Towards the mathematical model of rim structure formation

The high burnup LWR UO2 fuels show a notable micro-structural change around the pellet outer zone which is called the rim structure. It is observed at temperatures as low as 400 C so that fission track and cascade mixing could be the key mechanism. SEM observation revealed that the structure primarily appears on free surfaces of UO2, indicating that strong sink for point defects may play a big role. And as generic observations, increase of lattice parameter indicates extensive amounts of vacancies are stored in high burnup fuel, which may induce the restructuring interacting with dislocations of high density at high burnup. Considering these observations a model of reaction-diffusion process of defects with irradiation induced transport is proposed. The equations are investigated numerically. The model indicates that an instability starts when the dislocation network starts intensive interaction with vacancy flux which is modified by interstitial diffusion between spatial segments. It appeared to be similar to the Turing type instability which indicates that the rim structure formation is one kind of the self-organizing processes of open reaction-diffusion systems. (orig.)