Assessment of the impact of Zr-Cr eutectic reaction on the structural integrity of Cr-coated ATF cladding

Abstract

The potential impact of the Zr-Cr eutectic reaction on the structural integrity of Cr-coated ATF cladding under prolonged high-temperature steam exposure was comprehensively and systematically assessed. Cr-coated Zr-Nb alloys with various coating thicknesses (5, 8, 10, 12, 16, 18 µm) were subjected to steam oxidation at temperatures above the Zr-Cr eutectic onset for up to 2 h. No evidence of melting flow or structural collapse attributable to the eutectic mixture (β-Zr+ZrCr₂) was observed, even after extended exposure under steam-oxidizing conditions. As the oxygen concentration in the Zr matrix increased, the corresponding rise in eutectic onset temperature led to solidification of the eutectic mixture, which remained as ZrCr₂ within the matrix. Furthermore, even in an oxygen-free inert environment, the amount of Cr—within the tested coating thickness range—was insufficient to drive eutectic mixture through the entire cladding thickness (570 µm). To assess safety margins related to potential structural failure, a straightforward yet accurate model was developed to predict the Cr-limited eutectic thickness as a function of initial coating thickness, and was experimentally validated. According to the model, a Cr coating thickness of ∼72 µm would be required to form the eutectic mixture across the entire cladding wall. Even when accounting for the partially thickened eutectic reaction region (‘Thick zone’), the coating would need to exceed ∼49 µm reach full thickness. These findings suggest that, at coating thicknesses currently considered for commercial application (∼15–20 µm), safety concerns at the eutectic onset temperature should focus on oxidation-induced embrittlement rather than eutectic melting-induced collapse.