Characterization the microstructure and defects of matrix graphite irradiated with Xe ions

Abstract

The matrix graphite of pebble fuel elements was irradiated with 1MeV Xe ions at room temperature to fluences of 5.8×1014ions/cm2 and 2.9×1015ions/cm2, respectively. The microstructure and defects of matrix graphite samples were characterized by using scanning electron microscopy (SEM), Raman spectroscopy and slow positron beam techniques. The SEM result reveals that hundred-nanometer sized pores appear at the surface after irradiation and the density of pore increases with fluence. Raman results show that D peak (1350cm−1) and G peak (1580cm−1) are broadened after irradiation. In addition, the G peak position shifts from 1580cm−1 to 1560cm−1 with the linewidth increases from 21cm−1 to 132cm−1, corresponding to the increase in bond-angle disorder as the matrix graphite transforms from microcrystalline to amorphous carbon(a-C). The slow positron beam study shows that the defects-trapped positron S parameter increases with fluence, suggesting that the vacancy-type defects concentration or size of open volume defects increases. The analysis of Raman and slow positron beam consistently conclude that the reason for the phase transition after irradiation is the increase in irradiation-induced vacancy defects accompanied by the overlap of disordered regions.