Behavior of fission gases in nuclear fuel: XAS characterization of Kr in UO$_2$

International audienceX-ray Absorption Spectroscopy (XAS) was used to study the behavior of krypton as a function of its concentration in UO$_2$ samples implanted with Kr ions. For a 0.5 at.% krypton local concentration, by combining XAS results and DFT + $U$ calculations, we show that without any thermal treatment Kr atoms are mainly incorporated in the UO$_2$ lattice as single atoms inside a neutral bound Schottky defect with O vacancies aligned along the (100) direction (BSD1). A thermal treatment at 1273 K induces the precipitation of dense Kr nano-aggregates, most probably solid at room temperature. In addition, 26 ± 2% of the Kr atoms remain inside BSD1 showing that Kr-BSD1 complex is stable up to this temperature. Consequently , the (in-)solubility of krypton in UO$_2$ has to be re-evaluated. For high Kr concentration (8 at.%), XAS signals show that Kr atoms have precipitated in nanometer-sized aggregates with internal densities ranging between 4.15(7) g cm$^{-3}$ and 3.98(5) g cm$^{-3}$ even after annealing at 873 K. By neglecting the effect due to the UO$_2$ matrix, the corresponding krypton pressures at 300 K were equal to 2.6(3) GPa and 2.0(2) GPa, respectively. After annealing at 1673 K, regardless of the initial Kr concentration, a bi-modal distribution is observed with solid nano-aggregates even at room temperature and larger cavities only partially filled with Kr. These results are very close to those observed in UO$_2$ fuel irradiated in reactor. In this study we show that a rare gas can be used as a probe to investigate the defect creation and their stability in UO$_2$