Brendan Ensor

To better understand the effect of hydrides on accelerating Zircaloy-4 oxide growth in-reactor, two distinct studies were performed that utilized the Advanced Test Reactor. These and previous studies used three different ways of accelerating the accumulation of hydrogen: the use of samples precharged with hydrogen, adding nickel to accelerate the hydrogen pickup, and altering coupon dimensions (thickness) to reduce the volume. Results show that the previously observed out-of-reactor effect of increasing hydrogen content above solubility leading to an increased corrosion rate in Zircaloy-4 was also present in-reactor (for exposures for up to 1,746 days between 310°C and 356°C and with an average neutron flux of 0.08–1.4 × 1014 n/cm2/s). When compared to autoclave results and accounting for neutron flux effects (which for a given temperature leads to an absolute increase in the corrosion rate), the relative corrosion rate increase for the in-reactor material is similar as a function of hydrogen content. Due to limited data, future work is recommended on the determination of a synergistic effect of hydrogen and neutron flux leading to higher corrosion-rate increases. In-reactor hydrogen pickup as a function of nickel content was lower compared with autoclave-exposed materials; however, the trend was consistent with that observed in the autoclave-exposed samples. Comparisons of different hydrogen-charging methods suggests that precharged samples may have lower corrosion rates as a function of hydrogen content, although more study is needed. The results of these experiments were used to help hypothesize possible mechanisms for hydride accelerated corrosion.

Eleven Zircaloy-4 samples were irradiated in the Advanced Test Reactor at a variety of temperatures and neutron flux levels for up to 6.5 years. Subsequently, the coupons were characterized with complementary techniques to understand the mechanisms behind oxide growth as a function of different corrosion environments. Samples were examined using synchrotron X-ray diffraction/fluorescence, traditional X-ray diffraction, focused ion beam/scanning electron microscopy serial sectioning, and three-dimensional reconstruction to develop an improved understanding of the influence of the underlying oxide microstructure on oxide growth. The oxide microstructure formed under irradiation was compared to that in samples corroded in an autoclave to discern the impact of neutron irradiation and temperature on corrosion rate, oxide kinetic transition, irradiation-induced breakaway corrosion, stress development, phase formation, and oxide grain size. The microstructure of the oxide changed with the corrosion temperature, with larger crack spacing (characteristic of kinetic transition) and larger monoclinic oxide grains formed during higher temperature corrosion. The specimens that were exposed to a neutron flux exhibited larger oxide grains and an increase in the fraction of tetragonal phase at the metal-oxide interface (but less tetragonal phase in the bulk oxide) compared to those exposed in autoclave. Data obtained from electron microscopy demonstrated the effect of irradiation and corrosion temperature on oxide morphology. One specimen underwent an irradiated-induced breakaway oxidation that was characterized by a sharp change in the corrosion rate and a decrease in the spacing between adjacent crack layers in the oxide film. Stress is hypothesized to be a key driver in the oxide growth formation, with samples nearer transition having more plastic deformation in the metal and increased elastic strain. These observations lead to a theory of oxide growth on zirconium alloys that attempts to connect and integrate the effects of stress, irradiation, temperature, phase formation, crystal orientation, porosity, and precipitate amorphization.