Nuclear Science User Facilities 104 Post-irradiation examinations of annular mixed oxide (MOX) fuel pins for sodium fast reactors Fabiola Cappia – Idaho National Laboratory – Jason Harp – Idaho National Laboratory – The primary goal of Generation IV nuclear energy systems is to improve resource utilization and to minimize the nuclear waste burden while improving safety, reliability and proliferation resistance. In addition, through innovation aimed at reducing capital costs and financial risks, Gen IV technologies will provide clean energy at competitive costs compared to other energy sources [1]. The sodium-cooled fast reactor (SFR) is one of the six technologies that has been chosen as a next-generation nuclear energy system to meet these goals. SFR technology has been historically established both through pioneering experimental reactors (e.g., Fermi, Experimental Breeder Reactor- II, Rapsodie) and prototype reactors (e.g., Phenix, BN-600, Monju). Past experience is an essential asset and provides the basis to further develop advanced SFR concepts [2]. In terms of fuel for SFR, initial efforts in the fifties were focused on metallic fuels, due to the highest heavy-metal density favoring the highest breeding ratio. However, metal fuels struggled in achieving high burnup due to excessive swelling and dimensional instability. In a short time, uranium and plutonium mixed oxide (MOX) fuels became the reference fuel for SFR, thanks to their good stability under irradiation. Since the sixties, hundreds of thousands of pins have been successfully irradiated up to burnups exceeding 20% fission of initial heavy-metal atom [3]. High linear heat-generation rates (>30 kW/m) and high burnup, typically above 15%, were the main objectives of SFR fuel research and development, because it was shown that increasing fuel burnup offers significant cost advantages.Annular pellets, which lower the smear density to compen- sate for increasing pellet swelling as burnup progresses and maximize power rating for a prescribed margin to fuel melting [4], are one of the promising designs for SFR driver fuel. Irradiation experiments of advanced MOX test assemblies were conducted in the Fast Flux Test Facility (FFTF) between 1980 and 1993 [5]. Assembly FO-2, which was one of the first assemblies to use HT-9 as cladding material, was designed to A more systematic study of the performance of annular fuel MOX pins has been undertaken in this project, with the specific objective both to ameliorate the understanding of the effects of irradiation temperature and burnup on the fuel microstructural evolution and to provide supporting data for validation of fuel-performance models.