Structure and properties of amorphous uranium dioxide

Irradiation-Driven Restructuring of UO2 Thin Films: Amorphization and Crystallization

“…However, the simulations also reveal critical differences between amorphous and crystalline UO 2 . In the amorphous structure, anion spacing (O–O) is 0.285 nm, which is longer than (0.273 nm) in the UO 2 crystal.…”

“…Middleburgh et al also predicted a ∼13% difference in the densities of the amorphous (9.73 g/cm 3 ) and crystalline (10.97 g/cm 3 ) UO 2 . This prediction is similar to density differences between disordered and ordered phases for ZrO 2 (∼18%), Al 2 O 3 (∼20%), and SiO 2 (14%) systems.…”

“…In general, coarse-grained (5–10 μm) UO 2 is highly stable and accommodates significant radiation damage without amorphization. − However, irradiation of bulk UO 2 samples with nanocrystalline (∼20 nm) features exhibits significant changes in the unit cell parameter (swelling) attributed to the decreased oxygen content and increased disorder at the grain boundaries . Based on XPS results, Teterin et al also noted that irradiation destroys long-range ordering in UO 2 films .…”

“…In general, the propensity for crystallization depends on similarities in the local atomic structures of the amorphous solids and their crystalline phases. , Middleburgh and coauthors recently reported density functional theory and molecular dynamics simulations on the local structure of amorphous UO 2 . They show that the same basic building blocks (OU 4 tetrahedra) describe both amorphous and crystalline structures of UO 2 .…”

“…Uranium dioxide (UO 2 ) is the most commonly used fuel for nuclear energy, and its mineral analogue uraninite is the dominant ore of uranium. − UO 2 exists in a single-phase cubic fluorite structure at temperatures up to the melting point (2865 °C), − and hyperstoichiometric (UO 2+ x , 0 < x < 0.25) cubic structures are also important. ,, UO 2 is stable during reactor operation and accommodates substantial radiation damage without amorphization. − The fission products, neutrons, and large thermal gradients generate a complex network of defects within the large (5–10 μm) UO 2 grains and fission gas (Kr, Xe) bubbles at grain boundaries. ,,− Prolonged radiation exposure leads to considerable restructuring at the outer edges of UO 2 fuel pellets. , The resulting morphology, known as high-burnup structure (HBS), consists of nanoscale grains and pores. − The HBS changes the thermophysical properties of fuel pellets and increases radiation tolerance. A better understanding of HBS formation mechanisms is essential for predicting fuel performance for UO 2 and developing new nanostructured fuels for nuclear energy.…”