A team of researchers in the USA has come up with a revolutionary method capable of tracking chemical changes in molten salts – ionically bonded chemicals with high melting ranges – in real time. This innovative approach is expected to advance the development of molten salt reactors for energy production.
The scientists at Oak Ridge National Laboratory (ORNL) explain how when heated beyond its melting point, salt dissolves uranium, creating a mixture that serves as both the coolant and the fuel in a molten salt reactor.
However, while the federally funded research and development center operated two experimental molten salt reactors in the last century, neither remains in operation today.
In recent years, the interest in molten salt reactors has resurged due to their safety, efficiency, and ability to produce radioisotopes, yet, their intricate chemistry requires advanced sensing technologies for precise monitoring.
Now, to address this challenge, the researchers used laser-induced breakdown spectroscopy (LIBS) to measure elements and identify isotopes in molten salt for the first time.
Exploring LIBS technology for molten salt reactor
With LIBS, a form of atomic emission spectroscopy that uses a high-energy laser pulse as an excitation source, the laser is directed at the material, generating plasma that emits light.
In turn, analyzing this emitted light allows scientists to identify and quantify the elements and isotopes in the salt. In the current study, the research team utilized a modular LIBS system, allowing multiple spectrometers to collect data at the same time.
Researchers are searching for the ideal characteristics of molten salt, which can serve as both coolant and fuel in advanced nuclear reactors.
Credit: Argonne National Laboratory
“LIBS has been used before for investigating the elemental composition of solid samples like plant roots, solid nuclear fuel and geological samples; however, these samples do not change in time,” Hunger Andrews, PhD, ORNL research and development staff member and lead author of the study, says. “Here, we wanted to demonstrate the combined elemental and isotopic power of LIBS and harness its rapid measurement speed on the scale of milliseconds.”
Andrews explains that he and his team used LIBS to measure various elements an isotopes in a molten salt by first combining sodium nitrate (NaNO₃) and potassium nitrate (KNO₃). They then heated the mixture to 662 degrees Fahrenheit (350 degrees Celsius) and used argon gas to send two isotopes of hydrogen through the molten salt.
Results and evaluation
The measurements allowed the team to estimate the diffusion rate – the speed at which the gases spread through the molten salt – as well as how much gas the salt could hold.
This helped them thoroughly analyze the chemical reactions and determine how well the gas dissolves in the salt. Additionally, LIBS successfully distinguished between hydrogen and water in the gas by simultaneously detecting oxygen.
“We’ve performed several proof-of-concept experiments with LIBS to track aerosols and gases, finding it extremely insightful,” Andrews says in a press release.
“By making the jump to real molten salts, we were able to demonstrate in a more realistic system how LIBS could not only be used by researchers to better understand their experiments, but also monitor a reactor,” he concludes.
Unlike light-water reactors, which use water to cool fuel rods and slow down neutrons to sustain the chain reaction, molten salt reactors use liquid fuel and coolant that continuously circulate through the reactor’s core.
This means electricity can be generated more efficiently while radioisotopes are extracted during operation, with the new LIBS spectroscopy method enabling real-time isotope measurement in molten salt.
The study has been published in the Journal of the American Chemical Society.