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 coeficient, the bubble number density and the re-solution rate coeficient. 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 diffusion of fission gas is significantly faster than lattice diffusion. 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 justification 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 simplifies 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 assess its predictions against measurements. Reasonable agreement was found with respect to the experimental release in the temperature range where the release mechanism was originally interpreted to be due to volume diffusion of the gaseous fission products. In contrast to the original analysis, the developed model interprets the release mechanism as being due to pore growth, coalescence and, ultimately, venting. The model results for annealing conditions have been compared with available PIE data and this comparison has indicated that, for burnups examined to date, most of the fission gas is expected to be retained within the HBS porosity. The model of porosity growth and venting developed for annealing conditions has then been extended to take into account the effect of irradiation on the defect population and accumulation of fission gas in the HBS porosity. The power history of a high burnup fuel rod from a Swiss NPP was used to model the local behaviour of the HBS and good agreement was found between the calculation of porosity growth and results from open literature. A comparison between the calculated local gas release was found to be consistent with both the observations from the annealing calculations and measurements performed on the same fuel rod using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). To provide a more comprehensive analysis of the model predictions, a gas release correlation for calculating the release due to pore venting has been developed from the stand-alone in-pile modelling of porosity growth and this was implemented into the fuel performance code TRANSURANUS. The examined high burnup rod was modelled using the thus extended TRANSURANUS code so that the integral behaviour (rather than just the local HBS behaviour) could be calculated. The subsequent fuel performance analysis has revealed that venting from the HBS has a small effect on the integral fission gas release with the release increasing towards the pellet periphery. The principal effect of the HBS for the considered high-burnup irradiation is the presence of the high porosity, which increases the fuel centre temperatures and therefore the fission gas release from the centre of the rod. For the base case analysis 22% of the integral fission gas was found to be released. However, this is less than the quantity measured by rod puncturing. To address this discrepancy several sensitivity studies have been performed to investigate as to where the additional fission gas release originates, specifically the fuel centre and/or the HBS zones in the pellet periphery. It has been shown from this sensitivity analysis that extra release from just one of these regions is unlikely to account for the discrepancy between measured and calculated fission gas release. It is more likely that the uncertainties for each of the individual regions, when considered together, could account for the observed integral release. In particular the presence of an athermal gas release from within the HBS has been explored in detail and it has been shown that, while significant release could occur, there is still significant retention. Previous examinations of fuel behaviour have assumed little to no release in the HBS; however it is clear from this work that at burnups as high as considered currently, the gas release does have a signi cant component coming from the pellet periphery.

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