Thermal recovery mechanisms of UO2 lattices by defect annihilation

Abstract

Thermal recovery of irradiated uranium dioxide lattice have been investigated by experimental groups and defects related to each recovery stages are estimated based on the indirect observations like activation energies. In this study, thermal recovery mechanisms of both neutron and alpha-particle irradiated uranium dioxide lattice is investigated by atomistic simulation methods. Isochronal and isothermal annealing have been applied to the irradiated samples. Following the same procedures of the experimental groups, activation energies are calculated with molecular dynamics simulations and compared with empirical data which show a good agreement. Migration energy barrier and the recovery energies of the obstruction type defects are calculated with molecular dynamics simulation and density functional theory.Recovery stages of the irradiated crystals are discussed however unlike the predictions of the most experimental groups, it is shown that neutron and alpha-particle irradiated crystal structures do not thermally recover for the same reason. All three recovery stages of neutron irradiated uranium dioxide crystal is attributed to oxygen interstitials while the three recovery stages of the alpha-particle irradiated crystals is considered to the oxygen -uranium, uranium and uranium interstitial defects respectively. Finally, the reason and the main driving force of thermal recovery of irradiated crystals is the obstruction type defects which give a lead to all other types of defects recover with themselves in each lattice recovery stages.

Actinide and lanthanide dioxide lattice dilatation mechanisms with defect ingrowths

“…Fission and α-particle irradiation effects and thermal recovery of uranium dioxide were investigated by the author of this article in previous studies. 7,8 In this study, calculations were further expanded by investigating the effects of α-particles on isostructural oxides. Numerous experiments have been conducted on fluorite-structured compounds, which were exposed to α-particle irradiation, He ion implantation, Ce irradiation or α-decay events.…”

“…The first method is the "manual defect creation" method used by the author of this article in his previous works. 7,8 The second is the frequently used "DC" method. Besides being a more realistic method, the second method was applied to dispel doubts about the validity of the first method.…”

“…Based on these experiments, all cations were shifted in the Y-Z layer to model homogeneous irradiation from one side of the crystal. 7,8 Defects were not placed in the successive layers to prevent instant annihilations. At some point, for a high number of IFPs, the artificially created defects were so close to each other that it became difficult to maintain this limitation.…”

“…The data obtained from our previous studies 7,8 provide information about the defects responsible for the irradiation-induced swelling in uranium dioxide crystal lattice structures. It was concluded that the amount of lattice dilatation depends on the number of OT defects.…”

“…Results were compared with available experimental and molecular dynamics (MD) simulation results. [1][2][3][4][5][6] The effect of irradiation on the crystal was modeled by two techniques: "manual defect creation" (MDC) method 7,8 and a widely used "displacement cascade" (DC) method. In our previous study, 7 as a result of the calcu-lations made with the first method, the obstruction type (OT) defect was directly responsible for the change in the UO 2 lattice parameter.…”