Molecular dynamics studies of displacement cascades in the uranium dioxide matrix

Abstract

In the context of studies on long-time storage of irradiated spent fuel, molecular dynamics simulations have been carried out in order to understand the physical phenomena, on the atomic scale, linked to modifications and damage that the uranium dioxide structure undergoes during a-decay irradiation in repository conditions. Simulations of atomic displacement cascades over an energy range from 1 to 20 keV for the initial primary knock-on atom (PKA) do not show any amorphization of the structure in agreement with what has been found experimentally, and there is very little correlation between the initial orientation of the PKA and the cascade morphology. The number of Frenkel pairs, as a function of the initial energy of the PKA, exhibits a power-law behaviour with an exponent of 0.9 which is contrary to the theoretical linear Norgett-Robinson-Torrens law. Finally, for both species the vacancies have a tendency to aggregate and cluster near the core of the cascade while interstitial atoms are preferentially located at the periphery of the branches corresponding to subcascades.

Calculation of the threshold displacement energies in UO2 using ionic potentials

“…Conversely, for the uranium the calculations show that the mean threshold displacement energy is higher than the mean value estimated by Soullard et al, being rather close to E d [U] $ 50 eV. The values in parenthesis come from Van Brutzel et al [28]. The mean values estimated by Soullard et al [8,49] …”

“…They evaluated the threshold displacement energies of oxygen and uranium, to be roughly 20 eV and 40 eV respectively, since, according to their calculations, these values permitted to obtain consistent results with the experimental observations. Since then, quite a few theoretical works, mainly based on molecular dynamics MD simulations [28,29] However, the threshold displacement energy, which is defined as the minimum energy transferred to a lattice atom along a given crystallographic direction yielding the creation of a stable Frenkel defect is considered as a parameter of high importance for quantifying the number of displaced atoms during irradiation induced displacement cascades.…”

Atomic scale mechanisms for the amorphisation of irradiated graphite

“…This is done via molecular dynamics calculations, following a standard procedure [53] that we recall hereafter. First, each supercell is relaxed in the NPT ensemble at 300 K and at room pressure for 10 ps in order to reach an equilibrated state for graphite.…”

Molecular dynamics study on planar clustering of xenon in UO2

“…However, most of these studies focused on the stable fluorite phase, and bears a limitation that actual UO 2 might deviate from this structure locally due to irradiation damages. It is the case especially in cascade simulations [12,13]. This provokes a question about the applicability of classical potentials to model the behavior of UO 2 with lager-scale deformations with defects.…”