Within GEN-IV reactors, nuclear graphite plays a crucial role as both a moderator and reflector in an environment characterized by high temperatures and intense fast neutron irradiation. The exposure to fast neutron irradiation induces the formation of numerous Frankel defects in the nuclear graphite. These defects undergo processes of annihilation and diffusion, ultimately giving rise to larger defect clusters. This transformation in the microstructure of nuclear graphite directly impacts its macroscopic properties, necessitating a thorough investigation.

This study aims to comprehensively explore the evolution of defects in nuclear graphite under conditions of high-temperature irradiation which is essential for advancing reactor safety.

Firstly, the 30 MeV ¹⁰⁷Ag⁵⁺ ion source was employe to irradiate IG-110 nuclear graphite at 420 ℃, simulating the defect evolution behavior during fast neutron irradiation of nuclear graphite. Then, the energy loss, defect distribution, and ion implantation profiles of 30 MeV ⁵⁸Ni⁵⁺ and ¹⁰⁷Ag⁵⁺ ion beams bombarding standard nuclear graphite ICRU-906 (density of 2.26 g?cm^-3, displacement energy of 28 eV) were calculated using the full cascade damage model in the SRIM (Stopping and Range of Ions in Matter) software. The cross-sectional structure of IG-110 nuclear graphite was characterized using micro-Raman spectroscopy. Finally, the relationship between the Raman spectroscopic features at various depths of IG-110 nuclear graphite and the irradiation damage dose was compared to investigate the evolution of IG-110 nuclear graphite microstructure with increasing irradiation damage dose (Displacements Per Atom, DPA).

With the increase in particle fluence, the characteristic parameters of the Raman spectra of nuclear graphite, including the I_D/I_G ratio (the ratio of the D peak height to the G peak height), the Full Width at Half Maximum of the G peak (FWHM(G)), and the shift of the G peak, all show significant increments. Compared to samples irradiated with ⁵⁸Ni⁵⁺ at the same irradiation damage dose, the graphite Raman spectra irradiated with ¹⁰⁷Ag⁵⁺ demonstrate higher I_D/I_G ratios and FWHM(G). At the same FWHM(G) level, the I_D/I_G ratio of the graphite Raman spectra irradiated with ¹⁰⁷Ag⁵⁺ is greater than that of the samples irradiated with ⁵⁸Ni⁵⁺.

The results of this study suggest that irradiation with heavier ions induces a higher rate of defect accumulation in nuclear graphite, leading to a more rapid reduction in graphite grain size and promoting the progression towards nanocrystallization.