Materials tested at 1,400°F to make molten salt reactors safer

Research led by scientists from various institutes in China has provided important insights into the synergistic damage caused by radiation and corrosion in molten salt reactors (MSR).

After studying silicon carbide (SiC) materials at temperatures of nearly 1,400 degrees Fahrenheit (750 degrees Celsius), the data obtained by the researchers may contribute to safer use of fission reactors in the near future.

Molten salt reactors, or MSRs, are classified as Generation IV nuclear reactors due to their high fuel efficiency, low nuclear waste generation, and safer operation. The reactor’s name derives from the use of molten salt as a coolant and fuel, as fissile material is mixed in the salt.

The reactor design is inherently safe because excessive heat during the reaction process expands and expels the salt from the reactor. This negative feedback loop cools the reactor, reducing the risk of a meltdown. However, the technology still needs to overcome other problems before it can be used widely.

Challenging conditions

The use of salt, extreme temperatures and the release of neutrons during the fission reaction present some challenging conditions for reactor vessels. To overcome these, ceramic materials such as silicon carbide (SiC) are used for the structural components of MSRs. These materials are chemically inert, have favorable neutron properties and can withstand high temperatures.

However, the synergistic effect of radiation and corrosion and high temperatures is still a problem that requires further study. The synergistic effect can lead to a hardening of the material, making it brittle and prone to breakage.

While the impact of these disorders on MSRs is well known, the underlying mechanism is not well understood. Researchers from various institutes in China came together to study this further.

Examination of SiC at 1,400 Fahrenheit

In their study, a research team at the Shanghai Institute of Applied Physics subjected SiC samples held at temperatures of 1382 degrees Fahrenheit (750 degrees Celsius) to irradiation in two doses, one with 2 x 10^16 ions per cm2 and one with 1 x 10^16 ions per cm2 17 ions per cm2.

Additionally, irradiated and unirradiated SiC samples were exposed to the FLiNaK molten salt at these extreme temperatures to understand the synergistic damage behavior.

By examining these samples under a transmission electron microscope (TEM), the researchers found that 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 created by the irradiation, further supporting the corrosion process.

“After examining the number density and size of the He bubbles in the C-rich phase and in the surviving SiC near the corrosion boundary, as well as in the SiC region away from the boundary, it was found that the vacancies result from this.” The Si loss during The corrosion contributed to the migration and fusion of He bubbles,” said Jianjian Li, research leader at the Shanghai Institute of Applied Physics, in a press release.

“The collected results provide compelling evidence for the synergistic damage behavior of radiation and corrosion,” Li added.

The research results not only help make molten salt reactors safer, but also help develop SiC fiber-reinforced SiC matrix composites and determine the precise composition of Ni-Si compounds at the corrosion boundaries.

The research results were published in Journal for Advanced Ceramics.