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| Fig. 1: Hyperbolic Cooling Towers that most people associate with Nuclear Power. (Source: Wikimedia Commons) |
Nuclear power was one of the most influential and controversial products of science in the twentieth century. While nuclear energy offers large power generation with little to no carbon emissions, it remains one of the most dangerous ways to produce energy in the world. From Three Mile Island to Chernobyl and Fukushima Daiichi, nuclear power has faced very public safety failures and has been responsible for severe environmental radiation poisoning. However, for all the setbacks of these early attempts, there may be hope on the horizon in the form of Molten Salt Reactors or MSRs for short. In order to comprehend the potential of MSRs, we will first explore their history to establish a framework for understanding them. Next, we will examine their method of operation, with a focus on how they differ from what the public considers "normal" nuclear reactors. Finally, we will compare the potential advantages and drawbacks of MSRs against their "normal" counterparts.
The first successful development of an MSR was completed by the Air Force in 1946. The experiment, called the Aircraft Reactor Experiment or ARE for short, successfully sustained a nuclear reaction for 1,000 hours. [1,2] While none of the energy produced was converted into electricity, the ARE did demonstrate MSR's to be viable sources of nuclear energy.
When it comes to the first molten salt reactor that actually produced power, Oak Ridge National Laboratory near Knoxville, Tennessee pioneered the technology. Starting in 1965 the project, called the Molten Salt Reactor Experiment, ran without a hitch for five years and produced around 7.4 MW continuously during that time. [1,2]
Despite the successes of the MSRE, the United States government chose to pursue other forms of nuclear energy to power the nation. However, this decision has recently come under scrutiny by leading scientists. [3] In their paper on the subject, Robert Hargraves and Ralph Moir posit the following question: "Knowing what we know now about climate change, [peak oil, and the numerous nuclear meltdowns we've experienced], what if we could have taken a different energy path?". [1] Ultimately, a thorough examination of the issue will show a strong argument in favor of molten salt reactors as the best solution to society's nuclear energy needs.
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| Fig. 2: A typical molten salt reactor. [6] Source: Wikimedia Commons) |
Before we can answer why molten salt reactors are a better solution than the current form of nuclear power we rely on, we must first establish exactly what the United States' "current" nuclear energy capabilities are. What is the most common form of electricity-producing nuclear reactor in use? When most people think of a nuclear reactor, they imagine the giant concrete cooling towers depicted in Fig. 1. These reactors are, in fact, the most common around the world, accounting for the majority ofl nuclear reactors employed worldwide. [4] Called Light Water Reactors (or LWRs), they use regular water as both a coolant and a neutron moderator. Essentially, these nuclear devices employ a set of uranium control rods to induce a fission reaction, heating up water to a boil and then capturing its energy as steam by spinning a turbine. [4]
Where light water reactors use uranium control rods for fuel and water as a coolant, molten salt reactors employ a variety of different fuels but all rely on molten salts as a coolant. The most promising MSRs use a mixture of liquid fluoride, U-233, and thorium as both the coolant and the fuel. These nuclear devices, called Liquid Fluoride Thorium Reactors, work on a very straightforward cycle. Essentially, nuclear fission is induced in the core, which is cooled and moderated by the liquid mixture. This process is illustrated in Fig. 2. The primary liquid, having absorbed heat, is then transported away and cooled by a second set of liquid salts, which then power a turbine. Interestingly, the primary liquid, after moderating the reaction, experiences a series of chemical changes that produce both waste and fuel. These products are separated in a series of cleansing steps and are either removed from the mixture or added back to the core to facilitate fission. The Liquid Fluoride Thorium Reactor, while complex in design, offers serious advantages to the more popular LWRs currently in use.
In order to demonstrate the potential of Liquid Fluoride Thorium Reactors, this report will evaluate the advantages and disadvantages of the technology when compared to LWRs in terms of its waste impact, safety, and simplicity - that is, the ease of production, operation, and procurement of materials. This will show that the LFTR offers legitimate hope for a future powered by low-impact, safe nuclear energy. However, it is important to remember that no working LFTR has been built yet, and the advantages are still theoretical.
Spent fuel from light water reactors is extremely radioactive, and takes thousands of years to become stable. [1]
500 tons of thorium can provide all energy in the U.S. for a year. [1]
LFTRs possess a fail safe core that is agreed to be more reliable than LWRs. If there is a leak in a LFTR, the fuel leaks as well as the coolant, stopping the reaction quickly. [1,2,5]
LFTRs are easily "turned off" by letting a freeze plug melt, draining the molten liquid from the core into a cooling chamber and shutting down the reaction. [5]
Light water reactors require complete shut down of operations every eighteen months or so, limiting power producing capabilities and adding to inefficiency. Molten salt reactors don't have to do this, as you can simply swap out the salt solution. [1]
No need to fabricate the fuel rods, which is an expensive and laborious process. [1,5]
Reaction products can be easily removed in molten salt reactors. That is, harmful xenon gas "bubbles" to the surface and can be whisked away. Also, new fissile material is produced by the thorium and salt mixture surrounding the core. This can be separated in-process and sent to the core to replenish fuel. [1]
© Charlie Furrer. 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] R. Hargraves and R. Moir, "Liquid Fluoride Thorium Reactors: An Old Idea in Nuclear Power Gets Reexamined," American Scientist 98, 304 (2010).
[2] Y. Kelaita, "Molten Salt Reactors," Physics 241, Stanford University, Winter 2015.
[3] A. Cannara, "Nuclear Waste: Thorium's Potential," Science 330, 447 (2010).
[4] B. Zarubin, "Introduction to Light Water Reactors," Physics 241, Stanford University, Winter 2015.
[5] M. W. Rosenthal, P. R. Kasten, and R. B. Briggs, "Molten-Salt Reactors: History, Status, and Potential," Nucl. Appl. Technol. 8, 107 (1970).
[6] "A Technology Roadmap for Generation IV Nuclear Energy Systems." U.S. DOE Nuclear Energy Research Advisory Committee, GIF-002-00, December 2002.
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