Development of a Chemistry-Based, Predictive Method for Determining the Amount of Non-Pertechnetate Technetium in the Hanford Tanks: FY 2012 Progress Report

Abstract

Executive SummaryThis report describes investigations directed toward understanding the extent of the presence of highly alkaline soluble, non-pertechnetate technetium (n-Tc) in the Hanford Tank supernatants. The goals of this report are to: a) present a review of the available literature relevant to the speciation of technetium in the Hanford tank supernatants, b) attempt to establish a chemically logical correlation between available Hanford tank measurements and the presence of supernatant soluble n-Tc, c) use existing measurement data to estimate the amount of n-Tc in the Hanford tank supernatants, and d) report on any likely, processfriendly methods to eventually sequester soluble n-Tc from Hanford tank supernatants.The work described herein follows on a substantial literature developed in the 1990s and early 2000s that first identified the problem of the presence of n-Tc and led to a tentative identification of the species in at least two Hanford tank waste supernatants. This project attempted to correlate chemically relevant indicators for which quality-valid data exists in the TWINS database suitable for eventual extrapolation to as-yet untested tank supernatants. In addition, two other tangential tasks were undertaken. First, a model was developed that expanded upon a previously published model by Lukens et al. (2001), for the rate of reduction of pertechnetate by radiolysis and inorganic chemical-based processes. Using information obtained from TWINS, this model was applied to the Hanford tank supernatants to identify the rate of pertechnetate reduction in the various Hanford tank supernatants. Second, one proposed species for n-Tc, the complex [(CO) 3 Tc(H 2 O) 3 ] + , was prepared, and initial studies into its spectroscopic signature and chemical stability were performed.From this work, the only acceptable chemically based correlations with data from the TWINS database were that the fraction of n-Tc present negatively correlates with either of two, closely related variables: total dose experienced in the tank, and 137 Cs concentration in the tank supernatants. The observed inverse correlation is counterintuitive, but a possible explanation is discussed. However, the correlation should lend some insight as to which tanks might be tested to further validate and enhance the predictive value of this correlation.As noted above, chemical data, radiolysis constants and known chemical rate constants for the reaction of water radiolysis products with inorganic Hanford tank waste supernatant constituents were used to evaluate the relative rates of pertechnetate reduction in the Hanford tank supernatants. These results are summarized in the report. Interestingly, the reduction rate for pertechnetate is greatest in two tanks, AZ-101 and AZ-102, where the presence of n-Tc is not detected. However, this reduction only concerns the rate of pertechnetate reduction; the speciation of the reduced product is not addressed. Consequently, reduction of pertechnetate to the poorly alkaline-soluble technetium dioxide could ...

Protocol for Identifying the Presence of and Understanding the Nature of Soluble, Non-pertechnetate Technetium in Hanford Tank Supernatants

“…Over the past two decades, considerable work has been done examining the effectiveness of anion exchange-like resins for their ability to remove soluble Tc from Hanford tank supernatants. As discussed in Rapko et al (2013) and elsewhere, in most cases, excellent removal of Tc was observed. However, in several instances, there appeared to be a source of Tc that behaves differently than the Tc present as pertechnetate, [TcO 4 ] -.…”

“…In addition, routine collection of such data in solution is relatively simple and inexpensive. UV-vis spectra for the Tc(I) compound, [Tc(CO) 3 (H 2 O) 3 ] + , and the Tc(VII) compound, pertechnetate, TcO 4 -, have been compared (Rapko et al 2013) but show almost identical spectra in the UV-vis region and so do not provide a signature distinct enough to be used to identify the presence of the n-Tc candidate Tc(I) compounds.…”

“…Reported Distribution of Non-pertechnetate Technetium Present in Various Hanford Tank Supernatants (fromRapko et al 2013) …”

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

“…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). 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).…”

“…In fiscal year 2012 (FY 2012), a study by Rapko et al reviewed prior work on the nature and extent of this non-pertechnetate, alkaline-soluble technetium in the Hanford waste tanks (Rapko et al 2013a), and tentatively identified a Tc(I) carbonyl type compound of the form [Tc(CO) 3 ] + as a predominant component of the non-pertechnetate species present in tank waste. 1 In FY 2013, research was initiated to investigate the chemistry of the Tc(I) carbonyl compound noted above.…”