Elucidating changes in thermal creep strain rate of Cr-coated Zr-Nb alloy Accident Tolerant Fuel (ATF) cladding via experiment and mechanical analysis

Abstract

Remarkable decrease in creep strain rate (86% decrease) was observed for Zr-1.1Nb alloy cladding coated with a thin Cr layer (16.8μm) compared to uncoated cladding in the present experiment at 380°C. The structural analysis with the in-house code and FEM analysis showed good agreement with the experimental results, demonstrating that the creep strain rate reduction was primarily occurred by mechanical cause. Cr coating layer induces extra compressive stress on the underlying cladding due to the strain compatibility, reducing the tensile stress level on the underlying cladding and thereby resulting in a decreased creep strain rate. This suggests the coating layer has non-negligible impact on the overall mechanical behavior of the underlying cladding despite of its significantly thin thickness. In addition, the XRD analysis found that the Cr coating layer has high compressive residual stress that also impedes the creep deformation of the underlying cladding. However, if the residual stress within the coating layer is limited or absent, the creep deformation will not be dramatically reduced. Inclusion of appreciable creep or plastic deformation of Cr coating led to the use of unrealistically high elastic modulus to match the experimental results. This result implies that creep or plastic deformation in the Cr coating layer is likely to be absent or negligible in the tested conditions. The magnitude of irradiation creep is markedly greater than that of thermal creep, and is less sensitive to the changes in stress which is induced by coating. Therefore, the extent of changes in creep strain rate in the reactor environment is anticipated to be less than that of an out-of-pile thermal creep experiments. It can be estimated that the creep strain rate will be decreased by 10–20% with Cr-coated cladding (Cr-thickness of 10μm - 20μm) at BOL.