Post Bachelors RA
Pacific Northwest National Laboratory
Nancy is a Post Bachelors Research Associate and is currently working on her Master’s degree in Environmental Science at Washington State University Tri-Cities. She joined the lab in June 2017 as a student under the Minority Serving Institutions Partnership Program (MSIPP). Since then, Nancy’s work has included research in several programs including the Fuel Cycle Technology Waste Management Research, Deep Vadose Zone (DVZ), Immobilized Low Activity Waste (ILAW) and Nuclear Process Science Initiative (NPSI). Nancy also serves as a support analyst in the Environmental Sciences Laboratory (ESL) operating instrumentation such as the ICP-OES.
Iodate Leach Behavior from Ettringite and Implications for Iodine Retention in Cementitious Waste Forms
Authors: Nancy Avalos, Elizabeth Gillispie, Nikolla Qafoku, Sarah Saslow
Nuclear waste generated during the Cold War era from plutonium production efforts at the US Department of Energy Hanford site (Washington State, USA) is currently being stored in 177 aging underground tanks. Nuclear waste treatment and immobilization will be primarily performed at the Hanford Tank Waste Treatment and Immobilization Plant (WTP), where the legacy waste will be vitrified into glass for safe storage and disposal once WTP becomes operational. However, secondary waste streams will be produced during vitrification that will contain volatile radionuclides, such as iodine-129, and will require their own immobilization pathway. One such method may include immobilization as a cementitious waste form (CWF). Typically, iodine is primarily present as iodide (I-) in secondary waste streams and CWFs, but may persist as other iodine species as well, e.g., iodate (IO3-), that require alternative approaches to immobilization due to different binding mechanisms and coordination environments. Ettringite [Ca6Al2(SO4)3(OH)12 · 26H2O] is an important mineral phase formed in hydrated cement that can incorporate IO3- by substituting for the SO42- site in the ettringite crystal structure. Previous studies using ettringite precipitation to remove aqueous IO3- have shown ~99% removal. For this reason, ettringite is an alternative immobilization method expected to improve iodine-129 retention in CWFs where IO3- is present under oxic conditions. In this study, the IO3- release behavior from ettringite in different, environmentally relevant leach solutions was investigated to determine if ettringite substitution is a viable immobilization pathway for long-term IO3- retention and to identify the aqueous conditions that dictate the rate of iodine release. The dissolution of ettringite and the release of iodine is reported to be promoted by the influence of CO32- in groundwater. Ettringite loaded with IO3- was tested in leaching tests using solutions of varying CO32- concentrations such as double deionized water (DDI), simulated Hanford groundwater and vadose zone pore water. The experiments were run for 56 days and aliquots of the leachant were collected at different time intervals to determine the concentration of iodine in solution by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Trends in mineral composition were identified before and after leaching using X-ray diffraction (XRD) analysis. Initial results showed that less iodine was released from ettringite and more CaCO3 was formed in solutions with pH values greater than 9. Also, preliminary results from tests using DDI indicated that the percent of ettringite and amorphous material decreased and increased with leaching, respectively. These results and those of ongoing experiments will allow us to better understand the release of iodine from cementitious waste forms, identify potential parameters contributing to its release and improve current cementitious formulations for treatment of secondary waste streams.