In response to a renewed international interest in molten salt reactors, researchers from the Department of Energy’s Oak Ridge National Laboratory have developed a novel technique to visualize molten salt intrusion in graphite.
During ORNL’s revolutionary Molten Salt Reactor Experiment, or MSRE, in the 1960s, scientists first demonstrated the feasibility of nuclear fission reactions with molten fluoride salt used both as a fuel carrier and as a coolant, substituting for the solid fuel and water used in traditional nuclear reactors. Molten salt reactor designs show great promise as a means of carbon-free power generation.
To slow down neutrons so they can easily promote nuclear fission, nuclear reactors use a material called a moderator. To moderate the MSRE, scientists used synthetic graphite, which is resistant to thermal shock and dimensionally stable because of its extensive pore system resulting from the manufacturing process. MSRE graphite was custom-made and specially coated to decrease porosity and defend against detrimental effects that may occur when hydraulic and gas pressures cause molten salt to seep into graphite’s pores. Moreover, preventing molten salt intrusion avoids additional issues with waste management during reactor decommissioning.
Following the conclusion of ORNL’s experiment in 1969, the potential of molten salt reactors was largely unexplored until the 21st century, and low demand for the specialized graphite led to the material’s discontinuation among domestic graphite manufacturers. With an uptick in molten salt reactor research but no MSRE graphite, today’s scientists must identify an alternative graphite to successfully moderate nuclear reactions in molten salts. However, ambiguity around the effects of molten salt intrusion poses a barrier to discovery. Scientists have a limited understanding of what microscopic features enable some graphite grades to withstand intrusion better than others and how salt intrusion affects graphite’s other properties.
Hoping to resolve these uncertainties, a team of ORNL scientists led by Nidia Gallego and Jisue Moon studied molten salt intrusion across various graphite grades and have validated the first technique to visualize and monitor molten salt penetration depth and distribution in graphite’s pores.
“It’s critical for us as materials scientists to help test and develop techniques that we can use or have a better perspective on what we need to do to understand how the salt that goes into the pores may affect the mechanical or the thermal properties of the graphite,” said Gallego, a distinguished R&D staff scientist in ORNL’s Chemical Sciences Division.
Ultimately, the team hopes to develop predictive models to describe how different graphite grades respond to salt intrusion and refine molten salt reactor operating standards, which denote material and technical requirements for reactors but lack specification about preferred graphite grades.
“Understanding how the graphite interacts with the salt is critical,” Gallego said.
Additional researchers who contributed to the project include ORNL’s Cristian I. Contescu, James R. Keiser, Dino Sulejmanovic, Yuxuan Zhang and Erik Stringfellow.