Client: U.S. Department of Energy (through subcontracts with CH2M Hill Plateau Remediation Company and Washington River Protection Solutions)
Location: Richland, Washington, USA
Challenge. Perform PAs to demonstrate long-term (up to 10,000 years) environmental protection after closure of waste management facilities
Solution. In accordance with DOE Order 435.1, Title 10 of the CFR, and 40 CFR 191, INTERA has conducted comprehensive updates of the PAs for waste management facilities at the Hanford Site that include the Environmental Restoration Disposal Facility (ERDF) and Waste Management Area C (WMA C). The ERDF is used to dispose of CERCLA waste, including hazardous chemicals and low-level radioactive waste, generated as part of the environmental restoration activities at Hanford. The facility’s disposal cells, encompassing over 100 acres, contain more than 15 million tons of waste material. WMA C contains 16 underground single-shell tanks that were used to store high-level radioactive waste resulting from irradiated fuel processing during weapons-grade plutonium production at Hanford. Although the waste is being removed, some residual waste will remain in the tanks which will subsequently be filled with grout. The PAs for both of these facilities involved evaluating groundwater, atmospheric, and inadvertent intruder exposure pathways. For the ERDF PA, dose calculations were generated for the groundwater pathway using a 3-dimensional model to evaluate vadose and saturated zone flow and transport. Sensitivity and uncertainty analysis for the groundwater pathway were performed through stochastic simulation of abstracted model results using the GoldSim code. Contaminant transport calculations using GoldSim were performed for various flow-fields that incorporated the temporal effects of degradation of a surface cover and double-leachate liner. Source-term modeling included release of carbon-14 from disposed graphite blocks and developing solubility limits for release of activated metals disposed of at ERDF. Atmospheric exposure, including radon transport, was evaluated using a model for diffusion of volatile radionuclides through the landfill cover. Doses for intruder scenarios were developed and evaluated, including an acute scenario (well driller) and three chronic scenarios (rural pasture, commercial farm, and garden). The results of the analyses showed that peak doses from both the atmospheric and groundwater pathways are significantly less than the defined performance objectives and that groundwater protection limits, based on federal drinking water standards, will not be exceeded at any time. The WMA C PA evaluated the inventory in the residual waste and modeled the release of radionuclides and chemicals from waste-form degradation via advection and diffusion. Surface barrier failure, degradation of grout within the tanks, and transport of contaminants out of the tanks through the vadose and saturated zones were also modeled. Abstraction of the flow-field in the vadose zone for the intact and degraded states of the tanks was used in a system-level model developed using GoldSim. Uncertainty distribution in various system-level parameters was developed to evaluate uncertainty in effective dose equivalent and associated risks/hazards to receptors within a probabilistic mode. A Monte-Carlo based sampling approach via multiple realizations was used to evaluate uncertainty. Results from multiple realizations led to a range of estimates in the dose and risk, thereby reflecting uncertainty that can be expected given the current state of knowledge. Compliance with DOE Order 435.1 requirements and applicable state laws were demonstrated for all exposure pathways.