Intermediate depth disposal operations were conducted by the U.S. Department of Energy (DOE) at the DOE's Nevada Test Site (NTS) from 1984 through 1989. These operations emplaced highspecific activity low-level wastes (LLW) and limited quantities of classified, "special case" transuranic (TRU) wastes in 37 m (120-ft) deep, 3 m (10 ft) diameter Greater Confinement Disposal (GCD) boreholes.Four boreholes contain about 60,000 kg (132,000 lb.) of classified TRU waste packages, containing less than 330 curies of Plutonium-239. All of the TRU wastes emplaced in the GCD boreholes are classified for national security reasons and cannot be disposed of in the DOE's Waste Isolation Pilot Plant.The U.S. Environmental Protection Agency's (EPA's) 40 CFR 191 defines the requirements for protection of human health from disposed TRU wastes. This EPA standard sets a number of requirements, including probabilistic limits on the cumulative releases of radionuclides to the accessible environment for 10,000 years.
This report presents the performance assessment (PA) that has been conducted to determine if disposal of TRU waste in the GCD boreholes complies with the EPA's 40 CFR 191 requirements.Sandia National Laboratories completed this PA using all available information and an Iterative PA Methodology, which focused work on uncertainty reduction in a cost-effective fashion that does not overestimate system performance and assured defensibility. The simplicity of the conceptual models of the GCD disposal system allowed them to be implemented in Microsoft ® Visual Basic™ macros in an Access™ database. This PA model is built from a mathematical expression for mass conservation that includes the operation of a number of transport processes, including dissolution, precipitation, reversible chemical sorption onto soil, advection, diffusion, dispersion, radioactive decay and ingrowth, plant uptake, and bioturbation. The mathematical model and implementing code was used to calculate a complementary cumulative distribution function of integrated normalized release to the accessible environment for 10,000 years and probability distributions of dose based on two exposure conditions for the 1,000 year individual protection requirements.The primary conclusions of this PA are that the disposal of TRU wastes in the GCD boreholes will, at most, result in minuscule doses to individuals, and that the GCD disposal system easily meets the EPA's 1985, 40 CFR 191 requirements for disposal of TRU waste. Further, there is a strong, reasonable expectation that actual system performance will be better than what is simulated in this PA.vi ACKNOWLEDGMENTS