Pipe and grain boundary diffusion of He in UO₂

Abstract

Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in [Formula: see text]. Calculations were carried out for the {1 0 0}, {1 1 0} and {1 1 1} [Formula: see text] edge dislocations, the screw [Formula: see text] dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300-3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundary.

Experimental and simulation study of grain boundaries in UO2

“…Moreover, our results are in good agreement with those obtained by Zhang et al [20], with Yakub and Morelon potentials, and with those of Chen et al [22], calculated with the Yakub potential. The same remark applies to the results obtained by Galvin et al [25] using the Cooper potential. Conversely, the results obtained by Van Brutzel et al [17] using the Morelon potential are always higher than ours.…”

“…The Cooper potential goes beyond the pair potential through an Embedded Atom Model (EAM) contribution. Note that Yakub, Morelon and Cooper potentials were already used to study grain boundaries in UO2 [17,20,22,25] making comparison with the present work possible. To make sure that these potentials are suited to simulate grain boundaries, we first calculated surface energies.…”

“…Formation energies were calculated and the thermal migration of grain boundaries was studied. Finally, in 2016, Galvin et al [25] published a paper where 4 grain boundaries were studied using the Cooper potential [26]. They calculated the formation energies and investigated the diffusivity of helium at the vicinity of the boundaries.…”

“…Some authors have used specific codes like GBStudio [27] or Metadise [28]. In some cases, a simple 0 K energy minimization was performed [10,20,24] whereas, in other cases, a more complex procedure was applied involving molecular dynamics relaxations at various temperatures (300 K [17], 1500 K [25], 2000 K [22], and even 3000 K [23]). These differences may explain why calculated energies values are scattered.…”

Molecular dynamics study of UO2 symmetric tilt grain boundaries around [001] axis

“…First‐principles atomistic methods are however impossible to use due to the system sizes to be considered, so a “classical” atomistic simulation method that uses empirical potentials to describe the interactions between atoms is required. Thus, several papers have been published since 12 years showing properties of UO 2 grain boundaries calculated at the atomic scale 11‐18 …”