Research led by scientists at various institutes in China has delivered key insights into the synergistic damage caused by irradiation and corrosion in molten salt reactors (MSR).
After studying silicon-carbide (SiC) materials at temperatures of nearly 1,400 degree Fahrenheit (750 degrees Celsius), the researcher-derived data can help in safer deployments of fission reactors in the near future.
Molten salt reactors or MSRs are classified as Generation IV nuclear reactors due to their capabilities of high fuel efficiency, low nuclear waste generation, and safer operation. The reactor derives its name from using molten salt as a coolant and a fuel since fissile material is mixed inside the salt.
The reactor design is inherently safe since excessive heat during the reaction process expands the salt and drives it out of the reactor. This negative feedback loop cools down the reactor, reducing the meltdown risks. Yet, the technology must overcome other issues before it can be widely deployed.
The use of salt, extreme temperatures, and the release of neutrons during the fission reaction create some challenging conditions for reactor vessels. To overcome these, ceramic materials such as silicon-carbide (SiC) are used on the structural components of MSRs. These materials are chemically inert, have favorable neutron characteristics, and can withstand high temperatures.
Yet the synergistic effect of irradiation and corrosion, along with high temperatures, is still a problem that needs closer inspection. The synergistic effect can cause hardening of the material, making it brittle and prone to fractures.
While the impact of these conditions is well known on MSRs, the underlying mechanism is not well understood. Researchers from various institutes in China came together to explore this further.
In their study, a research team at the Shanghai Institute of Applied Physics subjected SiC samples maintained at temperatures of 1382 degree Fahrenheit (750 degrees Celsius) to irradiation in two doses, one at 2 x 10^16 ions per cm2 and 1 x 10^17 ions per cm2.
Additionally, irradiated and non-irradiated SiC samples were exposed to FLiNaK molten salt at these extreme temperatures to understand the synergistic damage behavior.
Studying these samples under a transmission electron microscope (TEM), the researchers found that the corrosion of the FLiNAK molten salt formed a carbon-rich phase with a graphite structure in SiC. The team also found that Ni impurities in the salt reacted preferentially with the Si-Si bonds generated due to irradiation and further helped the corrosion process.
"Following the investigation of the number density and size of He bubbles in the C-rich phase and survived SiC in the vicinity of the corrosion boundary, as well as in the SiC region away from the boundary, it was determined that the vacancies resulting from Si loss during corrosion contributed to the migration and coalescence of He bubbles," said Jianjian Li, research lead at Shanghai Institute of Applied Physics in a press release.
"The collective findings offer compelling evidence of the synergistic damage behavior of irradiation and corrosion," Li added.
In addition to helping make molten salt reactors safer, the research findings will help develop SiC fiber-reinforced SiC matrix composites and determine the precise composition of Ni-Si compounds at the corrosion boundaries.