PNNL

Abstract

To facilitate its vitrification, a portion of the radioactive low-activity waste (LAW) currently stored in underground tanks at the Hanford Site will be staged and treated using tank-side operations to render their chemistry compliant with the requirements of Hanford’s LAW vitrification facility. Initial sampling of staged feeds indicates that planned dilution of the waste feeds with process water to reduce their sodium molarity to the required vitrification target of 5 to 6 M Na may cause precipitation of fine, difficult to settle solids that could affect downstream tank-side filtration and ion-exchange operations. However, the assertion of dilution-induced precipitation has not been demonstrated under controlled, rigorous laboratory conditions. This paper presents a set of qualitative and quantitative assessments of dilution-induced precipitation using a nonradioactive, 8.53 M Na Hanford Tank AP-105 simulated waste. The assessment findings indicate that dilution of AP-farm wastes does induce precipitation, regardless of whether dilution is done with process water (which contains, among other analytes, naturally occurring Ca and Mg) or ultrapure (18 MO cm) deionized (DI) water. Naturally occurring process-water analytes appear to greatly accelerate the rate (and possibly extent) of precipitation, leading to rapid (within 10 days of dilution) precipitation of solids rich in the analytes provided by the process water. A secondary, slower precipitate formation was also observed (taking place over the 30-to-100-day period following dilution) for both process and DI water dilutions; secondary precipitation produced solids with analytes associated primarily with the chemistry of the waste simulant itself (namely Na, Al, and Fe). Precipitation was directly linked to dilution because no solids formation was observed in undiluted control samples. Although the quantity of process water dilution-induced precipitates is small (150 ppm or less), testing confirmed they could significantly challenge prototypic LAW dead-end filtration operations. While the solid content was observed to increase as a result of continued precipitation during the 3-month filtration study, the overall fouling proclivity of the solids was observed to decline, likely as a result of solids growth and aging. Filtration testing also found settle/decant operations to be effective at reducing the suspended particle loadings by several orders of magnitude (to 2 ppm or less); however, testing of decanted samples also evidenced a small fraction of fine colloidal precipitates that do not readily settle, leading to measurable fouling of prototypic dead-end filters. Overall, the data presented herein confirm LAW dilution as part of waste feed staging operations may cause the immediate and lagging formation of filtration-averse solids but suggest that waste staging hold periods can be used to minimize transport of these precipitates to downstream operations.