We use the Second Moment Tight Binding with charge eQuilibration (SMTB-Q) advanced empirical potential to study clusters of oxygen interstitials in hyperstoichiometric UO2. This potential reproduces ...the trends obtained from density functional theory calculations. We exhibit a new form of cuboctahedron that proves to be the most stable oxygen cluster in UO2+x . Considering different types of random arrangements of these clusters, we reproduce the fact that UO2 and U4O9 are the only two stable phases at low temperature in their composition range. Focusing on U4O9 at low temperature, we obtain a model for the α-U4O9 phase, which exhibits the experimental R3c space group. We present the results of the original neutron diffraction experiments measured on U4O9 at 1.5 K and perform a Rietveld refinement of the calculated U4O9 structure, which enables us to propose a complete description of the α-U4O9 phase.
Silicon single crystals were irradiated at room temperature (RT) with single and dual low-velocity (i.e., 900 keV I) and high-velocity (i.e., 27 MeV Fe or 36 MeV W) ion beams in order to study ...synergistic effects between nuclear and electronic energy losses in semiconductors. The damage created by irradiation was quantified by using Rutherford backscattering spectrometry in the channeling mode and Raman spectroscopy, and it was visualized using transmission electron microscopy. Whereas single low-velocity ion irradiation leads to amorphization of the surface layer of Si crystals, the use of a dual low- and high-velocity ion beam prevents this phase transformation. However, a remaining disorder exists, the level of which depends on the ratio of the high- to low-velocity ion fluxes. The higher the ratio (1.6 in the present case), the lower the disorder level, with a 30% integrated disorder instead of 100% upon single low-energy irradiation. These results provide evidence that ionization-induced disorder annealing can occur at RT in Si.
We investigate the thermally activated glide mobility of dislocations in uranium dioxide (UO2) from an atomistic point of view using a variable charge many-body empirical potential, the Second Moment ...Tight-Binding potential with charge equilibration (SMTB-Q). In order to determine the main glide system, we model the dislocation core structures for edge and screw orientations lying in different glide planes. Uncommon core structures with a double periodicity and a charge alternation are obtained. Straight dislocations motion is first considered to obtain the Peierls stress of each dislocation. We then address the thermally activated motion of the dislocations by the nucleation of kink pairs. Atomistic simulations give us the structure as well as the formation and migration energies of kink pairs. This information is finally combined with an elastic interaction model for kink pairs to obtain the dislocation velocities and the evolution of the critical resolved shear stress as a function of temperature. These quantities are compared to experimental data on urania single crystals.
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A coupling between the nuclear and electronic energy losses occur in the nuclear fuel (UO2) during in-reactor operations. However, the underlying mechanisms involved are still to be investigated. In ...this work, synergistic effects of nuclear and electronic energy losses have been investigated by irradiating crystals with single (900 keV I ions or 27 MeV Fe ions) and dual (900 keV I ions and 27 MeV Fe ions, simultaneously) ion beams at the JANNUS-Saclay facility. The damage build-up kinetic was in situ characterized by Raman spectroscopy. The microstructure evolution was determined by transmission electron microscopy (TEM) observations and by X-ray diffraction (XRD) analysis. Results show that both crystalline disorder and strain level are lower under dual-beam compared to the single-beam ion irradiations. Indeed, the dual-beam irradiation induces a transition from the formation of dislocation loops to dislocation lines. This result can be explained, in the framework of the thermal spike model, by a local increase of the temperature along the high-energy ion path. This temperature increase likely induces an enhanced defect migration leading to defect rearrangement.
•Induced mechanisms under dual-beam ion irradiation were studied in UO2.•Electronic ionizations lead to an accelerated defect rearrangement in UO2.•Fission fragments could generate partial recovery of the damage in nuclear fuel.
The strain and stress build-up in 20-keV He-implanted UO
2
single crystals have been determined by means of X-ray diffraction through reciprocal space mapping, with the use of a model dedicated to ...the analysis of the strain/stress state of ion-irradiated materials. Results indicate that the undamaged part of the crystals exhibits no strain or stress; on the other hand, the implanted layer undergoes a tensile strain directed along the normal to the surface of the crystals and a compressive in-plane stress. The build-up of both strain and stress with He fluence exhibits a two-step process: (i) a progressive increase up to a maximum level of ~1% for the strain and ~−2.8 GPa for the stress, followed by (ii) a dramatic decrease. The origin of the strain and stress build-up is the formation of both self-interstitial defects and small He-vacancy clusters. The strain, and stress relief is tentatively attributed to the formation of extended defects (such as dislocations) that induce a plastic relaxation.
{111}- and {100}-oriented UO2 single crystals were irradiated with 500-keV Ce3+ ions in the 1014–9 × 1014 cm−2 fluence range. The irradiation-induced strain was monitored using high-resolution X-ray ...diffraction. A mechanical modelling dedicated to thin irradiated layers was applied to account for the reaction of the unirradiated part of the crystals. The elastic strain, which is confined along the surface normal of the samples, increases with ion fluence until it is dramatically relieved. This behaviour is observed for both orientations. While the measured elastic strain depends on the crystallographic direction, the strain due to irradiation defects only is found to be equal for both directions, with a maximum value of ∼0.5%. Strain relaxation takes place at the damage peak, but the in-plane lattice parameter of the irradiated layer remains unchanged and equal to that of the pristine material. Meanwhile, the strain at the damaged/pristine interface continues to increase.
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