The Effect of γ-Ray Irradiation on the Corrosion of Zirconium Alloys
Recent results obtained at the Naval Nuclear Laboratory (NNL) show that γ irradiation may have a significant effect on zirconium corrosion kinetics. Irradiation of γ rays may generate photocurrent or alter the corrosion potential, which may increase the susceptibility of zirconium alloys to corrosion. The lack of experimental study of γ irradiation effect is partially because of presence of both γ and neutrons in the nuclear reactor environment.
In order to study the effect of γ rays separately and investigate the effect of water radiolysis on corrosion of zirconium alloys, In-pile corrosion of Zircaloy-4 coupons was conducted at MIT Nuclear Reactor Laboratory’s Research Reactor (MITR). The specimens were placed strategically at three locations: (i) at the center of the core (exposed to neutrons + γ-rays, (ii) immediately above the core (exposed to γ-rays only), and (iii) outside the core (to simulate typical out-of-pile loop corrosion test). Transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM), and nanobeam electron diffraction (NanoMEGAS ASTAR) were carried out to characterize the oxide phase and grain orientation. Figure 1 shows annular bright field image, electron diffraction pattern, and identification of the oxide phase. Increase of tetragonal ZrO2 fraction, stronger monoclinic ZrO2 texture, and more twin grain boundaries have been found in the irradiated oxides.