Characterization of eutectic reaction of Cr and Cr/CrN coated zircaloy accident tolerant fuel cladding

Abstract

Eutectic reactions of five kinds of Cr-coated Zr alloy cladding with different base materials (Zr–Nb–Sn alloy or Zr–Nb alloy), different coating thicknesses (6∼22.5 μm), and different coating materials (Cr single layer or Cr/CrN bilayer) were studied using Differential Scanning Calorimetry (DSC). The DSC experiments demonstrated that the onset temperatures of the Cr single layer coated specimens were almost identical to ∼1308 °C, regardless of base materials or coating thicknesses. This study demonstrated that the Cr/CrN bilayer coated Zr–Nb–Sn alloy has a slightly (∼10 °C) higher eutectic onset temperature compared to the single Cr-coated specimen. The eutectic region characterized by post-eutectic microstructure proportionally increases with coating thickness. The post-eutectic characterization with different holding times at high temperature (1310–1330 °C) reveals that progression of Zr–Cr eutectic requires time, and it dramatically changed with exposure time and temperature. The practical value of the time gain in non-instantaneous eutectic formation in terms of safety margin, however, seems to be limited.