Thermal stability of co-extruded U–Zr/Zr–Nb alloys

A co-extruded U–80 at.%Zr/Zr–1 at.%Nb rod was heat-treated at 590 and 700 °C for up to 4000 h in order to investigate the thermal stability of a U–Zr fuel and the interface between the U–Zr fuel and the Zr–Nb clad. The U–Zr fuel was composed of the δ-UZr2 matrix and α-Zr precipitates. There was little change in the microstructure during a heat-treatment at 590 °C. However, after a heat-treatment for 4000 h, uranium-rich precipitates were formed at the interface near the grain boundary of the α-Zr phase possibly due to a diffusion from the uranium supersaturated in the α-Zr phase. A heat-treatment at 700 °C induced a considerable dissolution of α-Zr into the matrix, which continuously formed a single (γ-U, β-Zr) phase. After a heat-treatment for 2000 h, the thickness of the diffusion layer in the interface between the U–Zr fuel and Zr–Nb clad was observed to be about 150 and 250 μm at 590 and 700 °C, respectively.