Department of Chemistry Seminar – Dr. R. Matthew Asmussen
About the event
Speaker: Dr. R. Matthew Asmussen, Pacific Northwest National Laboratory
Host: Graduate Chemical Society (GCS)
Title: Unravelling the Complicated Story of Technetium Speciation in Hanford Wastes and its Relevancy to Disposal
Abstract: The nuclear wastes stored at the Hanford site are highly complex and can generate conditions that stabilize various compounds and alternate speciation of many elements. While Tc(VII) in the form of pertechnetate (TcO4-) is expected to dominate in most Hanford conditions, high fractions of Tc in low oxidation states have been evidenced to exist in the Hanford wastes; termed non-pertechnetate. Recent studies by national laboratories and companion studies by the National Academy of Sciences on supplemental low-activity waste (SLAW) have identified significant uncertainty related to the behavior of non-pertechnetate species where a pathway to potable water is present (Bates 2022). Technetium-99 (Tc) can adopt multiple oxidation states in aqueous solutions ranging from +1 to +7 which can be stabilized in Hanford waste conditions (Lukens et al. 2004, Chatterjee et al. 2020, Asmussen et al. 2024). These forms of Tc can incorporate into grout waste forms differently and therefore have different release rates, resulting in higher uncertainty around the predictability of the overall Tc behavior when non-pertechnetate is present.
This presentation will give an overview of on-going efforts to establish spectroscopic suites and test protocols to assess the behavior of alternate species of various radionuclides in Hanford wastes and possible processing technologies. To evaluate the behavior of non-pertechnetate forms of Tc in a grout waste form, two sources of non-pertechnetate were evaluated in grout fabrication and subsequent leaching experiments. A Tc(I) compound was synthesized and used to form four different grout formulations providing a range of redox conditions. A second source of non-pertechnetate was obtained from two treated actual Hanford tank waste samples. The tank waste was first treated using resin, removing all pertechnetate from the solution, leaving only non-pertechnetate forms of Tc, and then was concentrated through evaporation prior to grouting. After curing, the grout waste forms were leached following a modified ANSI/ANS16.1 leach test procedure. Tc speciation was monitored in the solid grout and leachate with nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, FTIR and X-ray absorption spectroscopy, including novel measurements at the Tc L-edge.
References
Asmussen, RM, AM Westesen, C Alvarez, RA Peterson, E Cordova, H Cho, SD Branch, ED Walter, GB Hall and AM Carney. 2024. “The simultaneous removal of technetium and iodine from Hanford tank waste.” Chemical Engineering Journal 490: 151449.
Bates, WF. 2022. “Follow-on Report of Analysis of Approaches to Supplemenral Treatment of Low-actiivty Waste a the Hanford Nuclear Reservation Vol I and II”. SRNL-STI-2023-00007. Savannah River National Laboratory. Aiken, SC.
Chatterjee, S, VE Holfeltz, GB Hall, IE Johnson, ED Walter, S Lee, B Reinhart, WW Lukens, NP Machara and TG Levitskaia. 2020. “Identification and quantification of technetium species in hanford waste tank AN-102.” Analytical Chemistry 92(20): 13961-13970.
Lukens, WW, DK Shuh, NC Schroeder and KR Ashley. 2004. “Identification of the Non-Pertechnetate Species in Hanford Waste Tanks, Tc(I)−Carbonyl Complexes.” Environmental Science & Technology 38(1): 229-233