Chapter 12
Radioanalytical Methods in the Discovery and Characterization of Non-Pertechnetate ( Tc) Species in Hanford Tank Wastes 99
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Rebecca M. Chamberlin *, Kenneth R. Ashley , Jason R. Ball , Eve Bauer , JonathanG.Bernard , DouglasE.Berning , NormanC.Schroeder, and Paul Sylvester 1
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Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545 Department of Chemistry, Texas A&M University at Commerce, Commerce, TX 75429 Lynntech, Inc., 7610 Eastmark Drive, College Station, TX 77840 Current address: Triton Systems Inc., 200 Turnpike Road, Chelmsford, MA 01824 *Corresponding author:
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The existence of reduced technetium complexes in Hanford nuclear tank wastes was first proven using radioanalytical methods in anion exchange experiments. Subsequent efforts to understand the generation and stability of non-pertechnetate species in alkaline solution used radiochemistry in conjunction with spectroscopic methods. Collectively, the results indicate that aminocarboxylates such as E D T A do not form stable Tc complexes in strong base, while sugar derivatives such as gluconate support Tc(VII) reduction under a wide range of conditions, including realistic tank waste simulants. 99
© 2004 American Chemical Society
In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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178 As the U.S. Department of Energy prepares to dispose of defense nuclear wastes that have been generated and stored at the Hanford Site since the Manhattan project, increasing attention has centered on the highly complex alkaline wastes stored in underground tanks. Hanford's tank wastes consist of neutralized effluents from several chemical processes designed to recover plutonium from irradiated fuel rods, along with a host of minor effluents from processes such as equipment flushing, fission product recovery, and sugar denization (/). While the reactivity of the initially acidic solutions was wellunderstood within the context of plutonium recovery processes, relatively little consideration was given until recently to the chemical dynamics of the wastes after neutralization to p H 11-14 for long-term storage in the tanks. O f particular concern in this context is the chemical evolution of the longlived fission product technetium-99. Removal of a significant fraction of the T c from the waste is currently required to meet N R C Class " A " limits, so the waste can be managed by vitrification and on-site storage. To support this plan, and earlier disposal concepts, significant effort has been directed at developing and characterizing methods to separate the pertechnetate ( T c 0 ) anion from highnitrate alkaline solutions typical of Hanford wastes. This focus on pertechnetate separation was based on the knowledge that technetium emerged from fuel rod dissolution and plutonium recovery operations as the fully oxidized anion. In recent years, however, it has been established (2-4) and generally accepted (J-7) that the T c in certain classes of Hanford waste has, over decades of storage, been converted to lower oxidation state complexes. These complexes resist conventional separations, and are difficult to selectively oxidize back to the more tractable T c C V anion. These reduced complexes are characteristic of the Envelope C tanks, which contain large quantities of organic compounds that can serve as both reducing and complexing agents for " T c . This discovery emerged from an experimental program that relied on radioanalytical chemistry as its primary methodology. Later studies using modern spectroscopic techniques have provided additional corroboration of these "non-pertechnetate" species. This paper describes the role of radiochemistry in the discovery of nonpertechnetate species in Hanford wastes, and presents an overview of the radioanalytical methods that have subsequently been used—alone or in concert with other techniques—to study their potential origin and identity. 99
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Discovery of the Non-Pertechnetate Problem Samples of supernate from Hanford tanks 101-SY and 103-SY were provided to Los Alamos National Laboratory in 1995 for validation of the performance of commercial anion exchange resin Reillex-HPQ in removing T c 0 ~ from alkaline wastes. Later, a sample of 107-AN supernate was also provided to L A N L by Pacific Northwest National Laboratory. Prior studies had established the sorption behavior of pertechnetate in the presence of strong 99
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In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
