Evolution of pressurized xenon bubble and response of uranium dioxide matrix: A molecular dynamics study

Abstract

To explore the microstructure evolution of uranium dioxide (UO2) due to accumulation of xenon (Xe) atoms at high burnup, the growth of highly pressurized Xe bubbles has been simulated in molecular dynamics (MD) by sequentially inserting Xe into a pre-existing equilibrium Xe bubble in UO2 using five different interatomic potentials at 1500 K. The results reveal that the characteristics of Xe bubbles and the microstructure evolution of UO2 due to the bubble growth can be divided into two stages in terms of the maximum pressure (Pmax) of bubbles. Before the bubble reaches Pmax (stage I), the characteristics of bubbles and the microstructure evolution of UO2 are relatively independent of the interatomic potentials used. The bubble volume and pressure increase with inserting additional Xe atoms, and the Xe atoms in the bubble evolve from an initial gaseous state to a glassy/amorphous state, a nearly fcc solid structure and subsequently a high density amorphous or glassy state. Moreover, the high density, over-pressurized bubbles displace U and O atoms, creating a larger volume to accommodate Xe atoms and correspondingly produce self-interstitial atoms (SIAs) with increasing Xe atoms. Most of the SIAs within stage I are distributed at the center of six {100} facets around the Xe bubble. However, after the bubble pressure reaches Pmax (stage II), the bubble characteristics predicted by MD simulations depend on both the UO2 interatomic potential and the Xe-UO2 potential. However, the microstructure evolution of UO2 is mainly determined by the UO2 potential since it determines the mobility of self-interstitial defects and the stability of the UO2 fluorite structure. ½<110> dislocations (loops) or UO2 phase transitions along with incipient boundaries (or interfaces) appear due to the accumulation of Xe in over-pressurized bubbles.