Corrosion Avoidance Seen as Key to the Future of Fusion

Experts Chip Away at Corrosion for Fusion's Future Practical fusion energy is more than just a dream at Energy's Oak Ridge National Laboratory. Experts in fusion and materials science are working together to develop solutions that will enable a fusion pilot facility and ultimately carbon-free, abundant fusion electricity. As head of the Fusion Nuclear Science, Technology and Engineering Division at the laboratory, Chuck Kessel is familiar with the materials challenges that must be addressed to build a power plant. Kessel did not need to look to anyone other than Bruce Pint, head of ORNL's Corrosion Science and Technology Group, to work with. Pint has worked for decades on corrosion-resistant, high-temperature materials for energy production applications. His work has largely focused on gas-metal or alloy corrosion and oxidation for coal, gas and nuclear power plants. Examining corrosive liquids in the context of fusion energy represents a different and more challenging struggle. Pint said, "Everything involves a little bit of science and a little bit of art." A critical challenge for fusion is how to produce and recover tritium, a heavy hydrogen isotope that, together with its lighter cousin deuterium, will serve as fuel for tomorrow's fusion reactors. In a fusion reaction, these isotopes are heated to sun-like temperatures in a plasma where they collide to create helium and a neutron, releasing energy in the form of kinetic energy. Scientists can produce tritium inside the reactor by directing these accelerating neutrons at the more common metal lithium. A promising strategy to produce tritium in a fusion reactor involves channeling liquid lead-lithium through the reactor "blanket" - inner walls made of special steel with silicon carbide flow channel inserts. However, a problem arises: the continuous lead-lithium flow will slowly eat away at the steel. Minimizing corrosion is a crucial step for a viable fusion power plant. Kessel said, "Having this kind of blanket with a fluid flowing through it that corrodes these materials is basically limited by this corrosion mechanism." Marie Romedenne, who worked on liquid metals for her doctorate and joined ORNL in 2019, is helping Pint and learning more about ORNL's liquid metal experimental methods used since the 1950s. Many factors contribute to corrosion rates, including the composition of the exposed materials. These factors include the composition of the exposed materials, how long they are exposed, how fast the fluid flows, the strong magnetic fields used to control and confine the plasma, temperature and impurities in the system. This corrosion problem gave Pint and Romedenne the opportunity to plan several experiments designed to address these factors while approaching the conditions of a real fusion reactor. The team created a series of flow loops that tested materials under various conditions, including temperatures up to 700 degrees Celsius. Scientists placed steel samples similar to those that would be used for components in a fusion device into the loop, as well as silicon carbide samples. According to current fusion designs, silicon carbide electrically isolates the liquid from the steel walls, reducing the pressure drop in the lead-lithium flow. This approach supports three materials that coexist and interact with the lead-lithium mediating between steel and silicon carbide. After each 1,000-hour experiment, samples were tested to see if they had become brittle and how much mass they lost by dissolving in liquid lead-lithium or, alternatively, whether they were added to by newly formed compounds. [caption id="attachment_144212" align="aligncenter"] This steel sample was used in corrosion studies. Image source:
ORNL, U.S. Department of Energy[/caption] [caption id="attachment_144213" align="aligncenter"] Bruce Pint from ORNL, left, and Marie Romedenne review experimental results. Image source: ORNL, U.S. Department of Energy[/caption] In the first experiment, Pint and Romedenne found that iron and chromium in the steel dissolved in the liquid and subsequently reacted with the silicon carbide samples to form intermetallic compounds, silicides and iron and chromium carbides. These newly formed compounds, flowing through the loop, accumulated on the silicon carbide samples at the cooler end of the loop, resulting in a relatively thick layer. Pint said, "It was actually quite remarkable - several hundred microns thick." "I thought it could react a bit. I didn't expect it to react that much." Pint and Romedenne also discovered that reducing the loop's high temperature from 700 to 650 degrees Celsius resulted in much slower accumulation of the newly formed compounds. Pint said, "If you only have silicon carbide and no source of iron and chromium to put in the liquid, you don't see this reaction." "No one had ever put all the pieces together before." Because iron and chromium reacted with silicon carbide, the lead-lithium corroded the steel samples significantly. "After the test was done, they were barely there," he said. In the second experiment, the team coated the steel with a thin aluminum layer to protect it from the corrosive liquid, which was done in a flow experiment for the first time. Pint said the results were encouraging. Pint said, "Despite trying to combine everything as much as possible, corrosion still continues." "But we brought things down to a more manageable level. None of our coated steel samples suffered significant damage." Pint and Romedenne plan to use a thinner aluminum coating in upcoming experiments to minimize how much of this element is consumed in the system. They also plan to double the length of the experiments and increase them to 2,000 hours to better examine the growth of the reactive layer on the cold side of the loop. Romedenne, going beyond the limitations of their experimental loops, uses models and simulations to predict the corrosion life of fusion materials over industrial timescales (50,000 hours or more). However, continuous experiments and new test environments are needed to validate and improve these models. Kessel is now laying the groundwork for the development of an advanced flow loop that will have magnets to help measure the effect of magnetic fields on corrosion rates. Kessel said, "We want to create as prototypical an environment as possible to allow us to identify, demonstrate and optimize real solutions for a fusion pilot facility." This research was funded by the DOE Fusion Energy Sciences program. UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science works to address some of the most pressing problems of our time.   Source: https://www.ornl.gov/news/experts-chip-away-corrosion-future-fusion