Polygonization of single crystals of the fluorite-type oxide UO2 due to high dose ion implantation

A transition of the type single crystal-polycrystal has recently gained large interest for the fluorite-type oxide UO2. UO2 is todays fuel for electricity producing nuclear power stations. It is used as sintered product with a typical grain size of 10 to 15 μm. Above a certain level of damage and/or fission products, a grain subdivision process is seen causing each grain to subdivide into 104 to 105 subgrains (so-called RIM effect). A parametric study with UO2 single crystals and in situ ion implantation/channeling analysis was performed to investigate the basic mechanisms of this phenomenon. For most experiments, Xe ions of up to 300 keV energy and up to doses of 1x1017 ions/cm2 were used. At a given critical dose, dramatic changes of the damage peak in the channeling spectra were seen. Though these peaks eventually reached the random yield, analysis of the peak shapes in X-ray diffraction measurements (Omega scans) and channeling angular scan measurements proved that polygonization (rather than amorphization) had occurred causing a fine-grained polycrystalline structure with a misalignment between grains of a few degrees only. The mechanism of this process is discussed in terms of overpressurized gas bubbles causing cleavage and microfractures and the relevance to the technological application is treated. ((orig.))