Characterization of Non-pertechnetate Species Relevant to the Hanford Tank Waste

Technical Report · Mon Mar 27 04:00:00 EDT 2017

DOI:https://doi.org/10.2172/1400346· OSTI ID:1400346

Chatterjee, Sayandev ^[1]; Andersen, Amity ^[1]; Du, Yingge ^[1]; Engelhard, Mark H. ^[1]; Hall, Gabriel B. ^[1]; Levitskaia, Tatiana G. ^[1]; Lukens, Wayne W. ^[1]; Shutthanandan, Vaithiyalingam ^[1]; Walter, Eric D. ^[1]; Washton, Nancy M. ^[1]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Among radioactive constituents present in the tank waste stored at the U.S. DOE Hanford Site, technetium-99 (Tc), which is generated from the fission of ²³⁵U and ²³⁹Pu in high yields, presents a unique challenge in that it has a long half-life ( = 292 keV; T1/2 = 2.11105 y) and exists predominately in soluble forms in the liquid supernatant and salt cake fractions of the waste. In the strongly alkaline environments prevalent in most of the tank waste, its dominant chemical form is pertechnetate (TcO₄^-, oxidation state +7). However, attempts to remove Tc from the Hanford tank waste using ion-exchange processes specific to TcO₄^- only met with limited success, particularly when processing tank waste samples containing elevated concentrations of organic complexants. This suggests that a significant fraction of the soluble Tc can be present as low-valent Tc (oxidation state < +7) (non-pertechnetate). The chemical identities of these non-pertechnetate species are poorly understood. Previous analysis of the SY-101 and SY-103 tank waste samples provided strong evidence that non-pertechnetate can be comprised of [fac-Tc(CO)₃]⁺ complexes containing Tc in oxidation state +1 (Lukens et al. 2004). During the last three years, our team has expanded this work and demonstrated that high-ionic-strength solutions typifying tank waste supernatants promote oxidative stability of the [fac-Tc(CO)₃]⁺ species (Rapko et al. 2013a; 2013b; Levitskaia et al. 2014; Chatterjee et al. 2015). Obtained results also suggest possible stabilization of Tc(VI) and potentially Tc(IV) oxidation states in the high-ionic-strength alkaline matrices particularly in the presence of organic chelators, so that Tc(IV, VI) can serve as important redox intermediates facilitating the reduction of Tc(VII) to Tc(I). Designing strategies for effective Tc management, including separation and immobilization, necessitates understanding the molecular structure of the non-pertechnetate species and their identification in the actual tank waste samples, which would facilitate development of new treatment technologies effective for dissimilar Tc species. The key FY 2016 results are summarized below.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1400346
• Report Number(s): PNNL--26265; EY4049110
• Country of Publication: United States
• Language: English

Related Subjects

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES