MODELING THE IRRADIATION SWELLING OF UO2 AT THE FUEL PELLET RIM

Irradiation swelling of UO2 at the fuel pellet rim was modeled based on the published theory and data of HBS (High Burnup Structure) formation. Fuel swelling was divided into two parts: fuel matrix swelling and porosity growth. Both solid fission products and fission gas contribute to the fuel matrix swelling prior to HBS transformation, resulting in relatively stable matrix swelling rate of around 1.0% per 10 GWd/tU, but the transformation accompanied by Xe depletion reduces the matrix swelling rate to approximately 0.3% per 10 GWd/tU, only attributed to solid fission products. Considering the direct impact of Xe depletion on the drop of matrix swelling rate, the exponential law of Xe depletion was applied to model the reduction of matrix swelling rate. Pore size and pore density evolution are the two main aspects of porosity growth. Pore size takes the form of lognormal distribution, whose parameters are obtained through fitting the experimental data. Pore density increases in the transformation process but goes down as a result of pore coarsening thereafter. Published data of three pellets were used to verify the correlations modeling pore growth, which were proven generally consistent with each other. The results of this work are ready to be incorporated into fuel performance modeling code as an option for detailed calculation of fuel swelling.