Strain relaxation in He implanted UO2 polycrystals under thermal treatment: An in situ XRD study

Within the frame of the long-term evolution of spent nuclear fuel in dry disposal, the behavior of He in UO2 polycrystals has to be studied. Here, strain relaxation in He implanted samples has 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 UO2 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 UO2 single crystals, or aged UO2 pellets doped with α-emitters. 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.