Molecular dynamics simulations of deuterium bubble evolution in tungsten under neutron irradiation and non-irradiation conditions

Abstract

Tungsten (W), the wall material in Tokamak fusion reactors, is subjected to irradiation from high-energy neutrons and high-flux deuterium (D) plasma. In particular, in the presence of deuterium bubbles, neutron irradiation can significantly exacerbate the deterioration of the mechanical properties of the material. However, few studies have focused on the effect of neutron irradiation on deuterium bubbles in tungsten, and the mechanism of deuterium bubble evolution under neutron irradiation remains unclear. In this study, we simulate the evolution of D bubbles using a molecular dynamics approach, revealing a complete evolutionary process: standing, expanding, and bursting. In addition, the results of primary knock-on atom (PKA) irradiation of D bubbles show that high-energy neutrons transfer energy to the D atoms inside the bubbles through collision cascades, leading to a surge in the total pressure inside the bubbles, which contributes to the evolution of the D bubbles. Temperature, D atom to vacancy ratio (D/V), and PKA energy determine the behavior of D bubbles in tungsten. The effects of these three variables on the evolution of D bubbles are summarized in detail by counting the states of the D bubbles and comparing the changes in pressure and volume inside the D bubbles under specific conditions. We observed a modification process by neutron irradiation on a pure W surface with collision cascades generating Frenkel defect pairs within the W lattice. By analyzing the concentration changes, the evolution of Frenkel defects is divided into three stages: growth, elimination, and stabilization. The present work proposes a possible mechanism to explain the evolution of D bubbles stimulated by PKA cascades induced by neutron irradiation, providing an important theoretical guideline for the improvement and selection of optimal materials oriented towards the plasma wall.