The structure evolution in neutron-Irradiated nuclear graphite and post-annealing

Abstract

The neutron irradiated microstructure and performance of nuclear graphite as a neutron moderator and structural materials are directly related to the safety and longevity of reactors. Nuclear-grade isotropic graphite IG-110U and ETP-10 were irradiated at 200 °C with low-dose (1.38 × 1023 n/m2, 0.02dpa) and high-dose (1.92 × 1024 n/m2, 0.25dpa) neutron and high-temperature annealed at high temperatures up to 1400 °C, with the whole process analyzed using X-ray diffractometer and CARBONX program. After neutron irradiation of both doses, the c-axis lattice constant <c> became significantly larger, while the a-axis lattice constant <a> only changed a little. Although the crystallite size along c-axis Lc and the number of internal atom layers M both increased significantly, the crystallite size along a-axis La changed slightly for the low-dose samples. However, all three parameters decreased significantly after high-dose irradiation. During the subsequent high-temperature annealing, there were "recovery" points where both the microscopic parameters and macroscopic dimensions were restored to the initial unirradiated value and kept mostly constant. The movement and recombination of interstitial atoms, vacancies and their clusters played a major role in this annealing process. However, the crystallite sizes of two graphites irradiated with high-dose neutrons did not reached their initial values even with annealing at 1400 °C, mainly due to the unrecoverable defects, such as interstitial loops, pores, and microcracks.