Scientists at Oak Ridge National Laboratory have developed a method that can track chemical changes in molten salt in real time - helping to pave the way for the deployment of molten salt reactors for energy production.
Salt, heated above its melted point, is used to dissolve uranium. The resulting mixture acts as both the coolant and fuel inside a molten salt reactor. ORNL operated two experimental molten salt reactors in the 1950s and 1960s, but the United States has none in operation now. While their safety, efficiency and potential to generate radioisotopes has brought renewed interest in recent years, their complex chemical makeup means new sensing technologies are needed to monitor the reactor's chemical state.
The article "Real-Time Elemental and Isotopic Measurements of Molten Salt Systems through Laser-Induced Breakdown Spectroscopy," published in the Journal of the American Chemical Society , describes the scientists' use of laser-induced breakdown spectroscopy, or LIBS, to measure elements and identify isotopes in molten salt for the first time.
With LIBS, a laser is focused into the material to create a plasma that gives off light. By analyzing the emitted light, scientists can identify and quantify the elements and isotopes in the salt. In this case, scientists used a modular LIBS platform that allowed several spectrometers to capture information simultaneously.
"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," said ORNL staff scientist Hunter Andrews, who wrote the paper with ORNL's Zechariah Kitzhaber, Daniel Orea and Joanna McFarlane. "Here, we wanted to demonstrate the combined elemental and isotopic power of LIBS and harness its rapid measurement speed on the scale of milliseconds."
Scientists at ORNL were able to use LIBS to measure various elements and isotopes in real time in a molten salt, something that is not easily done. After making a salt mixture of sodium nitrate and potassium nitrate and heating it to a liquid at 350 degrees Celsius, the scientists used argon gas (which alone doesn't cause a reaction) to send two isotopes of hydrogen through the molten salt.
With these measurements, they were also able to estimate the rate of diffusion, or how fast the gases spread through the molten salt, and how much gas the salt could hold. This helps them understand chemical reactions as well as the solubility of the gas in the salt - or how well it dissolves. In addition, LIBS was able to differentiate between hydrogen and water in the gas, because it's capable of detecting oxygen at the same time.
"We've performed several proof-of-concept experiments with LIBS to track aerosols and gases, finding it extremely insightful," Andrews said. "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."
In a light-water reactor, which all commercial reactors in the United States currently are, water is used in the reactor to cool the fuel rods and slow down the neutrons produced during fission, when atoms split and release energy, so that the chain reaction that provides power will keep going.
In molten salt reactors, both the coolant and fuel are liquids that circulate through the reactor's core. This means they can generate electricity more efficiently, and radioisotopes can be harvested during operation. The new LIBS spectroscopy method can help researchers measure and identify the isotopes present in molten salt in real time.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science . - Kristi Bumpus
