The Swedish Radiation Safety Authority (SSM) follows the research on fuel performance closely. One aspect that has been the main question of several research projects is the fragmentation of fuel pellets during abnormal heat up. In this project the oxidation of UO2 is scrutinized and its effect on pellet fragmentation, fission gas release and damage of fuel rods has been investigated.
Many phenomena that affect a fuel rod are described mathematically in analysis tools (computer codes) like FRAPCON and FRAPTRAN, but fuel oxidation is not commonly included in such tools. Current analysis tools usually have an empirical description of the state of fuel pellets where the oxidation of UO2 would be a part of a rough model of porosity and heat transfer of the pellet. Better descriptions of essential phenomena in fuel analysis tools will lead to more accurate analysis of fuel behaviour.
Knowledge of what is happening in a fuel rod during an event and how it is implemented in analytical tools is essential to SSM for our supervision of nuclear power plants. This project has contributed to the development of knowledge at SSM regarding the phenomena of fuel oxidation and how it affects the risks and consequences of fuel damage. It is has also provided an insight into how phenomena like fuel oxidation can be included in analysis tools.
In this project the understanding of how to describe fuel oxidation during normal and abnormal conditions has been investigated. Starting from current knowledge of fuel oxidation a model has been suggested that can be implemented in FRAPTRAN for further analysis of fuel behaviour. In this project, which includes a first evaluation of the model, the results indicate that fuel oxidation has a large impact on fission gas release.