Ancient natural uranium oxide as analog of spent fuel: Contribution to assessing the impact of helium production on long term evolution of UO2 matrix

Nuclear waste management in particular for spent fuel disposal option, needs an improvement of the knowledge concerning the fate of helium produced by actinide decrease and its impact on the physical and structural changes on the fuel matrix in a repository. An original alternative to α ageing simulation concerns the study and characterization of different old natural uranium oxides, with increasing age of crystallization. Since 2005, in the framework of a collaboration between Cea DEN/DTCD, CREGU and CNRS a methodology devoted to the study of helium behavior in this kind of samples has been developed and applied to three natural uranium oxides. Coupling results from gas release mass spectrometry measurements and from HR-TEM showed: -) high helium mobility at low temperature, -) an helium retention always lower than 0,9 at% in the uranium oxide grains, and -) high concentration of nanometer size helium bubbles, poorly pressurized and released at high temperature (above 1300 deg. C). Predictable evolutions of spent fuel matrix under self irradiation are presented within the domain of physical, chemical and structural analogy between natural uranium oxides and spent fuels. (authors)