Performance Modeling to Support Decision-making for the Management and Disposition of Low-Activity Radioactive Waste

Abstract

The Agency has been examining the potential for the use of the RCRA-C disposal unit technology for the disposal of "low-activity" radioactive wastes (considered as a subset of Class A wastes as defined by the NRC). To determine waste acceptance criteria (permissible radionuclide concentrations in the wastes), preliminary performance modeling of the RCRA-C disposal unit was conducted for an expected case (slow degradation of the disposal unit), as well as exposures through three human intrusion scenarios and worker exposures during waste handling, treatment, storage and transportation. The expected case modeling examined disposal units in arid and humid climates and in a variety of hydrologic settings. A set of linked codes were used to assess stochastic performance within the waste unit, along the ground water travel path and the biosphere to calculate doses to a near-by off-site receptor. Results showed that arid site locations offered exceptional performance due to the low infiltration and generally deeper water tables. Preliminary modeling showed that worker exposures and human intrusion scenarios strongly dominate the comparative dose assessments due to direct exposures to the receptors in these analyses. The robust design of the RCRA-C units can exert a significant counterbalance to the limitations imposed by the direct exposure scenarios however. Determining permissible waste concentrations for use in rule making or guidance for this potential disposal option will consider site restrictions, waste form requirements and other factors before decisions are made on final criteria.