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Environmental Sustainability Division

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Technetium Uptake by Iron-Based Materials

Iron Oxide Corrosion
EDS element distribution

The objective of this project is to study and characterize the surface-mediated, heterogeneous reduction/sorption reactions that affect the sequestration of technetium-99 (99TTc) by iron oxide and iron hydroxide solids that precipitate during corrosion of iron-based materials (such as carbon steel) that may be used in high-level radioactive waste packages. In the early years, PNNL collaborated on this project with researchers from the In-Package Sequestration of Radionuclides at Yucca Mountain Project, led by Dr. Pat Brady from Sandia National Laboratories (SNL). In later years the work was funded directly by DOE-EM headquarters Washington, DC. The process being investigated is the sequestration of radionuclides onto structural materials of the nuclear waste package, the corrosion products that will occur after breach of waste packages for conditions associated with the U.S. Department of Energy’s Yucca Mountain geologic repository program and more recently using ferrous iron hydrous oxides as a sequestration agent for Tc in Hanford Site low activity and off-gas scrubbed secondary liquid wastes. PNNL’s studies focus on the uptake of pertechnetate by metal corrosion products that will occur after waste packages are breached and ferrous hydrous oxides added to liquid wastes followed by pH neutralization and exposure to air (oxidation). Batch reaction experiments and chemical and solid-phase analyses have been completed to measure the uptake of pertechnetate and perrhenate (as a nonradioactive surrogate for pertechnetate) by iron-oxide corrosion products that precipitated when carbon steel coupons were reacted with synthetic groundwater or dilute water spiked with stable perrhenate or radioactive pertechnetate. Future studies will include similar experiments using flow-through column systems. Fluid samples from the sorption experiments are collected periodically and analyzed for pH and composition. The compositions of rhenium- and 99TTc containing precipitates on the corroded steel coupons and within the final hydrous ferric oxides formed after contact with Hanford liquid wastes are being studied using various techniques such as X-ray diffraction; scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS); synchrotron-based X-ray techniques, including X-ray microfluorescence, X-ray absorption spectroscopy, and X-ray microdiffraction and other methods.

Reduction of redox-sensitive radionuclides (such as 99TTc) can occur by surface-mediated, heterogeneous reduction and sorption reactions on Fe(II) solids such as from corrosion of steel components used in high-level radioactive waste packages or ferrous hydrous oxides contacted with Hanford Site liquid wastes. This sequestration/sorption process involves electron transfer reactions, resulting in oxidation of Fe(II) to Fe(III) and the ensuing formation of Fe(III)-oxides and reduction of mobile Tc(VII) to sparingly soluble Tc(IV) that will precipitate as either hydrous Tc(IV) dioxide and/or possibly a Tc(IV)/Fe(III) oxide co-precipitate. Due to the thermodynamic stability of Fe(III) oxides, 99TTc trapped as a Tc(IV)/Fe(III) oxide may be irreversibly sorbed. Defining the mechanisms that control the sorption and desorption of 99TTc onto or off steel-corrosion products or Fe(II)-treated iron oxides purposefully added to Hanford liquid wastes is therefore essential to understanding the fate of 99TTc after breach of nuclear waste packages in conditions associated with the Yucca Mountain geologic repository program and solidified Hanford low activity waste forms. Results of PNNL’s experiments are being used in concert with results from SNL and Argonne National Laboratory project collaborators to construct a surface complexation-based, reactive transport predictor of radionuclide sequestration on waste package components. This information is crucial in developing accurate models for predicting dose contributions from 99Tc releases from waste packages and low activity solidified Hanford wastes to the environment.

Publications

PNNL study results continue to be published in a PNNL technical reports, journal articles and proceedings publications.

  • Um W, G Wang, HB Jung, and RA Peterson. 2013. Technetium Removal Using Tc-Goethite Coprecipitation. PNNL-22967; EMSP-RPT-017, Pacific Northwest National Laboratory, Richland, WA.
  • Um W, HS Chang, JP Icenhower, WW Lukens, RJ Serne, N Qafoku, RK Kukkadapu, and JH Westsik, Jr. 2012. "Iron Oxide Waste Form for Stabilizing 99Tc." Journal of Nuclear Materials 429(1-3):201-209. doi:10.1016/j.jnucmat.2012.06.004
  • Heald SM, KM Krupka, and CF Brown. 2012. "Incorporation of Pertechnetate and Perrhenate into Corroded Steel Surfaces Studied by X-ray Absorption Fine Structure Spectroscopy." Radiochimica Acta 100(4):243-253. doi:10.1524/ract.2012.1912
  • Um W, HS Chang, JP Icenhower, WW Lukens, RJ Serne, N Qafoku, JH Westsik, Jr, EC Buck, and SC Steven. 2011. "Immobilization of 99-Technetium (VII) by Fe(II)-Goethite and Limited Reoxidation." Environmental Science & Technology 45(11):4904-4913. doi:10.1021/es104343p
  • Um W, MM Valenta, CW Chung, J Yang, MH Engelhard, RJ Serne, KE Parker, G Wang, KJ Cantrell, and JH Westsik, Jr. 2011. Radionuclide Retention Mechanisms in Secondary Waste-Form Testing: Phase II . PNNL-20753, Pacific Northwest National Laboratory, Richland, WA.
  • Skomurski FN, KM Rosso, KM Krupka, and BP McGrail. 2010. "Technetium incorporation into hematite (α-Fe2O3)." Environmental Science & Technology 44(15):5855-5861.
  • Um W, H Chang, JP Icenhower, N Qafoku, SC Smith, RJ Serne, EC Buck, RK Kukkadapu, ME Bowden, JH Westsik, Jr, and WW Lukens. 2010. Immobilization and Limited Reoxidation of Technetium-99 by Fe(II)-Goethite . PNNL-19833, Pacific Northwest National Laboratory, Richland, WA.
  • Krupka KM, CF Brown, HT Schaef, SM Heald, MM Valenta, and BW Arey. 2006. "Rhenium Uptake, as Analogue for Tc-99, by Steel Corrosion Products." In Proceedings of the 11th International High-Level Radioactive Waste Management Conference (IHLRWM), April 30 - May 4, 2006, Las Vegas, Nevada , ed. B. Sagar (Technical Program Chair), pp. 905-912. American Nuclear Society, La Grange Park, IL.

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