Molecular dynamics study of fission gas bubble nucleation in UO2

Abstract

Molecular dynamics (MD) simulations are used to study helium and xenon gas bubble nucleation in UO 2 . For helium bubbles, the pressure release mechanism is by creating defects on the oxygen sublattice. Helium atoms diffuse away from the bubbles into nearby bulk UO 2 , thus forming a diffuse interface. For xenon bubbles, over-pressurized bubbles containing xenon can displace uranium atoms, which tend to aggregate around the xenon bubble as a pressure release mechanism.MD simulations of xenon atoms in pre-existing voids suggest that xenon atoms and the replaced uranium atoms occur in a 1:1 ratio, although kinetic factors may reduce that ratio depending on availability of xenon atoms and vacancies around the bubble. Finally, MD simulations suggest that for small bubbles (1-5 xenon atoms), the xenon bubble nucleus at UO 2 grain-boundaries has much lower formation energy compared to that of bubbles of similar sizes in the bulk. However, when the xenon bubble grows into larger sizes, this energy difference is reduced.1

Effect of Xe bubble size and pressure on the thermal conductivity of UO2—A molecular dynamics study

“…In that work, they found that helium atoms can diffuse away from a helium bubble into the UO 2 matrix. The formation of such a diffuse interface was also confirmed by Liu et al's MD simulations [40]. As a result, thermal conductivity is further reduced with respect to an empty void due to the formation of this diffuse interface.…”

“…Generally speaking, the bubble pressure in a full-filled bubble is about 4 times higher than that in a half-filled bubble at a given bubble size. Since small bubbles are over-pressurized [40], bubble pressure decreases with increasing bubble size. For the full-filled bubbles, bubble pressure is in the range of 16-23 GPa, which agrees well with Liu et al's MD results of 15-25 GPa [40] using a different interatomic potential.…”

“…Andersson [40], in which they found that uranium atoms around a Xe bubble were displaced to release the bubble pressure for Xe:V U 5 1.0. The interface distortion around an over-pressurized bubble can cause additional phonon scattering.…”

A new equation of state for helium nanobubbles embedded in UO2 matrix calculated via molecular dynamics simulations

“…To the best of our knowledge, no equivalent simulations have been yet carried out for UO 2 . Nonetheless, MD studies of He solubility [16] and bubble nucleation [17] show that in this ceramic material the He behavior is significantly different than in metals. In both studies, the authors observe an upper He concentration inside the bubble beyond which He atoms diffuse into the surrounding matrix creating a diffuse interface at the edge of the over-pressurized bubbles.…”

Mechanistic materials modeling for nuclear fuel performance

“…Efforts to develop new mechanistic fission gas release models based on multiscale simulations have focused on understanding the behavior of fission gas atoms and point defects in bulk UO 2 [49,[51][52][53][54][55] as well as how their properties are influenced by fuel chemistry [49,53,78,82] and irradiation conditions [73], segregation to grain boundaries [72,93,94], simulation of percolation networks at grain boundaries [95], nucleation and growth of inter-and intra-granular bubbles [85,[96][97][98][99][100][101][102][103] and bubble resolution [100,[104][105][106]. Attempts to use lower length scale simulations of fission gas properties in fuel performance simulations have so far relied on hierarchical approaches where certain parameters in the engineering scale models, such as the fission gas diffusion rate, are informed by lower length scale simulations [73,107].…”