The formation mechanism of the Zr rind in U-Zr fuels

Abstract

The Zr-rich rind that forms at the fuel-cladding interface in U-Zr nuclear fuels has been known for many years, although it has never been explained. It acts as an effective diffusion barrier preventing the diffusion of fission product lanthanides to the cladding, but the Zr rind breaks down and does not remain intact throughout irradiations. The current study is the investigation of the Zr rind formed in U-10Zr (wt. %). A series of experiments varied the conditions to explore the effects of heat and force on the alloy. The samples were analyzed using scanning electron microscopy and transmission electron microscopy to determine the microstructural changes and phases present. Based on the results presented, a mechanism for the formation of the Zr rind is proposed along with the Zr rind structure. The Zr rind is the metastable face-centered cubic (FCC) Zr phase formed due to δ-UZr2 decomposing and subsequent diffusion of Zr to the interface when the sample is subjected to both a compressive force and heat. If either heat or force are removed from the experiment, the FCC Zr rind does not form.