If you are an engineer, you must be very well aware of CRUD, yeah its a metallic oxide particles in the nuclear energy world, which build up directly on reactor fuel rods, impeding the plant’s ability to generate heat. These foulants cost the nuclear energy industry millions of dollars annually.
This issue has vexed the nuclear energy industry since its start in the 1960s, and scientists have only found ways to mitigate, but not cure, CRUD buildup. But that may be about to change.
The recent paper of authors describes about their work, which offers a novel approach to design fouling-resistant materials for use in nuclear reactors and other large-scale energy systems.
The team’s research goes beyond theory and lays out specific design principles for anti-foulant materials.
Short and his students built a flow loop (a way of recreating reactor conditions without radiation), and conducted a series of experiments to see which materials encouraged, and which discouraged, the growth of CRUD.
Researchers have floated a host of surface forces as candidates for causing the stickiness behind CRUD: hydrogen bonding, magnetism, electrostatic charges. But through experimentation and computational analysis, Short and his team began to suspect an overlooked contender: van der Waals forces. Discovered by 19th-century Dutch physicist Johannes Diderik van der Waals, these are weak electric forces that account for some of the attraction of molecules to each other in liquid, solids, and gases.
Then came a major breakthrough: Carlson recalled a 50-year-old equation developed by Russian physicist Evgeny Lifshitz that he had come across during a review of materials science literature.
If you dont know about Lifshitz’s theory let me tell you, it describes the magnitude of van der Waals forces according to electron vibrations, where electrons in different materials vibrate at different frequencies and at different amplitudes, such as the stuff floating in coolant water, and fuel rod materials. His math tells us if the solid materials have the same electronic vibrations as water, nothing will stick to them.
And that’s what short and his team thought.. What if cladding, the outer layer of fuel rods, could be coated with a material that matched the electronic frequency spectrum of coolant water, yeah, then these particles would slip right past the fuel rod.
The researchers got to work demonstrating that van der Waals was the single most important surface force behind the stickiness of CRUD. In search of a simple and uniform way of calculating materials’ molecular frequencies, they seized on the refractive light index—a measure of the amount light bends as it passes through a material. Shining calibrated LED light on material samples, they created a map of the optical properties of nuclear fuel and cladding materials. This enabled them to rate materials on a stickiness scale. Materials sharing the same optical properties, according to the Lifshitz theory, would prove slippery to each other, while those far apart on the refractive light scale would stick together.
Fig: Multi-foulant materials used by short and his team. Amorphous 2% fluorine-doped tin oxide, crystalline SiO2, CaF2, and Na3AlF6, which all nearly match the refractive index spectrum of water, successfully resisted adhesion of six diverse foulant materials in aqueous AFM measurements.
By the end of their studies, as the paper describes, Short’s team had not only come up with a design principle for multi-foulant materials but a group of candidate coatings whose optical properties made them a good (slippery) match for coolant fluids. But in actual experiments, some of their coatings didn’t work. Yeah, it wasn’t enough to get the refractive index. Materials need to be hard, resistant to radiation, hydrogen, and corrosion, and capable of being fabricated at large scale.
Additional trials, including time in the harsh environment of MIT’s Nuclear Reactor Laboratory, have yielded a few coating materials that meet most of these tough criteria. The final step is determining if these materials can stop CRUD from growing in a real reactor. It is a test with a start date expected next year, at an Exelon commercial nuclear plant.
References: Cigdem Toparli et al. Multi-Foulant-Resistant Material Design by Matching Coating-Fluid Optical Properties, Langmuir (2020). DOI: 10.1021/acs.langmuir.9b03903 link: https://pubs.acs.org/doi/abs/10.1021/acs.langmuir.9b03903