Modelling of fission gas behaviour in high burnup nuclear fuel

The safe and economic operation of nuclear power plants (NPPs) requires that the behaviour and performance of the fuel can be calculated reliably over its expected lifetime. This requires highly developed codes that treat the nuclear fuel in a general manner and which take into account the large number of influences on fuel behaviour, e.g. thermal, mechanical, chemical, etc. Although many mature fuel performance codes are in active use, there are still significant incentives to improve their predictive capability. One particular aspect is related to the strong trend of NPP operators to try and extend discharge burnups beyond current licensing limits. With increasing burnup, more and more fission events impact the material characteristics of the fuel, as well as the cladding, and significant restructuring can be observed in the fuel. At local burnups in excess of 60-75MWd/kgU, the microstructure of nuclear fuel pellets differs markedly from the as fabricated structure. This "high burnup structure" (HBS) is characterised by three principal features: (1) low matrix xenon concentration, (2) sub-micron grains and (3) a high volume fraction of micrometer sized pores. The peculiar features of the HBS have resulted in a significant effort to understand the consequences for fuel performance and safety. In particular there is the concern that the large retention of fission gas within the HBS could lead to significant gas release at high burnups, either through the degradation of thermal conductivity or through direct release. While for the normal fuel microstructure numerous models, investigations and codes exist, only a few models for simulating ssion gas behaviour in the HBS have been developed. Consequently, in this context, it is fair to say that reliable mechanistic models are largely missing today. In line with this situation the present doctoral work has focussed on the development and evaluation of HBS fission gas transport models, with a view to improve upon the current gap in fuel performance modelling. In particular two features of the HBS have been focussed on, viz. the equilibrium xenon concentration in the matrix of the HBS in UO2 fuel pellets, and the growth of the HBS porosity and its effect on fission gas release. In a first step a steady state fission gas model has been developed to examine the importance of grain boundary diffusion for the gas dynamics in the HBS. With this model it was possible to simulate the ≈0.2 wt% experimentally observed xenon concentration under certain conditions, viz. fast grain boundary di usion and a reduced grain di usion coeficient. A sensitivity study has been conducted for the principal parameters of the model and it has been shown that the value of the grain boundary diffusion coeficient is not important for diffusion coeficient ratios in excess of ~104. Within this grain boundary diffusion saturation regime the model exhibits a high sensitivity to principally three other parameters: the grain diffusion c