Application of ion beams in materials science of radioactive waste forms: focus on the performance of spent nuclear fuel

Ion beam techniques provide unique tools for the qualification of radioactive waste forms. They address three major issues: (i) the simulation by ion irradiation of the stability of a matrix submitted to radiative environment; (ii) the doping of a material with stable or radioactive elements which simulate the species to be confined; (iii) the characterisation of a material via nuclear microanalysis techniques. Among various classes of nuclear matrices the spent nuclear fuel is widely considered as a potential candidate for the stabilisation of radioactive wastes in scenarios of long term interim storage or final geological disposal. Illustrative examples revealing the potentialities of the use of ion beams either as a pure characterisation tool - to investigate the chemical stability of the UO2 matrix under an oxygen potential - or in a combined way (e.g. irradiation/characterisation, doping/characterisation) - to explore the radiation stability and the behaviour of foreign species - are presented. Transformations (stoichiometry, depth and structure of growing hyperstoichiometric U4O9/U3O7 oxides) occurring during low-temperature air oxidation of uranium dioxide single crystals are reported. Swift heavy ion irradiation of UO2 single crystals leads to a peculiar single crystal-polycrystal transformation (i.e. polygonisation of the fluorite-type structure of the material). Irradiation of UO2 at low energy shows that the damage production is directly linked to the energy deposited in nuclear elastic collisions. The lattice location of helium atoms (generated in large amount during the storage period) in interstitial octahedral positions is discussed.