Transmission electron microscopy investigation of phase transformation and fuel constituent redistribution in neutron irradiated U-10wt.%Zr fuel

Abstract

Uranium-10 wt.% zirconium (U-10 wt.%Zr) is a primary candidate for fast reactor nuclear fuels. However, there is a lack of data characterizing the crystallographic phases and chemistry of the neutron irradiated fuel. In the current study, the microstructural evolution of a U-10 wt.%Zr fuel neutron irradiated to a burnup of 5.7 at.% was investigated using scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction to determine the major phases, alterations in microstructure, and variations in local chemical composition at different localities of a fuel cross-section. The current study revealed that the irradiated U-10 wt.%Zr fuel was comprised of various major phases, including α-U, β-U, and δ-UZr2, as well as amorphous and crystalline solid fission product (FP) precipitates within different regions of the fuel cross-section. Regions A and A/B in the center of the fuel were comprised of U-rich, U-intermediate, and U-lean localities with α-U and δ-UZr2 composing the major phases. Regions B and C in the intermediate and peripheral fuel localities, respectively, were comprised of α-U grains, U-rich (β-U) grains with Zr-rich precipitates, U-intermediate grains (δ-UZr2), and solid FP precipitates. The major phases identified were associated with the nanoscopic chemical concentrations, the phase diagram, and the as-characterized specimen temperature, with the exception of the non-equilibrium β-U phase identified in Region B. The U-lean localities in Regions A, A/B, B, and C were Zr-enriched pathways along subgrain/grain boundaries, suggesting that Zr is susceptible to radiation-induced segregation in U-Zr fuels and indicating a new mechanism for constituent redistribution.