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| Fig. 1: Diagram of a closed, two-fluid molten salt reactor. (Source: Wikimedia Commons) |
Molten Salt Reactors (MSRs) are a half-century old design of nuclear reactor which, due to being selected as one of six Generation IV reactor concepts, is beginning to receive renewed attention. [1] MSRs are built on the concept of using uranium fuel dissolved into a salt as both fuel and coolant for the reactor - this design enables significant advantages over traditional reactors powered by solid fuel.
Early work on MSRs began in the 1950s with the Aircraft Reactor Experiment at Oakridge National Laboratories. The aim of this experiment was to construct a nuclear reactor which could be used to power an aircraft, enabling aircraft to enjoy a flight range twice that of the standard flight range of the day. In the design of the reactor, the positive temperature coefficient of reactivity of xenon at high temperatures meant that solid fuel designs were too unstable. [2] Contrastingly, a liquid fuel design would enable a self-stabilizing reactor model: given that a liquid fuel would expand when heated, eventually expanding beyond the active core region in which reactions take place. Another design consideration involved reducing the high temperature gradient of stagnant liquid fuel. To solve this, fuel was circulated through the core and through a heat exchanger in such a manner as to ensure turbulent flow - a core design with 66 passages was deemed optimal for ensuring turbulent flow through the core. Ultimately, the ARE was operated at 1-3 MWt, used 93.4% enriched UF4 fuel, and reactions were moderated by BeO blocks. [3]
Beyond the Aircraft Reactor Experiment, Oak Ridge National Laboratory performed the Molten-Salt Reactor Experiment (MSRE) in the 1960s. The MSRE was a graphite-moderated 8MW(th) reactor, cooled with F-Li-Be salt, designed to demonstrate reliability of the molten salt reactor concept. [4] The MSRE was operated in two different trials. One trial (June 1965) operated the reactor with 33% enriched U-235, which successfully demonstrated reactor reliability by operating the reactor at criticality for over 80% of a 15-month test period. The second trial utilized U-233 in the carrier salt, which demonstrated the viability of operating a molten salt reactor with fissile U-233. [4]
Molten salt reactors operate on similar principles as light and heavy water reactors, with several critical distinctions. Unlike light and heavy water reactors, fuel in MSRs is a fluid of U, Pu, or Th, salts. [1] Fig. 1 illustrates a closed two-fluid design: the fuel salt enters the reactor core, then moves into a heat exchanger to transmit thermal energy to the coolant salt. The coolant salt removes thermal energy from the fuel salt (the fuel salt, in the closed design, is then pumped back into the core), then moves into another heat exchanger to transmit thermal energy which is used to drive a turbine to generate electrical power. [1]
MSRs have unique design considerations when compared to light and heavy water reactors. One key consideration in the design of MSRs is the corrosive nature of the liquid salts that are used as fuel and coolant. Molten salts can corrode steel and melt aluminum, so innovation in materials science was required in order to create viable materials for the reactor. [5] Olson et al. found that nickel and molybdenum alloys have minimal corrosion: only several mils of material loss per year of operation. [5,6] In the MSRE, for example, Hastelloy-N was used as a salt containment vessel; modifications needed to be made to the Hastelloy-N alloy to account for corrosion suffered over the course of the experiment. [1,4]
MSRs have three key safety advantages when compared to conventional reactor designs. First, the aforementioned negative temperature coefficient of reactivity allows for self-regulating of temperature within the reactor: if the reactor overheats, the expansion of the salt pushes fuel outside of the core region, reducing the overall reaction rate. [2,7] Second, unlike heavy water and light water reactors, MSRs do not use water, and function with minimal pressure, meaning there is no risk of a steam explosion. [7] Molten salts are also a superior reactor coolant, as molten flourine salts have a 25% higher heat capacity than the pressurized water used to cool light and heavy water reactors. [1] Third, as shown in Fig. 1, MSRs are designed with a freeze valve which, in emergency situations, melts, and the liquid fuel is channeled from the moderated core into another chamber in which reactions are limited. [1,7,8]
Though MSRs enjoy safety advantages compared to conventional reactor designs, it is important to weigh these safety advantages relative to the risk of contemporary light water reactor designs. Given that light water reactors are very safe - estimates by Kessler suggest that European light water reactors undergo core meltdown with a frequency between 10-5 and 10-6 meltdowns per reactor per year - the safety advantages of MSRs may well be solving a problem which is dubious in its significance. [9]
© Michael Cooper. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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] D. LeBlanc, "Molten Salt Reactors: A New Beginning For an Old Idea," Nucl. Eng. Des. 240, 1644 (2010).
[2] E. S. Bettis et al., "The Aircraft Reactor Experiment - Design and Construction," Nucl. Sci. Eng. 2, 804 (1957).
[3] S. Omar, "The Aircraft Reactor Experiment," Physics 241, Stanford University, Winter 2012.
[4] P. N. Haubenreich and J. R. Engel, "Experience with the Molten-Salt Reactor Experiment," Nucl. Appl. Technol. 8, 118 (1970).
[5] J. Sunde, "Material Corrosion in Molten Salt Reactors," Physics 241, Stanford University, Winter 2017.
[6] L. C. Olson et al., "Materials Corrosion in Molten LiF-NaF-KF Salt," J. Fluorine Chem. 130, 67 (2009).
[7] B. M. Elsheikh, "Safety Assessment of Molten Salt Reactors in Comparison with Light Water Reactors," J. Radiat. Res. Appl. Sci. 6, 63 (2013).
[8] J. Dodaro, "Transatomic Power," Physics 241, Stanford University, Winter 2016.
[9] G. Kessler, Sustainable and Safe Nuclear Fission Energy (Springer, 2012), pp. 313-415.
