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

Abstract

Executive SummaryThe purpose of this report is twofold: first, to propose a test protocol for identifying the presence of soluble, non-pertechnetate Tc (n-Tc) in alkaline, high ionic strength, Hanford tank supernatants, and second, to evaluate various methods of obtaining information about the nature of the identified, non-pertechnetate species.The proposed protocol addresses three questions: 1) is n-Tc present and what fraction of the total soluble Tc is present in this form, 2) is there an interference at m/e 99 in the inductively coupled plasma mass spectrometry (ICP-MS) that gives false positives in a test for n-Tc, and 3) is there another isotope that could act as an interference in a test for n-Tc by liquid scintillation counting.For ALARA (as low as reasonably achievable) reasons, the protocol involves an initial Cs decontamination by contact with the Cs selective IE-911 ion exchanger. It is recommended that the Cs-depleted supernatant be spiked with a non-Tc-99 source (likely Tc-99m) of pertechnetate to allow an independent assessment of pertechnetate and total technetium behaviors during the test. The solution is then passed through a column containing a pertechnetate getter. Because of its extensive use with Hanford tank waste supernatants, SuperLig® 639 is recommended as the pertechnetate getter. The resulting effluent is sampled for analysis by gamma energy analysis, liquid scintillation counting for beta activity, and ICP-MS for its mass 99 concentration.Should there be a significant difference in pertechnetate versus total technetium behavior, the effluent is exhaustively oxidized by a persulfate/microwave digestion, adjusted as needed in composition, passed again through an appropriate pertechnetate getter, and resampled. From analysis of these sample results, the questions posed above can be addressed.The report concludes with an evaluation of a variety of spectroscopic methods that might be used either to identify n-Tc spectroscopically in Hanford tank waste supernatant or to gain insight into the chemical structure of n-Tc. It is concluded that most methods evaluated either are unsuitable, sufficiently insensitive to preclude use at the technetium concentrations present in Hanford tank waste supernatant, or would require significant methods development before they might be effective in n-Tc species analysis. The two methods currently available that can supply useful speciation information are X-ray absorption spectroscopy (of various types) and Tc-99 nuclear magnetic resonance spectroscopy.

Extended Leach Testing of Simulated LAW Cast Stone Monoliths

“…The objective of the Tc-Gluconate Suite of monolith leach testing was to evaluate the impacts of using a non-pertechnetate form of Tc that appears to be present in Hanford double-shell tanks (DSTs) (see Rapko et al 2013aRapko et al , 2013bRapko et al , 2014 and Serne and Rapko 2014 for more details). In this activity, the LAW 7.8 M Na Average simulant was spiked with a Tc(I)-gluconate species then the spiked simulant was solidified into Cast Stone monoliths and subsequently leached using the EPA-1315 leach protocol with DIW and VZP leachants.…”

Technetium Inventory, Distribution, and Speciation in Hanford Tanks (Rev.1)

“…Final observations and recommendations for determining the amount and forms of n-Tc in Hanford tank wastes from this report and the companion document (Rapko 2014) follow. Any proposed characterization of n-Tc in Hanford tank waste supernate must consider that no means of isolating or concentrating n-Tc has been discovered (aside from evaporation-which also concentrates all other species present except water), although efforts in this area have been made (Schroeder and Ashley 2005).…”

“…Further, the HDW model that estimates the fate of 99 Tc as it is released from the dissolved fuel elements and cladding during the reprocessing of irradiated fuel makes no attempts to speciate the 99 Tc. The aforementioned companion document (Rapko 2014) and an earlier report (Rapko et al 2013a) introduce the 99 Tc speciation issue with emphasis on the unexpected finding of soluble n-Tc species in supernate within DSTs. Both Rapko reports discuss the importance of the n-Tc species after waste retrieval and most of the available 99 Tc speciation data.…”

Preliminary Tests For Development Of A Non-Pertechnetate Analysis Method

“…The primary way that non-pertechnetate was quantified in earlier experiments was by passing a large volume of liquid (~1 L) through an ion exchange column containing resin that selectively removed pertechnetate [10]. That work led to a recommended sample preparation method using an ion exchange column comparable to that used for WTP flowsheet testing with tank waste samples [11]. Although that method builds on prior methods shown successful, it requires a large amount of sample, special ion exchange media, equipment, and experience at handling the resin that would be burdensome to the 222-S personnel.…”