'No clad lift-off' criterion and associated methodology developed by EDF and FRAMATOME-ANP on the basis of IFA-610 experiments

At high burn-up, fuel rod internal pressure increase due to fission gas release along irradiation time can potentially bring fuel rod into 'cladding lift-off' situation, where pellet-clad gap is supposed to re-open and to increase leading to positive thermal feedback on fuel behaviour. In this situation, a rather quick degradation of the thermo-mechanical fuel rod behaviour is expected to occur and could cause the loss of rod integrity. IFA-610 test series was launched in 1997 by HRP in order to address clad 'lift-off' and thermal feedback phenomena in a high burn-up PWR fuel rod under overpressure conditions. Up to now seven different experiments have been performed in Halden using IFA-610 rig with UO2 and MOX fuel rods pre-irradiated in PWR up to high burn-ups. These experiments provided many relevant and consistent data showing that cladding lift-off is possible in a high burn-up fuel rod and that a high overpressure level, at least +130 bar, is required to induce any noticeable effect on fuel thermal behaviour. At high burn-up, fuel rod behaviour under overpressure conditions is similar for both UO2 and MOX fuels. Based on IFA-610 results, EDF, in cooperation with FRAMATOME-ANP, has developed an alternative rod pressure design methodology based on the 'no gap re-opening' criterion with its thermo-mechanical code CYRANO3 validated for IFA-610 simulation. This methodology has been licensed in 2001 by the French safety authorities and it allows EDF to benefit from additional margins for its current and future fuel managements. (Author)