NSUF 17-1119: Characterization of Oxide Layer on the Surface of High Temperature Water Corroded Zircaloy-4 In the Presence of Neutron+Gamma and Gamma Only
Although there is a correlation between autoclave and in-reactor zirconium alloy corrosion behavior; consistent discrepancies in oxidation kinetics between these two environments have been observed. Data obtained recently at the Advanced Test Reactor (ATR), have clearly demonstrated that the measured oxidation rates were significantly higher than the predicted oxidation rates in regions where the gamma to the neutron flux ratio is the highest. In nuclear reactors, various radiation sources are present in the core from low energy UV light (via the Cherenkov effect) to hard ?-rays (from various nuclear reactions in the core). ZrO2 formation requires concurrent transport of oxygen anions from the oxide/water interface to the metal/oxide interface and of electrons in the opposite direction. It has also been shown that oxide electronic conductivity affects oxidation kinetics and hydrogen pickup, suggesting that electron transport is related to the rate limiting step of the oxidation mechanism. Considering that when radiation of a suitable energy is absorbed by the oxide film trapped electrons can be excited into the conduction band, it is thus reasonable to suppose that gamma irradiation affects the oxide electrochemical response and impact the oxidation and hydrogen pickup kinetics. In addition, radiolysis resulting from high energy photon irradiation of the coolant is known to have a significant impact on fuel cladding corrosion kinetics. Finally, possible oxide photo-dissolution processes have been hypothesized as well. Consequently, while irradiation damage due to neutrons is surely to play a role, gamma rays appear to also have a significant impact on corrosion kinetics. The goal of this experiment is to investigate the effects of neutron and gamma irradiation on oxidation behavior of Zr-based alloys separately. Zircaloy-4 samples were irradiated at various locations inside the MIT reactor. The specimens were all in an in-pile autoclave loop extending throughout and outside of the core. The samples were placed strategically in three locations: 1) at the center of core (exposed to neutrons + gamma), 2) immediately above the core (exposed to gamma only), and 3) outside the core (to simulate typical out-of-pile autoclave test). The in-core specimens were exposed to two cycles of irradiation totaling ~1021 n/m2 fast neutron fluence. The project objective is to characterize the nature of the oxide formed on the surface of these specimens after irradiation in the presence of neutron+gamma and gamma only. FIB techniques will be used to prepare thin foils of the oxide layer and the base metal from the surface of the irradiated specimens. Once the thin foils are prepared, transmission kikuchi diffraction (tKD) will be used to examine the texture of the oxide layers. The thin foils will then be further analyzed inside the FEI Talos TEM using TEM/STEM/EDS techniques to fully investigate the morphology of the oxide grains, the oxidation state and distribution of the minor alloying elements (Fe,Cr) and the thickness of the sub oxide (ZrO) layer. Thus, the effect of gamma irradiation only on oxide microstructure and microchemistry will be precisely assessed and decouple form neutron irradiation effects.
Additional Info
| Field | Value |
|---|---|
| Abstract | Although there is a correlation between autoclave and in-reactor zirconium alloy corrosion behavior; consistent discrepancies in oxidation kinetics between these two environments have been observed. Data obtained recently at the Advanced Test Reactor (ATR), have clearly demonstrated that the measured oxidation rates were significantly higher than the predicted oxidation rates in regions where the gamma to the neutron flux ratio is the highest. In nuclear reactors, various radiation sources are present in the core from low energy UV light (via the Cherenkov effect) to hard ?-rays (from various nuclear reactions in the core). ZrO2 formation requires concurrent transport of oxygen anions from the oxide/water interface to the metal/oxide interface and of electrons in the opposite direction. It has also been shown that oxide electronic conductivity affects oxidation kinetics and hydrogen pickup, suggesting that electron transport is related to the rate limiting step of the oxidation mechanism. Considering that when radiation of a suitable energy is absorbed by the oxide film trapped electrons can be excited into the conduction band, it is thus reasonable to suppose that gamma irradiation affects the oxide electrochemical response and impact the oxidation and hydrogen pickup kinetics. In addition, radiolysis resulting from high energy photon irradiation of the coolant is known to have a significant impact on fuel cladding corrosion kinetics. Finally, possible oxide photo-dissolution processes have been hypothesized as well. Consequently, while irradiation damage due to neutrons is surely to play a role, gamma rays appear to also have a significant impact on corrosion kinetics. The goal of this experiment is to investigate the effects of neutron and gamma irradiation on oxidation behavior of Zr-based alloys separately. Zircaloy-4 samples were irradiated at various locations inside the MIT reactor. The specimens were all in an in-pile autoclave loop extending throughout and outside of the core. The samples were placed strategically in three locations: 1) at the center of core (exposed to neutrons + gamma), 2) immediately above the core (exposed to gamma only), and 3) outside the core (to simulate typical out-of-pile autoclave test). The in-core specimens were exposed to two cycles of irradiation totaling ~1021 n/m2 fast neutron fluence. The project objective is to characterize the nature of the oxide formed on the surface of these specimens after irradiation in the presence of neutron+gamma and gamma only. FIB techniques will be used to prepare thin foils of the oxide layer and the base metal from the surface of the irradiated specimens. Once the thin foils are prepared, transmission kikuchi diffraction (tKD) will be used to examine the texture of the oxide layers. The thin foils will then be further analyzed inside the FEI Talos TEM using TEM/STEM/EDS techniques to fully investigate the morphology of the oxide grains, the oxidation state and distribution of the minor alloying elements (Fe,Cr) and the thickness of the sub oxide (ZrO) layer. Thus, the effect of gamma irradiation only on oxide microstructure and microchemistry will be precisely assessed and decouple form neutron irradiation effects. |
| Award Announced Date | 2017-09-20T12:30:28.24 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Kory Linton |
| Irradiation Facility | None |
| PI | Adrien Couet |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1119 |