Study tracks chromium chemistry in irradiated molten salts

"Molten salt reactors are an emerging technology for safer, scalable nuclear energy production. These advanced reactors can operate at higher, more efficient temperatures than traditional water-cooled reactor technologies while maintaining relatively ambient pressure," explained James Wishart, a distinguished chemist at Brookhaven Lab and leader of the research.

Unlike water-cooled reactors, molten salt reactors use a coolant made entirely of positively and negatively charged ions, which remain in a liquid state only at high temperatures. It's similar to melting table salt crystals until they flow without adding any other liquid.

"To assure the long-term reliability of these new reactors, we have to understand how molten salts interact with other elements in a radiation environment," Wishart said.

The scientists were particularly concerned with tracking chromium, a frequent constituent of the metal alloys proposed for molten salt nuclear reactors.

"Chromium tends to be the easiest element to corrode from most alloys and will ultimately accumulate in the coolant of molten salt reactors," Wishart said.

As chromium dissolves into the molten salt, some of its chemical forms can accelerate corrosion processes, compromising the structural integrity and performance of the reactor. The distribution of chromium ion oxidation states -- how many electron vacancies these ions have available for chemical reactions -- may be the factor that determines whether corrosion occurs.

"The presence of dissolved trivalent chromium [Cr³⁺, with three electron vacancies] can accelerate corrosion in some cases, whereas divalent chromium [Cr²⁺, with just two vacancies] does not," Wishart said.

Since chromium is stable as both Cr³⁺ and Cr²⁺ in most molten salts, "it is essential to understand how Cr³⁺ and Cr²⁺ react chemically with species produced in the radiation field of a reactor, and what products they make," explained Wishart.

Brookhaven Lab is the perfect place to investigate these processes because it houses facilities that can trigger radiation-induced reactions and track them in real time, from trillionths of a second to minutes. These facilities are the Laser Electron Accelerator Facility and the two-million-electron-volt Van de Graaff accelerator, both in the Lab's Chemistry Division.

Wishart and his collaborators used these facilities to measure the rates and temperature dependencies of reactions of the two types of chromium ions with reactive species generated by radiation in molten salt. "Our analysis indicated that the net effect of radiation in the molten salt environment is to favor the conversion of corrosive Cr³⁺ to less-corrosive Cr²⁺," Wishart said.

This research was a product of the Molten Salts in Extreme Environments Energy Frontier Research Center established at Brookhaven National Laboratory by the DOE Office of Science in 2018 to explore the fundamental properties and potential applications of molten salts in nuclear environments.