Projects
Technetium Uptake by Iron-Based Materials
The objective of this project is to study and characterize the surface-mediated, heterogeneous reduction/sorption reactions that affect the sequestration of technetium-99 (99Tc) by iron oxide and hydroxide solids that precipitate during corrosion of iron-based materials (such as carbon steel) that may be used in high-level radioactive waste packages.
PNNL is collaborating 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). The process being investigated is the sequestration of radionuclides onto structural materials of the nuclear waste package and 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. PNNL’s studies focus on the uptake of pertechnetate by corrosion products that will occur after breach. 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 99Tc containing precipitates on the corroded steel coupons 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 99Tc) 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. This sorption process involves electron transfer reactions, resulting in oxidation of Fe(II) to Fe(III) and the ensuing formation of Fe(III)-oxide surface coatings, 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 coprecipitate. Due to the thermodynamic stability of Fe(III) oxides, 99Tc trapped as a Tc(IV)/Fe(III) oxide may be irreversibly sorbed. Defining the mechanisms that control the sorption and desorption of 99Tc onto or off steel-corrosion products is therefore essential to understanding the fate of 99Tc after breach of nuclear waste packages in conditions associated with the Yucca Mountain geologic repository program. Results of PNNL’s experiments will be 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 to the environment.
PNNL study results will be published in a PNNL technical report, journal articles and proceedings publications. PNNL's results are expected to be used eventually by SNL's project collaborators in conjunction with their results to construct a surface complexation-based, reactive transport predictor of radionuclide sequestration on waste package components.