Nuclear reactors are traditionally powered with dense fuel rods that can produce about one gigawatt of carbon-free electricity daily, enough to power about 100,000,000 lightbulbs. Newer power plant designs using molten salt for cooling instead of the water found in traditional reactors could offer even better efficiency and stability, but they face a problem - the extreme chemical environment created by the molten salt can corrode the metal comprising the reactor.
A team led by engineers at Penn State found that adjusting the subtle atomic arrangement of structural metals can significantly affect the rate and extent of this corrosion, even with identical baseline chemical compositions. They did this by creating a series of reactive simulations to isolate and study this corrosion mechanism. Their findings are available online now, ahead of publication in the August issue of Corrosion Science.
Nuclear power plants use fission reactions to produce electricity, traditionally controlling these powerful reactions by submerging uranium, packed together in dense rods, into a large vat of water. Fission produces large amounts of heat, so the water stops the fuel rods from overheating while also producing large amounts of steam, which then turns turbines to generate power.
Molten salt reactors could eliminate the need to cool and power the fission reaction separately. A prevalent design utilizes liquid fuel, usually uranium, blended with a mixture of molten salts, removing the need to pack the fuel into rods and immerse them into a highly pressurized pool of water. This improves reactor stability and safety, while also meaning fuel and waste can be continuously cycled in and out of the system while the reactor runs, improving efficiency.
Despite potential efficiency and safety improvements, Miaomiao Jin, assistant professor of nuclear engineering at Penn State and co-author on the paper, explained that these reactors run much hotter than traditional designs. Local temperatures inside can exceed 800 degrees Celsius, or about 1,500 degrees Fahrenheit. Molten salts at this temperature remain chemically stable, but they can start to significantly corrode the metal alloys making up the structural components of the reactor.
"Maintaining material stability is a challenge for any reactor, but molten salt reactors specifically struggle with corrosion," Jin said. "Many factors play a role in this corrosion, many of which we don't entirely understand, including the temperatures, the specific salts or even the chemical makeup of the materials at the microstructure level."
According to Hamdy Arkoub, a nuclear engineering doctoral candidate and co-corresponding author on the paper, previous work had sought to better understand why and how this corrosion happens. In 2024, the team introduced a highly detailed chemical model of how FLiNaK salt, a popular salt used in molten salt reactors, interacts with nickel and chromium alloy, or nichrome - a heat-resistant metal used in reactor construction.
This model has allowed the team to build highly detailed simulations of how the elements making up nichrome, specifically chromium, corrode when exposed to FLiNaK salt under different levels of mechanical stress or even with changes to the orientation of the metal's surface.
"The high temperatures and the presence of radiation in these reactors make it difficult to experimentally study how corrosion starts and propagates in nichrome," Arkoub said. "Our work aims to use modeling and simulation to fill in the gaps."
A characteristic that had been observed through these studies, but Arkoub said wasn't understood, was that individual samples of nichrome seemed to be much more prone to corrosion, even when compared to other alloys with the same chemical composition. This relationship had been previously connected with mechanical stress, but the team wanted to investigate if the microscopic structure of the metal itself was playing a role.
Usually, the chromium atoms in nichrome scatter randomly throughout the alloy. A process known as atomic ordering, where the alloy is specially heated and processed, can influence this sorting and sway how atoms travel, or percolate, and arrange in the material. This can be manifested at the short-range, where just a few individual atoms are adjusted to create clustered pockets of interconnected energy pathways, or at the long-range, where the entire atomic makeup of the material is specifically tailored to create a network of patterned connections throughout the alloy.
