An Insight into Annealing Mechanism of Graphitized Structures after Irradiation

Abstract

Graphite components are constantly subjected to a combined actions of annealing and irradiation due to high temperatures and thermal effects caused by irradiation when reactors are operating, resulting in complex microstructures along with matching changes in the material properties and dimensions. This study uses random primary knock-on atom (PKA) cascades to create different types of irradiation-induced damage structures, with the goal of investigating the evolution process of irradiation defects at different annealing temperatures and their impact on graphite crystal dimensions and elastic properties. The findings suggest that both reactor-temperature annealing and high-temperature annealing can quickly restore isolated self-interstitial atoms and small-sized point defect clusters produced by irradiation types such electron irradiation to intralayer locations, mitigating the dimension contraction and stiffness reduction along to the a-axis of graphite crystal caused by vacancies. However, multi-interlayer penetration damages and localized point defect aggregation from high-energy irradiation can lead to the disappearance of layered structural information, which allows these damage structures to develop into interlayer dislocations during annealing process. These interlayer dislocations further exacerbate the interlayer expansion of graphite crystal. Fully amorphized regions can only regain a layered structure approximately when guided by residual layered structures, while chaotic regions or disordered layered structures form in the absence of guidance. The study provides insights into the transformation of initial irradiation defects into different types of defects under different temperatures and damage states, which serve as a critical foundation for assessing the evolution of irradiation defects in nuclear graphite for reactor applications and annealing studies.

Keywords: Nuclear graphite, neutron irradiation, Molecular dynamics, Annealing evolutionary mechanisms