Rhenium and Technetium Uptake by Iron-Based Materials
EMSL Project ID
16299
Abstract
PNNL is studying uptake of contaminants in corrosion products that will occur after breach of nuclear waste packages for conditions associated with DOE’s Yucca Mountain geologic repository program. Batch reaction experiments and chemical and solid-phase analyses are currently underway to measure the uptake of Re or Tc-99 on coupons of A-516 carbon steel reacted with synthetic ground water or dilute water spiked with stable perrhenate (nonradioactive surrogate for Tc-99) or pertechnetate (Tc-99). Analyses by X-ray photoelectron spectrometry (XPS) are proposed to investigate the speciation and redox state of the Re and Fe in the oxide phases present on the corroded A-516 carbon steel coupons with synthetic ground water or dilute water spiked with stable nonradioactive perrhenate Re(VII). Experiments with Tc-99 spiked solutions are currently ongoing, but analyses of those corroded coupons will be conducted at a future date after review of the requirements of EMSL facilities for handling materials containing low concentrations of radionuclides and review and approval by appropriate EMSL facility and safety staff. Reduction of redox-sensitive radionuclides, such as Tc(VII), can occur by surface-mediated, heterogeneous reduction/sorption reactions on Fe(II) solids, such as from corrosion of steel components used in 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) [as determined by analog studies using nonradioactive Re(VII) reduced to lower oxidation states such as Re(IV)] to sparingly soluble Tc(IV) which will precipitate as either TcO2∙H2O and/or possibly a Tc(IV)/Fe(III) oxide coprecipitate. Due to the thermodynamic stability of Fe(III) oxides, Tc-99 trapped as a Tc(IV)/Fe(III) oxide may be irreversibly sorbed. The proposed XPS analyses of Re and Fe speciation supplement other analyses being completed on these same materials using bulk XRD, SEM/EDS, XANES, microXRD, and microXRF. Defining the mechanisms that control the sorption and desorption of Re (as a surrogate for Tc-99) onto or off of steel corrosion products is essential to understanding the fate of Tc-99 in the repository environment and to developing accurate models for predicting dose contributions from Tc-99 release from waste package to the environment. Tc-99 is one of the top two dose-critical isotopes listed in the Environmental Impact Statement for the proposed HLW repository at Yucca Mountain over the first 50,000 years. The nominal dose is over predicted, possibly by orders of magnitude, because reduction and sorption of Tc-99 onto corrosion products in the waste package were neglected in the performance assessment. The results of these experiments and solid-phase analyses will be used in concert with results from SNL and ANL project collaborators to construct a surface complexation-based, reactive transport predictor of radionuclide sequestration on waste package components.
Project Details
Project type
Exploratory Research
Start Date
2005-08-09
End Date
2007-03-22
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members