Crystal structure and thermal stability of uranium nitride*

Uranium nitrides are among the most promising fuels for Generation IV nuclear reactors, but until now, very little has been known about their thermal stability properties under nonequilibrium conditions. In this work, thermal decomposition of nitrogen-rich uranium nitride (denoted as UN_2−x) under ultrahigh-vacuum (UHV) conditions was investigated by thermal desorption spectroscopy (TDS). It has been shown that the nitrogen TDS spectrum consists of two peaks at about 723 and 1038 K. The X-ray diffraction, scanning electron microscopy, and X-ray photoelectron microscopy results indicate that UN_2−x (UN₂ phase) decomposed into the α-U₂N₃ phase in the first step and the α-U₂N₃ phase decomposed into the UN phase in the second step. Further studies are needed to fully understand the thermal decomposition kinetics of the UN_2−x film, and work is in progress.