Cascade-driven mixing at metal oxide interfaces

Advanced nuclear fuel concepts sometimes involve metal oxide interfaces between fissile and nonfissile phases. During operation, cascade damage as secondary events from a fission track will occur throughout the material. Some of that damage will take place at the interface between phases. Here we simulate representative secondary events of this nature. As a model system, ongoing experiments consider a composite in which the nonfissile material is magnesia and the fissile phase is modeled via hafnia as a surrogate. In correspondence the experiments, the atomistic simulation cells are composed of hafnia in the fluorite structure and magnesia in the rocksalt structure. Molecular dynamics simulations of cascade damage across interfaces of these materials shows Hf cations becoming kinetically trapped in the magnesia phase. The Hf cations remained trapped for the duration of the 20-ps simulations. When the primary-knock-on atom energy is above a few hundred eV in the direction of the interface and is within five lattice spacings, the propensity for trapping is very high. Under these same conditions, an Mg cation will occasionally become trapped in the hafnia. Complementary electronic structure calculations indicate that Hf cations are thermodynamically unstable in magnesia. Furthermore, these calculations indicate that the charge on the Hf ions reduces by one electron if no compensating defect is present, but reverts to the charge in HfO₂ bulk in the presence of a defect such as an oxygen interstitial. Extensions of these observations to the behavior of urania and ceria are mentioned.