A SIMPLE KINETIC MODEL OF ZIRCALOY Zr(Fe,Cr)2 PRECIPITATE AMORPHIZATION DURING NEUTRON IRRADIATION

At neutron flux levels typical for Zircaloy fuel cladding in commercial power reactors, insufficient thermal energy below about 600°K fails to maintain long-range order in hexagonal close packed Zr(Fe,Cr)2 precipitates, leading to their gradual amorphization. This transformation, characterized as homogeneous, yields no compositional change at low temperatures. However, above 500°K, an inward-growing amorphous zone with 10 at% Fe emerges from the periphery, as Fe diffuses outward to the alloy matrix. The behavior of Zr(Fe,Cr)2 precipitates differs between Zircaloy-4, where central cores remain crystalline, and Zircaloy-2, where partial transformation occurs and the amorphous front progresses into a mix of amorphous and crystalline regions retaining the original composition. Above 600°K, both structure and composition of Zr(Fe,Cr)2 are preserved. The results indicate a dynamic interplay between the kinetic excitation toward amorphous states and thermal recrystallization, facilitating flux-assisted diffusion of Fe. With a uniform set of kinetic constants, a simple analytic model successfully forecasts precipitate amorphization based on neutron flux, temperature, and time for both Zircaloy variants, suggesting stability of hep Zr(Fe,Cr)2 with approximately 33 at% Fe for Zircaloy-2, while amorphous forms exhibit lower activation energy for recrystallization with higher Fe content in Zircaloy-4.