In Situ EIS Measurements of Irradiated Zircaloy-4 Post-Transition Corrosion Kinetic Behavior

Under irradiation the post-transition corrosion rates of Zircaloy are enhanced relative to out-of-flux rates. In an effort to better understand the effects of different mixed-field radiation environments and the variables that may contribute to this enhancement a series of in situ electrochemical impedance spectroscopy (EIS) tests were conducted in the Halden Test Reactor under PWR type conditions. The EIS tests were conducted using five identically fabricated electrode cells, which employed Zircaloy-4 tipped working electrodes. Approximately 30 μn thick oxides were prefilmed on the electrode tips prior to test insertion. The electrode test train was designed in a manner that enabled a wide range of gamma and neutron radiation flux conditions to be evaluated. Gamma fluxes (E ≥ MeV) ranged from 1.7×1013 to 1.3×1014 cm−2s−1 while the fast neutron fluxes (E ≥1 MeV) ranged from 2.9×10010 to 3.2×1013 cm−2s−1. In addition, the EIS tests were conducted over temperatures ranging from 250 to 335°C. Electrode sets were situated into two basic radiation environments, which were differentiated by the magnitude and relative proportion of the mixed radiation field. In one environment, where the fast neutron flux was low, the relative gamma radiation component was high (i.e., high gamma-to-neutron ratio environment). In the other environment, where the fast neutron flux was high, the relative gamma radiation component was low (i.e., low gamma-to-neutron ratio environment). Based on the low frequency limits, the impedance moduli and inferred corrosion rates were found to be a complex function of temperature, and the relative proportion of gamma and neutron radiation. At low temperatures of 250°C it was found that the electrode sets which were exposed under high gamma-to-neutron ratios exhibited low frequency impedances that were almost 2.5 times lower than the case for the electrode sets which were placed in low gamma-to-neutron ratio. This occurred even though the absolute magnitude of the gamma and neutron flux fields decreased. This would imply that the corrosion rate is higher under high gamma-to-neutron ratios than under low gamma-to- neutron ratios. Conversely, at high temperatures, 335°C, only very small differences in impedance were noted as a function of radiation environment. Post-test destructive examination of the Zircaloy tipped electrodes indicates that the growth of the oxide formed during the in-pile phase of testing is consistent with the in situ EIS measurements obtained in-pile. The porous nature of the post-transition oxide coupled with the apparent correlation with gamma radiation suggests that heterogeneous radiolysis within the corrosion product film plays an important role in enhancing post-transition corrosion kinetics.