Early stages of UO2+x sintering by in situ high-temperature environmental scanning electron microscopy

Abstract

Early stages of various uranium oxides sintering were monitored in situ through High Temperature Environmental Scanning Electron Microscopy. Systems composed of two UO 2+x microspheres were heated up to 900-1200°C under P O 2 ranging from 10-10 to 25 Pa. The oxide phases stabilized were assessed through thermodynamic calculations and powder X-ray diffraction. In all the conditions tested, the formation and the development of a neck was evidenced and image processing led to quantitative data describing the morphological changes. The evolution of the sintering degree was fitted using an exponential law and allowed the evaluation of the activation energy. When P O 2 led to stabilize hyper-stoichiometric UO 2+x oxides, the values obtained increased with x, typically in the 200-400 kJ•mol-1 range. Under air atmosphere, the stabilization of U 3 O 8 led to E A = 260 ± 40 kJ•mol-1. Finally, the main diffusion mechanism driving neck formation was found to be volume diffusion, independently from the oxide stoichiometry.

Evaluation and application of a new scintillator‐based heat‐resistant back‐scattered electron detector during heat treatment in the scanning electron microscope

“…Several experiments were performed in air, pure O 2 , vacuum and nitrogen. N 2 + 4 vol.%H 2 + 2000 ppm H 2 O was also used for specific experiments 44 …”

Hyperstoichiometric Uranium Dioxides: Rapid Synthesis and Irradiation-Induced Structural Changes

“…This mechanism is different from the thermally induced grain coarsening/sintering observed in bulk UO 2.12 materials. 42,43 Our results show that in thin films with ultrafine grains, the coarsening process occurs even at near ambient temperatures. TEM images and EDS analysis also show that irradiation facilitates the mixing of UO 2.12 with the Al 2 O 3 -based layer at the film/substrate interface.…”

“…32 Thermally induced grain coarsening for UO 2.12 is shown to be more extensive than UO 2 . 41,43 The current work aims to investigate the impact of radiation on structural changes of nanoscale hyperstoichiometric UO 2.12 . However, the lack of reliable and straightforward methods to prepare hyperstoichiometric oxides impedes radiation-damage investigations at the nanoscale level.…”

“…Irradiation-induced transformation in fine-grain stoichiometric UO 2 has been investigated extensively. ,,, Examples include spent fuel pellets consisting of small (100–300 nm) grains formed by the radiation-driven restructuring (grain subdivision or recrystallization). − These small grains appear free of extended defects, exhibiting high radiation tolerance and structural stability . However, further reduction of grain sizes could significantly influence stoichiometric and hyperstoichiometric uranium dioxide radiation stability and grain growth behavior. ,,− For example, synchrotron-based X-ray diffraction (XRD) measurements indicate that nanocrystalline UO 2 experiences a significant grain coarsening at 1000–1090 K as a result of the mobility difference of various grain boundaries . Thermally induced grain coarsening for UO 2.12 is shown to be more extensive than UO 2 . , The current work aims to investigate the impact of radiation on structural changes of nanoscale hyperstoichiometric UO 2.12 .…”

“…However, further reduction of grain sizes could significantly influence stoichiometric and hyperstoichiometric uranium dioxide radiation stability and grain growth behavior. ,,− For example, synchrotron-based X-ray diffraction (XRD) measurements indicate that nanocrystalline UO 2 experiences a significant grain coarsening at 1000–1090 K as a result of the mobility difference of various grain boundaries . Thermally induced grain coarsening for UO 2.12 is shown to be more extensive than UO 2 . , The current work aims to investigate the impact of radiation on structural changes of nanoscale hyperstoichiometric UO 2.12 . However, the lack of reliable and straightforward methods to prepare hyperstoichiometric oxides impedes radiation-damage investigations at the nanoscale level.…”