In the framework of the 1991's French Law which defines the 3 major research lines to manage of nuclear wastes, CEA (Commissariat à l'Energie Atomique) is supporting a wide R&D program dealing with the long term behavior of spent fuel in various boundary conditions representative of interim storage and geological disposal. One major issue concerns the potential evolution with time of the thermo-mechanical properties of the irradiated cladding, the question of its integrity and whether the cladding can be considered as a first confinement barrier for radionuclides. The answer will strongly influence the design and the safety analyses of the interim storage and potentially the first stage of a geological disposal.

After irradiation, the thermo-mechanical properties of the spent fuel cladding are altered compared to those of the non irradiated material: presence of numerous irradiation defects due to the high irradiation, presence of external zirconia layers and hydrogen within the Zircaloy due to the external corrosion. Furthermore, the cladding is submitted to a relative high internal pressure field which is related to the production and release in the free volumes of fission gases and helium. Since the cladding is expected to undergo a relative high temperature field (∼300 - 400°C), long term creep is expected to become a relevant deformation mechanism which can potentially lead to a breaching of the cladding.In order to deal with this strategic issue, experimentation on irradiated samples as well as modelling work are in progress in CEA with the support of EDF and FRAMATOME. The ambitious objectives are to define and qualify the long term mechanical properties of the cladding, in particular a long term creep law and an adapted breaching criterion. Long term creep properties are studied through a stepwise approach from short term (few days) and high stress field experiments to long term (few years) and low stress field experiments. The validation of the extrapolation is ensured by complete metallurgical characterizations (HRTEM, XRD, H2 content) performed before and after deformation. The first step of this work, medium term (∼ 1 month) creep experiments on irradiated cladding will be presented.