Identification and Quantification of Technetium Species in Hanford Waste Tank AN-102.

Technetium-99 (Tc), a high yield fission product generated in nuclear reactors, is one of the most difficult contaminants to address at the U.S. Department of Energy Hanford, Savannah River, and other sites. In strongly alkaline solutions typifying Hanford tank waste, Tc exists as pertechnetate (TcO4-) (oxidation state VII) as well as in reduced forms (oxidation state < VII), collectively known as non-pertechnetate (non-TcO4-) species. Designing strategies for effective Tc management, including separation and immobilization, necessitates understanding the molecular structure of the non-TcO4- species and their identification in actual tank waste samples. Identification of non-TcO4- species would facilitate the development of new treatment technologies effective for dissimilar Tc species. Toward this objective, a spectroscopic library of the Tc(I) [fac-Tc(CO)3]+ and Tc(II, IV, V, VII) compounds was generated and applied to the characterization of the actual Hanford AN-102 tank waste supernatant, which was processed to adjust Na concentration to ∼5.6 M and remove 137Cs by spherical resorcinol-formaldehyde (sRF) ion-exchange resin. Post 137Cs removal, the cesium-loaded sRF column was eluted with 0.45 M HNO3. As-received AN-102, Cs-depleted effluent, and sRF eluate fractions were comprehensively characterized for chemical composition and speciation of Tc using 99Tc nuclear magnetic resonance spectroscopy and X-ray absorption spectroscopy. It was demonstrated for the first time that non-TcO4- Tc present in the AN-102 tank waste is composed of several low-valent Tc species, including the Tc(I) [fac-Tc(CO)3]+ and Tc(IV) compounds. This is the first demonstration of multiple non-TcO4- species co-existing in the Hanford tank waste, highlighting their importance for the waste processing.

Evolution of Acid-Dependent Am3+ and Eu3+ Organic Coordination Environment: Effects on the Extraction Efficiency.

Coordination of trivalent lanthanide and actinide metal ions by lipophilic diglycolamides and phosphonic acids has been proposed for their separation through extraction from aqueous nitric acid solutions. However, the nature of M3+ coordination complexes in these combined solvent systems is not well understood, resulting in low predictability of their behavior. This work demonstrates that a combination of N,N,N',N'-tetrakis(2-ethylhexyl)diglycolamide (T2EHDGA) and weakly acidic 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) in n-dodecane exhibits a complicated extraction mechanism for Eu3+ and Am3+, which continuously evolves as a function of the aqueous phase acidity. At low aqueous phase nitric acid concentrations, M3+ ions are primarily extracted via exchange of the phosphonic acid proton and coordination with HEH[EHP]. At high aqueous phase nitric acid concentrations, HEH[EHP] remains protonated, and M3+ ions are transported to the organic phase by the coextraction of nitrate anions from the aqueous phase, thus forming complex species with T2EHDGA. At moderate acid regimes, both ligands participate in the coordination of M3+ ions and show a synergistic relationship resulting in considerable enhancement of M3+ transport into the combined solvent system over the simple sum of the individual extractants. The observed synergism is caused by differences in organic phase M3+ speciation and has a significant impact on the performance of the organic solvent. Distribution studies with Eu3+ indicate that nominally two or three T2EHDGA ligands participate in metal extraction in the presence of phosphonic acid, while nominally three diglycolamide ligands participate in the presence or absence of phosphonic acid. While synergistic behavior has been observed in many solvent-extraction processes, this system demonstrates a clear correlation between the continuously changing organic speciation of M3+ and its transport into the organic solvent. This paper reports the spectroscopic characterization of the organic phase M3+ species by IR, X-ray absorption, and visible spectroscopies. Spectroscopic evidence indicates a mixed-ligand complex, i.e., a ternary complex at the moderate acid regime, where the greatest degree of synergism is observed. Differences in synergistic extraction of Am3+ and Eu3+ at the low acid regime were observed, indicating their dissimilar extraction behavior.

Author Correction: Closing the Nuclear Fuel Cycle with a Simplified Minor Actinide Lanthanide Separation Process (ALSEP) and Additive Manufacturing.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Spontaneous redox continuum reveals sequestered technetium clusters and retarded mineral transformation of iron.

The sequestration of metal ions into the crystal structure of minerals is common in nature. To date, the incorporation of technetium(IV) into iron minerals has been studied predominantly for systems under carefully controlled anaerobic conditions. Mechanisms of the transformation of iron phases leading to incorporation of technetium(IV) under aerobic conditions remain poorly understood. Here we investigate granular metallic iron for reductive sequestration of technetium(VII) at elevated concentrations under ambient conditions. We report the retarded transformation of ferrihydrite to magnetite in the presence of technetium. We observe that quantitative reduction of pertechnetate with a fraction of technetium(IV) structurally incorporated into non-stoichiometric magnetite benefits from concomitant zero valent iron oxidative transformation. An in-depth profile of iron oxide reveals clusters of the incorporated technetium(IV), which account for 32% of the total retained technetium estimated via X-ray absorption and X-ray photoelectron spectroscopies. This corresponds to 1.86 wt.% technetium in magnetite, providing the experimental evidence to theoretical postulations on thermodynamically stable technetium(IV) being incorporated into magnetite under spontaneous aerobic redox conditions.

Closing the Nuclear Fuel Cycle with a Simplified Minor Actinide Lanthanide Separation Process (ALSEP) and Additive Manufacturing.

Expanded low-carbon baseload power production through the use of nuclear fission can be enabled by recycling long-lived actinide isotopes within the nuclear fuel cycle. This approach provides the benefits of (a) more completely utilizing the energy potential of mined uranium, (b) reducing the footprint of nuclear geological repositories, and (c) reducing the time required for the radiotoxicity of the disposed waste to decrease to the level of uranium ore from one hundred thousand years to a few hundred years. A key step in achieving this goal is the separation of long-lived isotopes of americium (Am) and curium (Cm) for recycle into fast reactors. To achieve this goal, a novel process was successfully demonstrated on a laboratory scale using a bank of 1.25-cm centrifugal contactors, fabricated by additive manufacturing, and a simulant containing the major fission product elements. Americium and Cm were separated from the lanthanides with over 99.9% completion. The sum of the impurities of the Am/Cm product stream using the simulated raffinate was found to be 3.2 × 10-3 g/L. The process performance was validated using a genuine high burnup used nuclear fuel raffinate in a batch regime. Separation factors of nearly 100 for 154Eu over 241Am were achieved. All these results indicate the process scalability to an engineering scale.