Researchers at the Lawrence Livermore National Laboratory have announced that they observed a net gain in nuclear fusion energy for the very first time at the end of 2022. The research is a huge milestone towards fusion energy that can power millions of homes and businesses with a carbon-neutral energy source. However, converting this achievement into a practical nuclear energy source requires innovative technologies to bring fusion-powered society to life.  

The scientists at the Virginia Polytechnic Institute and State University, and Pacific Northwest National Laboratory are working on making this goal a reality through their efforts in material research. Their recent work published in Scientific Reports included the case of tungsten alloys and showed how the metal could be improved for use in advanced nuclear fusion reactors by copying the structure of a seashell. 

Jacob Haag, the first author of the research, said this is the first study on such material interfaces at too-small length scales. He added they also revealed some fundamental mechanisms that govern the toughness and durability of materials. 

Withstanding the heat

The sun has a core temperature of about 27 million deg Fahrenheit and is powered by nuclear fusion. Thus, the fact that nuclear fusion reactions produce plenty of heat is understandable. Before scientists can harness the energy of these reactions and turn them into power, they need to develop advanced nuclear fusion reactors capable of withstanding high temperatures and irradiation conditions that develop in fusion reactions. 

Tungsten has the highest melting point among all the elements available on planet Earth. This makes it one of the best materials for nuclear fusion reactors. However, the metal can also be brittle, making it possible to mix with other metals. Mixing it with other metals, such as iron and nickel, can help create an alloy tougher than tungsten but retains its high melting properties. 

It is not merely the composition that offers these tungsten alloys their properties but the thermo-mechanical treatment of the metal that leads to the development of toughness and tensile strength. 

Using a specific hot-rolling method, tungsten-heavy alloys were made with microstructures that resemble the mother-of-pearl, or nacre, found in seashells. Nacre is prized for its stunning iridescent colours and amazing strength. The heavy tungsten alloys that approximate nacre were studied by the PNNL and Virginia Tech research teams for potential nuclear fusion applications. 

According to Haag: "We sought to know why these materials have practically unheard-of mechanical capabilities in the realm of metals and alloys."

Examining metal alloy for toughness

To examine the microstructure of the alloys, Haag and his team used advanced techniques like scanning transmission electron microscopy to analyse the atomic structure of the alloy. In addition, they also worked on mapping the nano-scale composition of material by combining atom probe tomography and energy dispersive X-ray spectroscopy. 

The heavy tungsten alloy is composed of two separate phases that coexist inside the nacre-like structure: a 'hard' phase that is nearly pure tungsten and a 'ductile' phase that is composed of a combination of nickel, iron, and tungsten. The results of the study point to an excellent link between the different phases, including the closely coupled 'hard' and 'ductile' phases, as the source of the high strength of tungsten-heavy alloys.

According to Wahyu Setyawan, a computational scientist at PNNL and a co-author of the study: "The two different stages generate a tough composite, but they pose major hurdles in generating high-quality specimens for characterisation. This allowed us to expose the precise structure of interphase boundaries and the chemical gradation across these boundaries, thanks to the superb work of our team members."

The work shows how strong material interactions in heavy tungsten alloys are influenced by the structure of crystals, geometry, and chemistry. It also shows ways to enhance the design and characteristics of materials for fusion applications.

The safety and durability of these bi-phase alloys must be optimised if they are to be utilised inside nuclear reactors, according to Haag.

Building the next-generation materials for nuclear fusion

The results of the study are already being expanded in multiple dimensions within PNNL and the community of scientific research. PNNL is also working on multiscale material modelling to optimise the structure and test the strength of materials with dissimilar interfaces. In addition, they are working on observing how these materials behave under extreme temperatures and irradiation conditions in a fusion reactor. 

Setyawan added that it's exciting for their team to work on fusion energy as the White House and private sector has shown interest in the research work. The research they do in discovering the material solutions for prolonged operations is required to accelerate the realisation of fusion reactors.