsorbing radionuclides beneath the facility. Facility performance for a hypothetical disposal facility has been compared for the hydraulic and steel containerization liner alternatives. Results were compared in terms of meeting the DOE Order 435.1 low-level waste performance objective of 25 mrem/yr all-pathways dose during the (1) institutional control period (0-100 years), compliance period (0-1000 years) and post-compliance period (>1000 years). Evaluation of the all pathway dose included the dose from ingestion and irrigation of contaminated groundwater extracted from a well 100 meters downgradient, in addition to the dose received from direct contact of radionuclides deposited near the surface resulting from facility overflow. Depending on the disposal facility radionuclide inventory, facility design, cover performance, and the location and environment where the facility is situated, the dose from exposure via direct contact of near surface deposited radionuclides can be much greater than the dose received via transport to the groundwater and subsequent ingestion.