Evaluating the Effects of Molten Salt on Graphite Properties: Gaps, Challenges, and Opportunities

The recent interest in developing molten salt reactors (MSRs) for energy production opens multiple new opportunities for graphite manufacturers, reactor vendors, and designers and creates new challenges for engineers and scientists. In MSRs, graphite is not only exposed to fast neutron irradiation but also in continuous contact with the coolant molten salt, the fuel salt, or both, depending on the design. The continuous operation in contact with the molten salts is expected to affect graphite’s local composition and microstructure, which in turn impacts the mechanical, thermal, and irradiation-resistance properties of the graphite. Most ASTM testing procedures developed so far apply to graphite for gas-cooled reactors. Although some effort has recently been directed to the development of standards applicable to graphite-molten salt systems (such as ASTM D8091 and ASTM D8377), knowledge gaps still remain in this area. Characterization of graphite properties in molten fluoride salts has many associated challenges. Most salts of interest are hygroscopic and must be handled in a protected atmosphere; some are highly toxic (containing beryllium) or may be radioactive if they contain fuel. The MSR community needs to quickly adapt existing standards or develop new testing methods to respond to the new demands of the MSR technology. This paper is intended to initiate a productive discussion and summarizes the challenges and opportunities in this field.