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 in uences on fuel behaviour, e.g. thermal, mechanical, chemical, etc. Although many mature fuel performance codes are in active use, there are still signi cant 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 ssion events impact the material characteristics of the fuel, as well as the cladding, and signi cant restructuring can be observed in the fuel. At local burnups in excess of 60 75MWd/kgU, the microstructure of nuclear fuel pellets di ers 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 signi cant e ort to understand the consequences for fuel performance and safety. In particular there is the concern that the large retention of ssion gas within the HBS could lead to signi cant 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 ssion 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 e ect on ssion gas release. In a rst step a steady state ssion gas model has been developed to examine the importance of grain boundary di usion for the gas dynamics in the HBS. With this model it was possible to simulate the ≈0.2wt% experimentally observed xenon concentration under certain conditions, viz. fast grain boundary di usion and a reduced grain di usion coe cient. 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 di usion coe cient is not important for di usion coe cient ratios in excess of ∼104 . Within this grain boundary di usion saturation regime the model exhibits a high sensitivity to principally three other parameters: the grain di usion coe cient, the bubble number density and the re solution rate coe cient. In spite of such sensitivity it has been shown that the model can reproduce the observed HBS xenon depletion with the assumption that grain boundary di usion of ssion gas is signi cantly faster than lattice di usion. In particular the results from this study have demonstrated that the release of produced gas from the grains to the HBS porosity corresponds to a dynamic equilibrium, providing a justi cation for the typical modelling approach used in HBS modelling, viz. fast transport to the porosity, which from a fuel performance modelling point of view largely simpli es the calculations. In a second step, a model describing the evolution of the HBS porosity under annealing conditions has been developed. The model was applied to a high burnup fuel annealing experiment to