|Fig. 1: Superphenix, a french fast breeder reactor in France that was shut down after a plethora of technical problems and strong opposition (Source: Wikimedia Commons)|
Countries around the world have turned to nuclear power as the answer to climate change. As a result, the risk of nuclear weapons proliferation has risen substantially. Is thorium - an alternative nuclear fuel that resists proliferation - the solution to this growing problem? Thorium's proliferation resistance, as well as other advantages including reduced volume and lifetime of waste and the abundance of naturally occurring reserves, inspired China, Russia and India to pour funding into thorium research as early as 2013.  However, why is it that three years later other countries remain reluctant to follow suit? Are they right in believing there to be some catch? The answer to this question is complex: while in some regards thorium fuel is safer than either uranium or plutonium fuel as a result of passive safety measures, in other regards it is potentially even more dangerous. While the latter should not wholly bar investment into thorium as an alternative fuel, it should inform the direction of future research.
There are three main safety features to thorium dioxide, the form of thorium used in nuclear reactors, which set it apart from conventional nuclear fuels: its melting point, stability, and thermal conductivity are all significantly higher than that of uranium oxide. Thorium dioxide has a melting point 500 degrees Celsius higher than the melting point of uranium oxide, reducing the risk that an accidental power surge or loss of coolant inside the reactor will result in temperatures high enough to trigger a meltdown.  As an extra margin of safety, the thermal conductivity of thorium dioxide is 30 percent higher than its uranium counterpart at 100°C, and 8 percent higher at 650°C. Thus the plant can be operated at lower temperatures, even further below the melting point of the fuel. Lastly, the compound's higher stability reduces the risk of the fuel pellets reacting to and therefore oxidizing their metal claddings, which would produce highly explosive hydrogen gas. Thorium dioxide is in fact the most stable solid oxide at high temperatures, making it the best choice to resist highly reactive metals. 
Why they are necessary: Unlike Uranium-235 and Plutonium-239, both isotopes commonly used in nuclear reactors, thorium is not itself fissile but instead fertile. This means that Th-232 must absorb a neutron from the fissioning of conventional nuclear fuel in order to generate fissile U-233, which can then fission to create power for the reactor. In order for the breeder plant to sustain itself, Th-232 must "breed" U-233 at a rate equal to or greater than the rate at which it consumes the conventional fuel. That way, after the initial reaction the subsequent fissioning of U-233 will create enough neutrons for Th-232 to absorb and continue the process.  Keeping this rate equal to or greater than one requires the use of a fast reactor breeder plant.
Why they are dangerous: Current fast reactor breeder plants employ molten sodium metal to cool their cores, because the molten metal does not inhibit the breeding process by moderating (slowing down) neutrons.  Since molten sodium explodes upon contact with either water or oxygen, the coolant renders the use of water, the most important safety feature of common reactor designs, unfeasible. The un-moderated, high-energy neutrons compound upon this danger. Due to these fast neutrons, the chain reaction of breeder plants can accelerate beyond what the plants can handle, significantly increasing the risk of runaway reactions and therefore a core meltdown. 
The renewed interest in thorium has opened the door for research into reactor designs that bypass the use of molten sodium as coolant. Already, companies have such designs underway. If these designs can be brought to light, thorium will become a safer alternative to conventional nuclear fuels, and hopefully an alternative that many countries pursue.
© Isabel Gueble. 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.
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