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| Fig. 1: Diagram of a Molten Salt Reactor (Source: Wikimedia Commons) |
Molten Salt Reactors are a relatively new type of nuclear reactor that use a molten salt such as uranium tetrafluoride as a coolant, with either a dissolved or solid fuel, and a graphite core as a moderator. This type of Generation IV reactor relies on a low pressure, high temperature environment (850-1000 degrees F) for stable energy production in a highly efficient, breeder type of fission reactor.
One major benefit of these reactors is that the simple fuel cycle and the negative coefficients of reactivity that keep the system within operating temperatures through a negative feedback loop of convection (as seen in Fig. 1). [1] This would mean that in the case of system failures there is low risk of explosion, radioactive steam and environmental damage. [2] While the salts have a low water solubility which would prevent contamination if there was a leak, one of the most notable problems with MSRs is their corrosive properties. The molten salts can easily corrode steel and melt aluminum, so non-traditional manufacturing methods must be used contain the highly corrosive, radioactive materials.
Due to the extremely corrosive nature of the extremely reducing molten salts, Nickel based alloys have been used in most MSRs that have been designed. It has been shown through testing in molten salt convection baths that alloys high in Iron and Chromium such as Inconel 106 and Type 316 Stainless corrode readily and lose significant amounts of mass within a few thousand hours. [3]
Alloys with high Nickel and high Molybdenum content tend to last much longer with only a few mils of material loss per year. There have also been experiments with increased Niobium and Silicon concentrations that have shown improvements in oxidation rate and material loss. An alloy that is considerably more resistant to molten salt's corrosive effects could lead to advances not only in MSR technology, but also heat storage and high temperature oil refinement among other industrial fields. [4]
These self-regulating, safe, and efficient reactors are a potentially useful as a portable or high efficiency choice as Generation IV reactors begin to move from R&D into production. While these reactors have numerous benefits when compared to current technologies, the corrosion and material loss of the containment system is one of the largest. Using high Nickel and Molybdenum content, experimenting with Manganese and other additive content, and reducing Iron and Chromium content has proven to be relatively effective for reducing corrosion. For MSRs to become a viable option, a more effective alloy or material should be used to lengthen the life of the containment structure and to maintain relatively pure salts.
© Joe Sunde. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] Y. Kelaita, "Molten Salt Reactors," Physics 241, Stanford University, Winter 2015.
[2] C. Renault et al., "The Molten Salt Reactor (MSR) in Generation 4: Overview and Perspectives," GIF Symposium Proceedings, Nuclear Energy Agency, September 2009, p. 191.
[3] J. R. Keiser, "Compatibility Studies of Potential Molten-Salt Breeder Reactor Materials in Molten Fluoride Salts," Oak Ridge National Laboratory, ORNL-TM-5783, May 1977.
[4] L. C. Olson et al., "Materials Corrosion in Molten LiF-NaF-KF Salt," J. Fluorine Chem. 130, 67 (2009).
