Fission gases inside a fuel rod plays an important role in the behaviour of the fuel, both during normal operation and during events and accidents. Fission gases are released from inside the fuel pellets to the gap between the pellet and the cladding tube and then flow to the plenum volume at the top of the fuel rod. In high burnup fuel, this axial flow to the plenum volume can be blocked because of pellet-cladding gap closure.
The presence of fission gases in the pellet-cladding gap is important for the rod internal pressure and for the pellet-cladding heat transfer. While these effects are considered in most computer codes for fuel rod thermal-mechanical analyses, the axial flow of fission gases is generally not. This flow can be important for several reasons, for example for removal of fission gases from the active region during normal operation, in the complex behaviour during load-follow operation and for how the fission gases propagates during an event.
The results include the development, implementation and verification of models for axial gas transport and gas mixing in the pellet-cladding gap. The models take into account gas transport both by axial pressure gradients and by diffusion, and they are intended for use in the FRAPCON and FRAPTRAN codes for further analysis of fuel behaviour. Calculations that verify the correctness of the numerical implementation and validation against a few experiments have been done.
With this project, SSM has obtained a computer code that can model axial fission gas transport in a fuel rod. SSM has also gained insight into how such a model is implemented in a computer code, with what assumptions and limitations. This is of great importance when reviewing safety analyses for nuclear fuel. Furthermore, this project is part of the international development work and enables active participation in international contexts.
Need for further research
The development and implementation of models for analysing axial gas flow and fission gas mixing needs to be complemented with validation against more complex tests on fuel with higher burnup. This validation will also reveal the need for further development. On a longer time scale, much research and development remains to fully understand the behaviour of high burnup fuel.