The Science   

The interior of a fusion reactor can be an extreme environment. Not many materials can withstand the high temperatures and intense radiation that occurs during fusion reactions. Tungsten heavy alloys (WHAs) emerged as a promising material for fusion reactors, as they combine the high melting point and thermal conductivity of tungsten with improved ductility and fracture toughness. However, little experimental data exists on WHA behavior in extreme environments. To address this, researchers subjected a tungsten-nickel-iron (W-Ni-Fe) WHA to a simulated fusion environment featuring elevated temperatures and ion irradiations to mimic the damage expected after 5 years of service inside a working fusion reactor. The researchers then analyzed the structure of the materials at the atomic scale, revealing selective embrittlement at the material interfaces caused by carbon impurities.

The Impact

Fusion energy has the potential to be a source of abundant, carbon-neutral energy. However, the extreme environments within fusion reactors can cause immense damage to the materials subjected to them. This research provides crucial insight into understanding the response of WHAs in high temperature irradiation environments by mimicking the damage expected after 5 years of use in a fusion reactor. The results of this study provide an essential understanding of how microstructures within WHAs may evolve in the interior of a fusion reactor and provide a basis for estimating how long these materials can safely withstand the fusion environment based on how well they tolerate high temperature irradiation conditions and impurities.

Summary

WHAs display high melting temperatures, high thermal conductivity, and high fracture toughness, making them an attractive option for fusion reactor materials.  However, little experimental data exists on the response of these materials to the extended high temperature irradiation environment of the reactor interior. Researchers prepared a WHA of 90% tungsten, 7% nickel, and 3% iron (90W-7Ni-3Fe) and subjected it to elevated temperatures and sequential nickel and helium ion irradiations to mimic the damage expected after 5 years of service as materials for plasma facing components.

The researchers performed atomic-scale structural analyses and nanoscale elemental mapping of the irradiated materials. From these experiments, the researchers identified the formation of two distinct precipitation structures, a surface localized η-carbide, and a hexagonal W₂C type tungsten carbide following the exposure of the WHA to the simulated fusion environment. This precipitate formation is anticipated to adversely affect the performance of materials by selective embrittlement at bi-phase interfaces leading to a reduction in the material’s overall fracture toughness during prolonged high temperature irradiation. It is asserted that any potential exposure to carbon during fusion reactor operational service should be minimized to prevent the formation of these phases.

A portion of this work used the facilities of the Environmental Molecular Sciences Laboratory, a Department of Energy, Office of Science user facility located at Pacific Northwest National Laboratory.  

Contact

James (Jacob) Haag

Pacific Northwest National Laboratory

jacob.haag@pnnl.gov

Funding

This work was supported by the Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research program and the Department of Energy, Office of Science, Fusion Energy Sciences program.