AsianScientist (July 7, 2017) – Researchers in Japan have used computer simulations to model the movement of impurities in tungsten used to line the walls of nuclear fusion reactors. Their findings, published in Nuclear Materials and Energy, bring practical fusion reactors one step closer to reality.

Instead of splitting atoms apart like in present day nuclear reactors, nuclear fusion generates energy by fusing atoms together. Although much more technically challenging than nuclear fission, nuclear fusion is safer, generates less radioactive waste and uses readily available fuel materials. Nonetheless, fusion reactors remain impractical despite more than seventy years of intense research.

One of the main problems is finding a suitable material for the vacuum vessel, the part of the system that comes into contact with the fuel materials that have been heated into a plasma state. Tungsten is a promising candidate thanks to its high melting point and high conductivity, properties required to withstand the extremely high heat generated during nuclear fusion and conduct the heat away in a useful manner.

Plasma ions enter the material of the vacuum vessel and remain there as impurities. Seeking to understand the impact of these impurities on the performance of tungsten as a plasma-facing material, researchers from the National Institutes of Natural Sciences developed a method to automatically trace the movements of impurities in tungsten.

Firstly, the team led by Professor Atushi Ito used molecular dynamics to compute the migration paths of impurities in a small domain of the entire tungsten crystal structure. Because the calculations in each small domain can be conducted independently, they performed a large amount of calculations in parallel across three different systems: the NIFS supercomputer, the Plasma Simulator and the HELIOS supercomputer system at the Computational Simulation Centre of International Fusion Energy Research Centre, Aomori, Japan.

On the Plasma Simulator, because it is possible to use 70,000 CPU cores, simultaneous calculations over 70,000 domains can be performed. By adding up all the calculation results from the small domains, the migration paths over the whole material were obtained.

By combining the parallelization with automation, the researchers developed a high-speed automatic search method for determining the migration path of impurities. With this method, it becomes possible to trace the path of impurities in actual materials that have crystal grain boundaries or even materials where the crystal structure has become disordered by long duration contact with plasma. This in turn is expected to improve our ability to control plasma in fusion reactors.

Apart from its implications on nuclear fusion research, the researchers anticipate that their results could be used to develop other plasma processing techniques in fields like semiconductors and surface coating.



The article can be found at: Ito et al. (2017) Automatic Kinetic Monte-Carlo Modeling for Impurity Atom Diffusion in Grain Boundary Structure of Tungsten Material.

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Source: National Institutes of Natural Sciences; Photo: Atsushi Ito.

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