Evidence on spent fuel aging: gas behaviour and alpha-damage

The safety assessment of spent nuclear fuel (SNF) during storage/disposal requires defining/extrapolating the behaviour of the fuel over the timescale of interest, to ensure that the mechanical integrity is retained, hence minimising the possible exposed surface to corrosion processes. Since no direct measurement of stored fuel can cover the full time extension of interest, additional studies aimed at understanding aging processes of the SNF expected to affect properties and behaviour of spent fuel during many decades of storage are necessary. Tests conducted under accelerated conditions can thus contribute to the safe implementation of extended storage concepts. Alpha-decay damage and helium accumulation are the key process affecting the microstructure evolution of properties and behaviour of spent fuel. The effects of alpha-decay damage and helium build-up during SNF storage are the object of a dedicated programme of studies carried out at JRC-ITU, which covers in particular the evolution of physical-chemical and mechanical properties as a function of accumulated radioactive decay damage and He. The investigations address processes and mechanisms from the microstructural level (lattice defects and swelling, He behaviour) up to macroscopic properties (fuel swelling, hardness, stored heat, thermal conductivity). Accelerated ageing conditions are obtained by using suitable UO2 matrices containing short-lived alpha-emitters (the so-called alpha-doped UO2). The evolution of the fuel is also directly connected to its microstructure and composition after irradiation. Knowing the location of the various elements present and their chemical nature but also their evolution or migration during storage through diffusion or alpha-radiation enhanced mobility will allow assessing the source term in case of contact with water. This paper addresses both fundamental studies of aging processes of spent fuel surrogate (the alpha-doped) and the investigation of irradiated nuclear fuel aiming at determining the location and behaviour of the volatile fission products in particular. Several techniques have been used to characterize these two types of material (XRD, TEM, helium thermal desorption, Knudsen-cell mass spectrometry, etc).JRC.E.3-Materials researc