The recent release of the film Oppenheimer has reawakened public interest in the intriguing world of nuclear energy. While the blockbuster film focuses on the world-ending ability that nuclear weapons hold, nuclear power has a more positive role. Nuclear power is a significant player in the global energy landscape, providing over ten percent of the world’s electricity without any carbon emissions. Nevertheless, the infamous Chernobyl Disaster of 1986 has since left a bad taste in the mouths of many, leading them to question the safety of nuclear power systems. Today, scientists are looking to improve this extraordinary power by developing materials that exhibit greater resistance to radiation.
Nuclear energy systems host some of Earth’s most radioactive environments, thanks to the natural phenomenon of nuclear fission. This phenomenon begins when a high-energy particle strikes the nucleus of an atom, causing the atomic nucleus to split in two. This process releases energy in the form of heat and even more high-energy particles (Lewis, 2008). These high-energy particles, known as radiation, pose risks not only to life but also to the structural integrity of nuclear energy systems.
In nuclear energy systems, the primary materials used for structural components, such as the vessels containing the nuclear fuel and coolant systems, are metal alloys: mixtures mainly composed of a metallic element, supplemented with smaller amounts of one or two other elements (George et al., 2019). Much like cooks crafting the perfect recipe, metallurgists blend different elements to create metal alloys tailored to specific needs and conditions.
The right blend is crucial because within nuclear energy systems, metal alloys are subject to an onslaught of high-energy particles produced by nuclear fission. In metal alloys, atoms are arranged in an ordered, repeating structure. When a high-energy particle strikes the metal alloy, it can dislodge atoms from their positions, a process known as displacement (Pickering et al., 2021). Displacement leads to a cascade in which these displaced atoms further knock others out of place, disrupting the alloy’s orderly atomic structure (Pickering et al., 2021). Over time, these defects, known as point defects, can travel in the alloy and combine into larger, more complex defects. This results in corrosion, cracking, and brittleness.
Metal alloys have a natural self-healing ability in which the alloy’s structure can reorganize itself and repair defects in a matter of picoseconds; nevertheless, defects in the atomic structure are not fully corrected and accumulate over time, gradually ruining the alloy’s overall integrity (Pickering et al., 2021). This leads to a pressing question that researchers are attempting to answer: How can we make metal alloys that can heal themselves more effectively after high-energy particle collisions? This is where the ground-breaking work of Dr. Jien-Wei Yeh comes into play.
In 2004, Dr. Jien-Wei Yeh, a researcher at the National Tsing Hua University, devised a novel class of metal alloys that he coined “High Entropy Alloys” (HEAs), which has since ushered in a new era of metallurgy. Dr. Yeh defined HEAs as single-phase metal alloys with five or more principal elements in relatively high concentrations, in contrast to conventional alloys, which consist of one base metal in high concentration and one or two other elements in relatively low concentration (George et al., 2019). Astoundingly, this simple formulation opens up an unbounded new world of metal alloys and enables scientists to develop alloy recipes with highly specialized properties.
HEAs have been shown to have an exceptional ability to self-heal point defects and resist the formation of larger defects. HEAs, due to the high number and variety of constituent elements, have a much more complicated and chaotic atomic structure than conventional alloys. This complex structure allows the atomic structure to reconfigure itself more easily, improving its ability to self-heal point defects. Moreover, the complexity of the atomic structure means that defects become trapped where they are, preventing them from combining into bigger, more problematic structures (Moschetti et al., 2022).
In 2020, researchers in the U.K. discovered that a specific HEA, V2.5Cr1.2WMoCo0.04, displays an extraordinary ability to self-heal radiation damage (Patel et al., 2020). When researchers bombarded the prepared alloy with gold ions, they observed that after radiation exposure, 96% of the mass of the sample retained its original structure, and the other 4% changed into a similar version of the original structure. This means the material retained its structural integrity. Additionally, the alloy became softer after radiation exposure, a clear divergence from conventional alloys, which become hard and brittle after radiation exposure. Generally, softness is preferred in structural components as softer materials can deform under stress without totally failing. All in all, this particular high entropy alloy showed exceptional behavior and is just a taste of the amazing abilities that are possible with the perfect alloy recipe.
Since J. Robert Oppenheimer and his team introduced the world to the might of nuclear energy with the atomic bomb, our capability to harness this power safely and efficiently has come a long way. High entropy alloys will serve a crucial role in enabling safer nuclear systems–ensuring that Chernobyl will never happen again.
References
George, E. P., Raabe, D., & Ritchie, R. O. (2019). High-entropy alloys. Nature Reviews.Materials, 4(8), 515-534. https://doi.org/10.1038/s41578-019-0121-4
Lewis, E. E. (2008). Fundamentals of Nuclear Reactor Physics. Elsevier Inc. https://doi.org/10.1016/B978-0-12-370631-7.X0001-0
Moschetti, M., Burr, P. A., Obbard, E., Kruzic, J. J., Hosemann, P., Gludovatz, B. (2022). Design Considerations for High Entropy Alloys in Advanced Nuclear Applications. Journal of Nuclear Materials, 567, 153814. https://doi.org/10.1016/j.jnucmat.2022.153814
Patel, D., Richardson, M. D., Jim, B., Akhmadaliev, S., Goodall, R., & Gandy, A. S. (2020). Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04. Journal of Nuclear Materials, 531, 152005. https://doi.org/10.1016/j.jnucmat.2020.152005
Pickering, E. J., Carruthers, A. W., Barron, P. J., Middleburgh, S. C., Armstrong, D. E. J., & Gandy, A. S. (2021). High-Entropy Alloys for Advanced Nuclear Applications. Entropy (Basel, Switzerland), 23(1), 98. https://doi.org/10.3390/e23010098
Nuclear energy - The World Factbook. (n.d.). https://www.cia.gov/the-world-factbook/field/nuclear-energy/