Within the frame of the long-term evolution of spent nuclear fuel in dry disposal, the behavior of He in UO 2 polycrystals has to be studied. In this paper, strain relaxation in He implanted samples have been characterized using in-situ X-ray diffraction during thermal annealing. The influence of a wide range of experimental parameters (annealing atmosphere, He ion energy, orientation of the UO 2 grains probed by X-rays) has been evaluated. If each of them contributes to the strain relaxation kinetics in the implanted layer, strain relaxation is not completed for temperatures below 900 °C which is equivalent to what has been found on He implanted UO 2 single crystals, or aged UO 2 pellets doped with α-emitters. This stands for an additional evidence of the interest of ion implantation to simulate self-irradiation in spent nuclear fuel. Moreover, in the case of implantation with 500 keV He ions, we clearly show that strain relaxation and He release are not correlated for temperatures below 750 °C and demonstrate that this applied in-situ approach is well-suited for measuring strains in ion implanted polycrystals.
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