Although its use has declined in recent decades, nuclear energy remains a viable source for carbon-neutral power. Since the late 1950s, commercial nuclear reactors have provided power to the U.S. electric grid, particularly for use in homes and businesses and in a variety of industrial applications. By the end of 2022, 93 commercial nuclear reactors operated at 55 nuclear power plants in 28 states, supplying about 20% of U.S. electricity since 1990.1
Nuclear reactors produce spent nuclear fuel (SNF) that can no longer sustain a nuclear reaction but remains highly radioactive. Some nuclear power plants in the United States housing SNF are located in coastal areas, where proximity to large bodies of water such as lakes, rivers, or oceans enable plant operators to both power and cool down the facilities.
These coastal sites are often susceptible to salty sea breezes that can, over time, impact steel surfaces, including canisters containing SNF. The chloride salt aerosols deposited on the metal absorb moisture, forming a brine in a process known as deliquescence. These chloride-rich corrosive brines can then result in pitting corrosion.
Additionally, the canisters are potentially prone to stress corrosion cracking (SCC), defined by the Association for Materials Protection and Performance (AMPP) as “the cracking induced from the combined influence of tensile stress and a corrosive environment.”2
To protect steel surfaces from these corrosive forces, researchers at Sandia National Laboratories tested several nickel and nickel-based alloy cold spray coatings to determine their ability to offer corrosion protection for 304L stainless steel in chloride-rich environments. Two Sandia researchers co-led the project: Charles Bryan, a geochemist, and Rebecca Schaller, an engineer and materials scientist.
Handling Spent Nuclear Fuel
Bryan has nearly three decades of expertise working in the nuclear fuel cycle, with the last decade or so spent on the potential corrosion of SNF dry storage canisters. In his role as geochemist, Bryan was charged with understanding the chemistry of the brines that form in chloride-rich environments in order to determine their corrosivity and their impact on canister lifetimes.
SNF is initially stored in pools of water in order to thermally cool it and decrease radioactivity levels. From there, it is moved into stainless-steel dry storage canisters. However, the process for depositing SNF into these canisters is both painstaking and time-intensive, as Bryan explains.
“Once it’s taken out of the reactor, [SNF] is placed in a cool pool for potentially several years or even decades—although for decommissioned plants it may be a very short time interval—and then placed within these stainless-steel dry storage canisters in the pool. The fuel is highly radioactive, so it can’t be taken out of the water because it would endanger the workers. So, all of this is done underwater using robots to transfer the fuel into the canisters. The canisters are sealed and pumped dry, and then a long drying process is used to remove any additional water that’s in the canisters. Then the canisters are placed inside overpacks, which are concrete and steel structures that provide both shielding and protection for the canisters.”
Along with shielding workers from radiation, the overpacks are also passively ventilated to cool the canisters and allow heat to escape. As this happens, however, potentially corrosive compounds and salts form on the canister surface, a fact that concerned Sandia researchers.
“As the canister environment changes, the salts could deliquesce and cause atmospheric corrosion of the steel and potentially stress corrosion cracking,” Bryan notes.
About a decade ago, Sandia researchers began to investigate whether corrosive salts deposited on dry storage containers resulted in SCC. Shortly thereafter, Sandia collaborated with the Electric Power Research Institute (EPRI), an independent, nonprofit energy research, development, and deployment organization that, according to Bryan, played a critical role in getting access to industry sites.
Sandia also worked with nuclear power companies to obtain dust samples from the exteriors of dry storage canisters. When Sandia analyzed the samples, it found chloride salts on all of them, with higher chloride salt concentrations on canisters stored near an ocean.
Using thermal models of the canisters developed by Pacific Northwest National Laboratory (PNNL), a U.S. Department of Energy (DOE) national laboratory, and thermodynamic modeling of brine compositions performed at Sandia, the researchers characterized canister surface conditions, including the temperature, ambient humidity, and chemical composition of the brines formed by deliquesced sea salts.
They also experimentally characterized the corrosivity of those environments using a suite of electrochemical tests, exposure testing, and SCC crack growth experiments. Further research along these lines is ongoing.
Additionally, models were created to study the formation of crack tip conditions and the rate at which those cracks grow. While research conducted over the past decade has not detected a crack caused by SCC, the researchers are not yet willing to disregard its potential presence over an extended period.
Other Sandia staff members who have worked on the geochemistry and coatings evaluation aspects of the project include Andrew Knight and Brendan Nation.
Applying Cold Spray Coatings
In 2020, researchers from Sandia and PNNL tested a variety of cold spray coatings to determine if they could protect 0.5-inch (12.5 m)-thick pieces of stainless steel from chloride corrosion.
Erin Karasz, a then-postdoctoral appointee at Sandia who aided in the cold spray tests, found that when various nickel and nickel-alloy-based coating mixtures were applied as a “patch,” rather than over the entire steel surface, that its porosity (i.e., the “empty” spaces in a material) impacted corrosion behavior. Moreover, with two dissimilar metals interfacing with one another, there is the possibility for galvanic corrosion.
As a Sandia corrosion expert and co-project lead, Schaller has spent much of the past few years on research identifying when and where SCC will occur. She worked alongside a team that also included Ryan Katona, who worked on pit-to-crack and crack growth rate testing.
“When we apply any coating to a canister, one of the first things we have to ensure is that we do no harm to the canister or exacerbate any further processes [like] corrosion that might occur across the surface,” Schaller says. “What we were really trying to do [with this study] is look at accelerated environments that might highlight where we have to tailor specific materials or coatings to the point that they can be protective.”
To further their cold spray experiments, the Sandia research team will conduct additional tests using cold spray and other polymer coatings to see if welded stainless steel will experience stress or corrosion under more relevant atmospheric conditions. Both Bryan and Schaller agree that the primary aim of their work is to understand the controlling factors for SCC and thereby make meaningful contributions to the wider body of corrosion literature and research.
“From a corrosion standpoint, what we’re trying to understand in these environments is what are the significant variables that enhance or reduce the corrosion that’s occurring,” Schaller says. “And if we can identify that, then we can help sites identify whether they’re in a more severe or less severe area [for corrosion].”
Karasz and Schaller were among the co-authors of a paper recently published in Metals and Alloys3 that detailed the research team’s use of cold spray techniques and their potential application as anticorrosive protective coverings for steel. Schaller says that PPNL supplied the cold spray coatings for the experiments. This Sandia research was supported by the DOE Office of Nuclear Energy.
References
1. U.S. Energy Information Administration, “Nuclear explained–U.S. nuclear industry,” https://www.eia.gov/energyexplained/nuclear/us-nuclear-industry.php#:~:text=According%20to%20the%20U.S.%20Nuclear,were%20104%20operating%20nuclear%20reactors (April 18, 2022)
2. AMPP, “Stress Corrosion Cracking (SCC),” https://www.ampp.org/technical-research/impact/corrosion-basics/group-3/stress-corrosion-cracking#:~:text=Stress%20corrosion%20cracking%20(SCC)%20is,fatigue%20threshold%20of%20that%20material
3. E. Karasz, et al., “Accelerated corrosion testing of cold spray coatings on 304L in chloride environments,” Metals and Alloys (November 2022) https://www.frontiersin.org/articles/10.3389/ftmal.2022.1021000/full
Editor’s note: This article first appeared in the October 2023 print issue of Materials Performance (MP) Magazine. Reprinted with permission.