Effect of Cr coating on the mechanical integrity of Accident Tolerant Fuel cladding under ring compression test

Abstract

The mechanical behavior of Cr-coated Zr-Nb alloy cladding was investigated with Ring Compression Test (RCT) and Finite Element Analysis (FEA). The post-RCT specimens were examined by SEM analysis with a focus on crack characterization. The experimental result shows that Cr-coating less than 20 μm does not introduce notable changes in the load-displacement behavior of the cladding structure until the point at which the Cr-coated cladding undergoes premature failure. FEA results demonstrate that the premature failure of the Cr-coated cladding cannot be explained by local stress and strain fields as they are not affected by the presence of the coating if the crack of the coating layer is not considered. Under RCT, a crack is initially formed across the coating thickness and acts as a pre-existing flaw that concentrates stresses, growing towards the underlying Zr-Nb alloy. Once the crack depth attains the critical size, it unstably grows, resulting in cladding failure which is shown as an earlier load drop compared to the uncoated cladding. FEA results using eXtended Finite Element Method (XFEM) demonstrate the stress concentration by the coating crack which results in the premature failure of the Cr-coated cladding in RCT. Increasing coating thickness decreases crack density, thereby reducing the uncovered area of the underlying cladding in case where the coating experiences extended cracking. On the other hand, the thicker coating, when cracked, can induce extended damage to the underlying cladding by introducing larger cracks. The net benefit of these two competing effects needs to be considered to optimize the thickness of Cr-coating.