Thermophysical investigations of the uranium-zirconium alloy system

The solid phase transformation behavior of uranium-zirconium (U-Zr) alloys (U-0.1, 2, 5, 10, 20, 30, 40, and 50 wt% Zr) was observed using differential scanning calorimetry (DSC) with thermogravimetric analysis (TGA). The phase transformation temperatures and enthalpies were measured from the alloys annealed at 600 °C for 72, 168, and 672 h. The observations indicated distinctive mismatches between the measured data and the existing U-Zr alloy phase diagram. Most notably, the phase transformation of the (α-U, γ₂) phase to the (β-U, γ₂) phase at ∼662 °C was not evident in Zr-rich (> 10 wt%) U-Zr alloys, while only two phase transformations were evident in the U-10Zr and U-20Zr alloys compared to the three isotherm lines extended over the two compositions in the current phase diagram. The absence of the phase transformation is rather consistent with the older U-Zr phase diagram that was experimentally assessed in the 1950s. This observation may lead to the conclusion that the (β-U, γ₂) phase region is not correctly represented in the Zr-rich portion, or the hyper-monotectoid region, of the current U-Zr alloy phase diagram. It is evident that the phase diagram needs to be experimentally revisited to provide more reliable information for the development of metallic nuclear fuel performance models, if such models are to include phase-relevant effects, such as fuel constituent redistribution and fission gas swelling.