The National Research Council, Study of the Isolation System for Geologic Disposal of Radioactive Wastes

ABSTRACTThis study was conducted for the U. S. Department of Energy by the Waste Isolation Systems Panel appointed by the National Academies of Science and Engineering. The panel was charged to review the alternative technologies available for Isolating of radioactive waste in mined geologic repositories, evaluate the performance benefits from these technologles as potential elements of a waste Isolation system, and identify appropriate technical criteria for satisfactory long-term performance of a geologic repository. Conceptual repositories in basalt, granite, salt, and tuff were considered. Site-specific data on geology, hydrology, and geochemical properties were evaluated and used to define parameters for estimating long-term environmental releases, supplemented when necessary by generic properties.The technology for solid waste forms and waste packages was reviewed and evaluated. Borosilicate glass and unreprocessed spent fuel are the waste forms appropriate for further testing and for repository designs. Testing in a simulated repository environment is necessary to develop an adeauate prediction of the long term performance of waste packages in a geologic repository. Back-up research and development on alternative waste forms should be continued. The expected functions of backfill placed between the rock and waste package need clearer definition and validation.The overall criterion to be used by federal agencies in designing a geologic waste-isolation system and in evaluating its nerformance has not yet been specified. As a guideline, the panel selected an average annual dose of 10-4 sieverts to a maximally exposed individual at any future time, if the exposure is from expected events such as the slow dissolution of waste solids in wet-rock repositories and the groundwater transport of dissolved radionuclides to the biosphere. Risks from unexpected events such as human intrusion were not evaluated.Calculations were made of the long-term isolation and environmental releases for conceptual repositories in basalt, granite, salt, and tuff. The major contributors to geologic isolation are the slow dissolution of key radioelements as limited by solubility and by diffusion and convection in groundwater surrounding the waste solids, long water travel times from the waste to the environment, and sorption retardation in the media surrounding the repository. Dilution by surface water can reduce the individual radiation exposures that can result from the small fraction of the waste radioactivity that may ultimately reach the environment. Estimates of environmental releases and individual doses were made both for unreprocessed spent fuel and for reprocessing wastes.Accelerated dissolution of waste exposed to groundwater during the period of repository heating was also considered. Long-term environmental releases of radioactivity from some repositories were calculated to cause doses to maximally exposed individuals that are several orders of magnitude below the Individual dose criterion of 10-4 Sieverts per year. Other conceptual repositories were found to not meet the individual dose criterion, although these repositories could still meet the radioactivity release limits in the standard proposed by the Environmental Protection Agency.The technology for geologic waste disposal has advanced to the state of a preliminary technical plan, suitable for testing, verification, and for pllot-facility confirmation. The waste Isolation program needs a reliable prediction of long-term performance that will serve as a basis for final design, construction, licensing, and waste emplacement.