The Role of Gamma Radiation on Zircaloy-4 Corrosion

A key microstructural feature of irradiated oxide films that form on Zircaloy-4 is the development of significant amounts of interconnected porosity or microcracking, particularly along oxide grain boundaries. This development disrupts the protective nature of the barrier layer, effectively resulting in an increase in the observed post-transition corrosion rates. This observation suggests that some aspect of oxide dissolution may be occurring. It is noteworthy that when exposed to photons with energies greater than that of the oxide band gap, n-type semiconducting oxides are prone to dissolution. Because zirconia (ZrO2) is an n-type semiconductor, it is plausible that high-energy photons may play an important role in corrosion. In an operating nuclear reactor, neutron and gamma flux levels are closely interrelated because they arise, for the most part, from the same originating process (i.e., the fissioning of nuclear fuel). Thus, it is difficult to isolate the independent effects of neutron and gamma radiation on corrosion. It is, however, theoretically conceivable to assess the role that gamma radiation may have on corrosion by comparing instances where variations in radiation environment exists; specifically by comparing where variations in gamma flux levels relative to neutron flux levels within different regions of a reactor or from reactor to reactor exist. Comparisons of the corrosion rates obtained between two different reactors (the Halden test reactor and the advanced test reactor [ATR]) that have differing gamma-to-neutron ratios were performed. In addition, an examination of corrosion rate data from the ATR, in which a variation in gamma-to-neutron flux ratio exists, was also conducted. Results are presented, from both assessments, which indicate that gamma radiation plays a role in the irradiated corrosion behavior of Zircaloy-4. In addition to a discussion of experimental data, potential mechanisms associated with photon-induced oxide dissolution of n-type semiconducting oxides, such as zirconia, is also provided.