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Zr(Fe,Cr)2 Precipitate Amorphization and Iron Redistribution in Zircaloy-4 Using a Two-Step Proton Irradiation

The corrosion of zirconium-based alloys is a service life-limiting factor in fuel rod performance. Mechanistic understanding of the corrosion process under reactor irradiation conditions still alludes to the nuclear industry. Pre-transition corrosion behavior of Zircaloy-4 has been reported to show a minimal effect from the irradiation environment, and the in-reactor corrosion kinetics is athermal and similar to the ex-situ autoclave corrosion exposure. However, the post-transition in-reactor corrosion kinetics depends on temperature and neutron flux. As discussed by Kammenzind et al. in Ref., the long-term post-transition corrosion rates of Zircaloy-4 are significantly accelerated in a PWR radiation environment over that observed with non-irradiated specimens in an autoclave environment.

Product Number: ED22-17211-SG
Author: Peng Wang, Gary S. Was
Publication Date: 2022
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Neutron irradiation alters the microstructure and microchemistry of Zircaloy-4 through amorphization and dissolution of second phase particles (SPPs) and redistribution of alloying elements from the SPP into the matrix. The changes in alloy microstructure and microchemistry caused by neutron irradiation impact the in-core corrosion behavior of the alloy. Hence, emulating neutron damage of the matrix and SPPs with proton irradiation was explored. However, previous work indicated that isothermal proton irradiation at reactor operating temperatures (250-350ºC) only produces slower amorphization and less iron redistribution out of the laves SPPs and into the Zircaloy-4 matrix compared to neutron irradiation. There is a desire to simulate this effect of neutron irradiation on SPPs with protons. A fully recrystallized Zircaloy-4 was irradiated with 2 MeV protons at a lower temperature, -10ºC (liquid nitrogen-cooled irradiation), resulting in complete amorphization of the Laves phase precipitates (approximately 200 nm in diameter) at a relatively low damaging level of 2.5 dpa with a damage rate of 1.65x10-5 dpa/s. The iron and chromium levels of the amorphized SPP remained consistent compared to the non-irradiated precipitates, which suggests amorphization of the precipitate is not necessarily driven by the depletion of alloying elements. Subsequent high-temperature irradiation, at 350ºC to an added 2.5 dpa of damage, leads to iron and chromium redistribution and depletion from the amorphous region into the matrix. This observation suggests that the amorphous structure of the precipitates appears to drive the alloying elements out of the structure when both active point defect creation and thermal diffusion are present. Hence, the rate of laves phase amorphization and the rate at which iron and chromium are rejected out of the laves phase into the surrounding zirconium matrix can be significantly enhanced under proton irradiation through a two-step irradiation sequence, with the first step occurring under liquid nitrogen cooling and the second step at 350ºC.

Neutron irradiation alters the microstructure and microchemistry of Zircaloy-4 through amorphization and dissolution of second phase particles (SPPs) and redistribution of alloying elements from the SPP into the matrix. The changes in alloy microstructure and microchemistry caused by neutron irradiation impact the in-core corrosion behavior of the alloy. Hence, emulating neutron damage of the matrix and SPPs with proton irradiation was explored. However, previous work indicated that isothermal proton irradiation at reactor operating temperatures (250-350ºC) only produces slower amorphization and less iron redistribution out of the laves SPPs and into the Zircaloy-4 matrix compared to neutron irradiation. There is a desire to simulate this effect of neutron irradiation on SPPs with protons. A fully recrystallized Zircaloy-4 was irradiated with 2 MeV protons at a lower temperature, -10ºC (liquid nitrogen-cooled irradiation), resulting in complete amorphization of the Laves phase precipitates (approximately 200 nm in diameter) at a relatively low damaging level of 2.5 dpa with a damage rate of 1.65x10-5 dpa/s. The iron and chromium levels of the amorphized SPP remained consistent compared to the non-irradiated precipitates, which suggests amorphization of the precipitate is not necessarily driven by the depletion of alloying elements. Subsequent high-temperature irradiation, at 350ºC to an added 2.5 dpa of damage, leads to iron and chromium redistribution and depletion from the amorphous region into the matrix. This observation suggests that the amorphous structure of the precipitates appears to drive the alloying elements out of the structure when both active point defect creation and thermal diffusion are present. Hence, the rate of laves phase amorphization and the rate at which iron and chromium are rejected out of the laves phase into the surrounding zirconium matrix can be significantly enhanced under proton irradiation through a two-step irradiation sequence, with the first step occurring under liquid nitrogen cooling and the second step at 350ºC.