DOE OSTI2016
The fission process in nuclear fuel produces volatile fission product gases, such as xenon, that migrate out of the fuel and into surrounding structures. Consequently, these fission product gases may affect thermal performance and integrity of the fuel. Fission gas release models, used in modern fuel performance codes such as BISON, are currently under investigation to improve their accuracy and ability to simulate the fission gas release phenomenon and its consequences. For uranium dioxide (UO{sub 2}), the most commonly used fuel in commercial power reactors, fission gas release models are known to have significant uncertainties. In the interest of improving such models, several complex diffusional mechanisms have been examined using a variety of techniques such as Nudge Elastic Band (NEB), utilizing both Density Functional Theory (DFT) and semi-empirical inter-atomic potential functions. One proposed diffusion mechanism in UO{sub 2} is through tetra-vacancy (i.e., 2U{sub V} + 2O{sub V}) structures, which is expected to provide a suitable means for the incorporation and diffusion of xenon in the fuel crystal. In this work, a model was developed to study the incorporation of xenon in UO{sub 2} via a tetra-vacancy using ab initio molecular dynamics (AIMD). This is a first principles technique that abandons the use empirical force fields as usually is the case of classical MD analysis. Furthermore, in contrast with static transition analysis tools such as NEB, AIMD is able to capture the dynamics of a given process. The translation of xenon within a tetra-vacancy has been investigated using ab-initio molecular dynamics. The transition frequency, barrier energy, and incorporation energy were extracted from the model, demonstrating its ability to capture a variety of parameters and its potential for use in studying fission gas diffusive behavior. Future refinements to the model will seek to develop additional insight into Xe and Kr mobility and will likely involve executing more elaborate AIMD simulations to investigate this phenomenon. (authors)
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