Mercury Speciation of Hanford 241-AP-105 Tank Waste Samples – 21108

Understanding the movement and fate of mercury, and its chemical consequences are important in the Hanford flowsheet. Necessary inputs to evaluate the impact of mercury include the magnitude of the mercury feed inventory, how the inventory is distributed throughout the tank farms, how the tank-by-tank mercury speciation inventory is distributed by waste phases (i.e., sludge, saltcake, and supernatant), the expected mercury species during waste processing, and finally, the degree of uncertainty in this information. Based on process knowledge, an estimated inventory of 2,100 kg of mercury is assumed to be distributed in varied amounts across 149 single-shell tanks (SSTs) and 28 double-shell tanks (DSTs) at the Hanford Site. Historically, the limited analyses of individual SSTs and DSTs for total mercury were below method detection, yielding upper limits in concentration estimates based solely on method detection limits (MDLs), resulting in an uncertain mercury inventory for Hanford tanks. Recently, Pacific Northwest National Laboratory (PNNL) researchers provided the first glimpse into the concentration of total and elemental mercury in 241-AP-107 (hereafter referred to as AP-107) tank waste feed and AP-107 treated effluents. Highly sensitive separation methods for total, elemental, and monomethyl mercury species, in tank waste, developed at PNNL’s Radiochemical Processing Laboratory, and non-radiological environmental methods to quantify mercury species have been adapted for application to Hanford tank wastes and waste at other sites. This method (separation and quantification), which has been previously demonstrated at PNNL to measure AP-107 waste, was applied to four sample types from Hanford waste tank 241-AP-105 (hereafter referred to as AP-105) and designated as raw, feed, Mott grade 5 filtered samples, and cesium-decontaminated samples. These samples represent feed inventory and pretreatment process effluents prior to immobilization by vitrification. Predicting and tracking mercury speciation and associated inventories mitigates risks associated with compliance with the mercury-to-sodium ratio waste acceptance criterion for the Hanford Waste Treatment and Immobilization Plant, secondary waste disposal land disposal restriction requirements, and air permit abatement assumptions during the Direct Feed Low-Activity Waste (DFLAW) mission and beyond. This work directly aides in the understanding of mercury movements, chemical consequences, and the fate of mercury in the Hanford flowsheet, enabling engineers to reliably anticipate and control the mass movements of various mercury species through the Hanford waste processing flowsheet, thereby reducing flowsheet risk and providing reference-case data to compare to actual future DFLAW operations.