179 nitrate and hydroxide solutions, and characterized ideal column load and stripping conditions (8). The key challenge in the radiochemical analysis of " T c in tank waste is the abundance of the -30-year nuclides C s and ^Sr, whose beta activity is several orders of magnitude greater than that of T c . Careful radiochemical purification allows the " T c to be separated from these interfering nuclides, but chemical losses during the purification must be accounted for by concurrently processing a thoroughly equilibrated tracer nuclide such as T c . The radiochemical method developed at L A N L (3) for total " T c analysis in waste samples, as well as in post-contact solutions for batch distribution coefficient (Kd) determinations, is summarized in Figure 1. In this analysis, a measured volume of waste solution is spiked with the gamma-emitting tracer T c , for subsequent chemical yield determination. To oxidize the technetium and organics, and fully equilibrate the Tc/"Tc isotopes, Ce(IV) ion and concentrated nitric acid are added and the solution is evaporated to incipient dryness. Two additional cycles of nitric acid addition and evaporation follow, then the sample is dissolved in water and passed through a column of cation exchange resin to remove cationic radionuclides such as C s . The Tc0 "-containing effluent is neutralized, then contacted for two hours with Reillex-HPQ to selectively sorb the technetium as Tc0 ~. After thorough column washes, the T c and T c are co-eluted by reductive complexation with alkaline Sn(II)/ethylenediamine solution. After measuring the final sample activity using both N a l gamma counting and liquid scintillation counting, the chemical yield (typically 65%) is calculated based on recovery of T c . The L S C results then provide an accurate measure of the total " T c present in the initial sample. Determination of the total " T c concentrations in the 101-SY and 103-SY samples by this method gave values of 5.9 x 10" and 7.2 x 10" mol/L, respectively (Table I). Significantly lower values of 1.7 x 10' and 4.3 x 10' mol/L, also determined radiochemically but using a milder equilibration with hydrogen peroxide in 4 mol/L H N 0 , had been reported by P N N L . Because of this disparity in the radiochemistry results, the " T c concentration was determined by two other methods. First, total T c was measured by ICP-MS using a standard addition technique, and assuming that all signal at mass 99 could be attributed to technetium. The species-independent ICP-MS determination gave values that agreed within uncertainty with the Ce(IV) radiochemistry results. Finally, the T c 0 " concentration in the unoxidized waste samples and Ce(IV)-oxidized samples was measured by " T c N M R using a signal-to-noise ratio method (4). The vigorously oxidized N M R sample indicated a pertechnetate concentration consistent with the ICP-MS and Ce(IV)/HN0 radiochemistry results. Collectively, these results indicate that pertechnetate-selective methods such as anion exchange and T c N M R greatly underestimate the technetium concentration in the wastes, unless extreme steps are taken to oxidize the samples. Similar results were obtained for the 107-AN samples. While strongly suggestive of the presence of reduced " T c species, further evidence was l37
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In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
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Figure 1. Radiochemical methodfor total Tc analysis in tank wastes.
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Table 1. Tc Concentrations i n 101-SY and 103-SY Samples, by Method. Analytical Method
103-SY
101-SY
Oxidation Method
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(Iff
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mol/L)
Radiochemistry
Ce(IV)/concHN03
5.92 ± 0 . 8 8
N M R , oxidized
Ce(IV)/concHN03
4.19 ± 1.22
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5.18 ± 0 . 3 0
7.76 ± 0.44
ICP-MS Radiochemistry N M R , unoxidized
H 0 /4mol/LHN0 2
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1.72
4.25
1.57 ± 0 . 4 6
2.30 ± 0 . 5 8
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provided by measuring the partitioning behavior of T c from the waste samples onto Reillex-HPQ resin. Batch distribution coefficients (Kd) were measured under experimental conditions developed in prior simulant studies (5). The "apparent" T c K4 for the waste samples is defined as the ratio of the post-contact T c activity sorbed onto the anion exchange resin (per g of resin), to the T c activity remaining in solution (per m L of solution), without regard to chemical speciation. In practice, the K
