Evidence of two gas release kinetics during the oxidation of an irradiated PWR UO2 fuel

The 'ADAGIO' (French acronym for Discriminating Analysis of Accumulation of Inter-granular and Occluded Gas) experiments are dedicated to the determination of the gas inventory located at inter-granular position thanks to a partial oxidation of an irradiated UO2 fuel. During these experiments the 85Kr gas release is measured as a function of time. The comparison of 85Kr release kinetic with the fuel oxidation kinetic brought into prominence two different gas release kinetics: the first one is proportional to the oxidation kinetic, the second one has a sigmoid shape and could be linked to the gas release of the so called 'High Burn-up Structure'.