Formation of ammonium uranate on uranium dioxide during aging under controlled storage conditions

Abstract

• UO 2 aging was characterized via microscopic and spectroscopic methods. • Saturated (NH 4 ) 2 SO 4 solutions were used to maintain constant relative humidity. • Storage conditions led to alteration regardless of temperature. • SEM and FTIR provide evidence of ammonium uranate formation on the surface of UO 2 . The analysis and characterization of uranium dioxide (UO 2 ) under controlled storage conditions can provide insight into potential alteration products and overall stability, which has relevance for nuclear fuel cycle processes, environmental management, and the stability of used nuclear fuel in a repository. In this work, UO 2 was monitored as a function of time (6–24 months) and temperature at constant relative humidity (81% RH) in the presence of ammonium sulfate. The morphology of the aged material was described using scanning electron microscopy and a previously published lexicon, while phase changes were confirmed using X-ray diffraction. Additional characterization was carried out using energy dispersive X-ray spectroscopy, transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy to further identify aging effects. Over time, UO 2 transitioned to a mixed phase assemblage. Among the alteration was ammonium uranate, a common fuel cycle compound, which was identified from its morphological and spectral characteristics. This work highlights the affinity and persistence of ammonium uranate compounds and emphasizes the need to continue investigating these alteration systems.