PNNL

Abstract

During eutectoid transformation of U-10Mo alloy, uniform metastable gamma UMo phase is expected to transform to a mixture of alpha-U and gamma prime-U2Mo phase. The presence of transformation products in final U 10Mo fuel, especially the alpha phase is considered detrimental for fuel irradiation performance, so it is critical to accurately evaluate the extent of transformation in the final U-10Mo alloy. This phase transformation can cause a volume change that induces a density change in final alloy. To understand this density and volume change, we developed a theoretical model to calculate the volume expansion and resultant density change of U-10Mo alloy as a function of the extent of eutectoid transformation. Based on the theoretically calculated density change for 0 to 100% transformation, we conclude that an experimental density measurement system will be challenging to employ to reliably detect and quantify the extent of transformation. Subsequently, to assess the ability of various methods to detect the transformation in U-10Mo, we annealed U-10Mo alloy samples at 500°C for various times to achieve in low, medium, and high extent of transformation. After the heat treatment at 500°C, the samples were metallographically polished and subjected to optical microscopy and x-ray diffraction (XRD) methods. Based on our assessment, optical microscopy and image processing can be used to determine the transformed area fraction, which can then be correlated with the alpha phase volume fraction measured by XRD analysis. XRD analysis of U 10Mo aged at 500°C detected only alpha phase and no gamma prime was detected. To further validate the XRD results, atom probe tomography (APT) was used to understand the composition of transformed regions in U-10Mo alloys aged at 500°C for 10 hours. Based on the APT results, the lamellar transformation product was found to comprise alpha phase with close to 0 at% Mo and gamma phase with 28–32 at% Mo, and the Mo concentration was highest at the alpha/gamma interface.