Dynamic Simulation of Multiplier Effects of Helium Plasma and Neutron Irradiation on Microstructural Evolution in Tungsten

Abstract

Plasma-facing and high heat flux materials in a fusion reactor suffer two types of damage: displacement damage caused mainly by highenergy neutrons and surface damage, such as erosion, sputtering, and blistering, caused by hydrogen and helium plasma. Usually, these two kinds of damage are investigated separately. In the present study, multiplier effects of helium plasma and neutron irradiation on microstructural evolution in tungsten were investigated using computer simulations based on a rate theory. Neutron irradiation with 10 À6 dpaÁs À1 at 873 K and a helium flux of 10 18 m À2 Ás À1 with 10-keV energy were used as typical irradiation conditions. The effects of irradiation temperature and defect production rate on the interaction between helium and defects were also investigated. The simulation results revealed the rapid diffusion of helium in tungsten. The helium concentration was saturated in tungsten 0.067 mm thick within 0.01 s at 873 K, and the formation of heliumvacancy clusters was nearly uniform in the matrix except at the surfaces facing towards and away from the irradiation after 0.01 s. This meant that the 10-keV helium plasma could enhance formation of helium-vacancy clusters in the region without damage produced by helium. The concentration of 6He-V clusters reached a very high level (10 À6 ) even after a 1-s irradiation with a defect production rate of 10 À6 dpaÁs À1 at 873 K. The concentration of helium-vacancy clusters decreased with increasing irradiation temperature and decreasing defect production rate. The accumulation of helium and the formation of helium-vacancy clusters depended on not only the vacancy concentration, which was determined by the irradiation temperature and defect production rate, and helium concentrations, but also irradiation time.

Accumulation of helium in tungsten irradiated by helium and neutrons

“…These results suggest that injected helium interacts with lattice defects produced by the neutron irradiation in the fusion reactor. Recent simulation results based on a simple model using rate theory, where the diffusion of helium and the point defects produced by neutron irradiation were considered, agreed with the suggestion and indicated that helium was trapped by vacancies to form helium-vacancy clusters in tungsten even in the region deeper than the range of helium [7]. In order to gain a better understanding of the performance of tungsten under the ITER condition, in the present work, we focused on the damage evolution in tungsten with simultaneous irradiation by helium and neutrons.…”

“…In the case of 1 keV-helium irradiation, defect production by the helium is taken into account, which is different from the model used in Ref. [7], but there are no displacement events for the 30 eV-helium irradiation. In contrast to the uniform damage produced by neutron irradiation, injected helium and associated damage is located close to the incident surface.…”

Interaction of tritium plasma and defects in tungsten irradiated with neutrons

“…Tritium, interstitials, vacancies and their clusters with tritium could migrate freely in the matrix, and their concentrations at both surfaces are zero at all times. The time evolution of the concentration of point defects, defect clusters and tritium were calculated using dynamic rate theory [6][7][8] with the following assumptions:…”

Effects of edge dislocations on interstitial helium and helium cluster behavior in α-Fe

“…Because of its extremely low solubility in materials, helium precipitates into bubbles by absorbing vacancies, which deteriorate the mechanical properties of materials [1][2][3][4][5]. The interaction between He and irradiation-induced defects, especially vacancies, has been investigated [6,7]. A molecular dynamics study [8] has shown that a single interstitial He atom can easily migrate through a perfect a-iron lattice owing to its small migration energy of 0.08 eV.…”