An attempt to simulate the porosity buildup in the rim at high burnup

The 'rim effect' was identified fifteen years ago with the examination of the first rods irradiated over 45 GWd/tU (pellet average burnup) in test reactors as well as in power plants. Detailed PIE have shown a progressive disappearance of the grain structure in the fuel periphery, in relation with a strong increase of the porosity. A scenario for the rim buildup at high burnup is proposed. Regarding the role of the porosity on the evolution of the physical properties, the first step has been an attempt to simulate the fission gas behaviour in the rim. In the proposed model, bubble growing is related to the volume fraction of the fuel restructured. An assumption is made on the creation of short cuts, increasing the average Xenon diffusion rate at very short distances. This model allows to evaluate the pore volume fraction in the fuel, the mean size of these pores and the fission gas volume repartition within the bubbles and the fuel matrix. The model presented herein was issued in the framework of a PhD, held on May 31st. 1996 in the Marseille II University by Dr. Bruna HERMITTE. (author)