Molecular Dynamics Simulations of Hydrogen Diffusion and Retention Behavior Near Sub-Surface Defects in Tungsten: Towards Predicting Tritium Retention in the ITER Divertor

Molecular Dynamics Simulations of hydrogen diffusion and retention behavior near subsurface defects in tungsten: Towards predicting tritium retention in the ITER divertor

Plasma-facing components within fusion reactors will be exposed to massive particle and thermal loads, which will pose serious material challenges. The plasma-facing component of interest in this project is the divertor, a device that allows for the online removal of the helium reaction product from the deuterium – tritium plasma. The ITER divertor will be constructed from tungsten, which is also a prime candidate for use in future demonstration fusion reactors, because of its superior mechanical properties and low tritium retention. However, recent computational modeling predictions and experimental observations indicate that helium bubbles rapidly form in helium plasma exposed tungsten, and that these bubbles can strongly trap hydrogen. This observation raises concerns about the possibility of larger than expected tritium retention, as well as how these bubbles may degrade the thermo-mechanical properties of the tungsten divertor, and possibly increase rate of tungsten erosion. We report on the use of molecular dynamics simulations to quantify the hydrogen behavior near sub-surface defects and bubbles in tungsten. In this study, we have simulated the dynamic evolution of tungsten containing either voids or helium bubbles to study how the hydrogen diffuses in the vicinity of these defects at 1200 degrees Kelvin. The simulation results will provide insight into the possibility of increased tritium retention within tungsten due to these defects, as well as the possibility that the tritium can be removed through isotope exchange with deuterium. As such, this work will further the understanding of the retention of radioactive tritium within tungsten to estimate the lifetime and possibly reduce gas-driven damage of the divertor in the ITER.