(214e) Dead-End Filtration of Dilute an-107 Hanford Tank Waste Supernatant at Reduced Temperature

The U.S. Department of Energy's Hanford Site is home to 56 million gallons of highly radioactive legacy waste stored in underground tanks. Washington River Protection Solutions (WRPS) manages the tank farms and has deployed the Tank-Side Cesium Removal (TSCR) system designed to treat the tank waste supernatant by removing suspended solids and cesium via filtration and ion exchange unit operations, respectively, before the supernatant is immobilized by vitrification for long-term storage. The Direct Feed Low-Activity Waste test platform developed and operated by Pacific Northwest National Laboratory consists of bench-scale purpose-built systems that mimic TSCR unit operations and is used to demonstrate the filtration, ion exchange, and vitrification of batches of aqueous tank waste supernatant provided by WRPS. The dilution and filtration steps are crucial in maintaining ion exchange cesium removal performance and robustness. The most recent demonstration began in 2023 with the dilution of 9 L of AN-107 supernatant from >9 M Na to 5.5 M Na with raw process water sourced from the Columbia River followed by reduced-temperature filtration utilizing a Mott Media Grade 5 sintered metal dead-end filter at 16 °C with a nominal flux rate of 0.065 gpm/ft² and a transmembrane pressure not to exceed 2 PSI – parameters and materials prototypic of the full-scale TSCR dilution and filtration processes. Filter maintenance procedures employed by TSCR including back-pulse to reverse fouling when the transmembrane pressure reaches 2 PSI, filter cleaning with 0.1 M NaOH, and clean water flux measurements pre- and post-filtration to compare filter performance before and after the test were also executed on the bench-scale system. The unique physical and chemical properties of the AN-107 supernatant resulted in filter fouling and eventual plugging that was resistant to back-pulsing and cleaning methods that had been successful with previous tank waste filtration campaigns, requiring the use of oxalic acid to restore filter performance. Multiple fouling mechanism functions were fit to filtration data to identify the predominant blocking regime in the filtration of AN-107 supernatant. Total alpha analysis conducted on permeate samples showed transuranic (TRU) content exceeded the 0.1 µCi/g threshold established by the Department of Energy to define TRU waste by a factor of 10. Scanning electron microscopy and scanning transmission electron microscopy examination showed the presence of fluorophosphates and Mn-Fe phases in the dewatered solids samples collected from back-pulses and scraped from the filter. The filtration test suggests the Mott Media Grade 5 filter may be prone to quick and sudden plugging in the TSCR processing of AN-107 supernatant that has not been given sufficient time to settle and may require cleaning and maintenance protocols that diverge from methods that have been effective with previous tank wastes.