PNNL

Abstract

The aim of work this year has been to investigate the role of alpha (a)-radiation and hydrogen (H2) on the corrosion of uranium oxide (UO2) using a microfluidic device. The microfluidic device, termed the Particle-Attached Microfluidic Electrochemical Cell, (PAMEC), enables monitoring of the UO2 electrochemical corrosion potential (Ecorr) under different environments, including de-aerated conditions and in the presence of dissolved H2. The Si3N4 window allows us to study morphological and chemical changes under an electron microscope. We have previously demonstrated that the PAMEC matches the results from bulk electrochemical tests with UO2 [1]. We used the high specific activity uranium (U) isotope, 233U, (t1/2 = 160,000 years) incorporated into UO2, to generate a localized a-field. The objective of the experiments were to mimic the radiation environment that would be experience at the surface of aged spent nuclear fuel (SNF) during long-term geologic disposal under anoxic conditions. Wittman et al. [2] predicted that in the presence of a pure a-radiation field and under H2 conditions, the concentration of the radiolytic oxidant H2O2 would be suppressed or even eliminated. In a UO2 corrosion experiment this would be exhibited through a lowering of the measured UO2 corrosion potential compared to identical conditions in the absence of dissolved H2. We found that the predictions of Wittman and co-workers were supported and that the corrosion potential of the 233U-doped UO2 in solution lowered with presence of H2 gas and increased in the absence of H2, when under anoxic conditions. The PAMEC experiments indicate that H2O2 has been eliminated in a solution sparged with H2 while exposed to an a-radiation field. The corrosion potential of the a-doped 233U(10%)- 238UO2 in a solution sparged with Ar/H2 matched the corrosion potential of 238UO2 in solution sparged with air. This clearly demonstrated that the H2O2 had been eliminated and that the only oxidant present was O2 in this system in complete agreement with the modeling results of Wittman et al. These results point to the need to improve the Fuel Matrix Degradation (FMD) Process Model training data set that is being used in the FMD surrogate model that is being developed for the repository program. The incorporation of realistic radiation chemistry will improve the scientific basis for the FMD Model.