|Fig. 1: A depiction of the atomic structure of thorium. (Source: Wikimedia Commons)|
The potential of Th-233 as a nuclear fuel has long been discussed. Thorium, a slightly radioactive element whose atomic structure is detailed in Fig. 1, is not fissile. However, it transmutes into a fissionable isotope of uranium, U-233, when struck by neutrons. This U-233 is typically contaminated with U-232, but can sustain the nuclear chain reaction and produce energy. The nuclear industry has experimented with thorium, resulting in thorium reactor configurations of "high-temperature gas reactors, pressurized water reactors, and molten salt reactors". 
Thorium is approximately three times more abundant in nature than uranium, and advocates of thorium energy support the implementation of thorium reactors due to thorium's availability. This is an advantage because most nuclear reactors utilize the U-235 isotope as a nuclear fuel, which is much less abundant than thorium.  And because U-235 makes up only one of every 139 uranium atoms, raw uranium needs enrichment to become a source of nuclear energy. However, advocates of thorium fuels state that thorium does not require the "enrichment process needed to concentrate U-235".  Therefore, enthusiasts believe that utilizing thorium could result in a more sustainable and cheaper nuclear fuel cycle.
Thorium has consistently been referenced as a more proliferation-resistant fuel. Ironically, articles state that this is because U-233 is more dangerous to handle than U-235, resulting in more difficulty whilst crafting a nuclear weapon.  U-233 is more risky because U-233 produced from the thorium decay cycle is tainted with U-232 and not easily separated from it. This is not ideal for weapons creation because U-232 releases dangerous decay products that emit gamma radiation, which can penetrate skin and damage cells. As a result, remote handling of the equipment is required. This is not an issue if thorium is in a reactor, as U-232 is eventually burned during the production of energy. However, it is hazardous when crafting a military bomb with U-233, as trace U-232 can damage underlying electronics. Furthermore, thorium is a chemically more stable fuel than uranium.  As a result, thorium as a nuclear fuel is deemed more proliferation-resistant than U-235. However, there have been early nuclear tests performed utilizing thorium, so there is still an underlying potential for danger. 
As a result of the abundance and proclaimed proliferation-resistant properties of thorium, there has been an increase in research on proliferation-resistant uranium-thorium dioxide and thorium-plutonium fuels. Uranium dioxide is the primary fuel used in existing light water reactors (LWRs). Vast amounts of plutonium are contained in spent uranium dioxide-based LWR fuel worldwide - approximately 1000 tons. This is because LWRs generate plutonium from U-238 neutron capture. As it only takes "6 kg of Pu-239 to build a weapon", it would be beneficial to utilize more proliferation-resistant fuel in commercial LWRs.  On another note, plutonium is separated into two primary categories: weapon-grade plutonium, where the concentration of Pu-240 is less than 7%, and reactor grade plutonium, where the concentration of Pu-240 is greater than 7%. The presence of Pu-240 has the potential to start nuclear chain reactions prematurely, and is therefore beneficial for plutonium non-proliferation.  Herring et al. have conducted research to develop a mixed thorium dioxide and uranium dioxide fuel for existing light water reactors (LWRs) that is less expensive, more sustainable, and more resistant to nuclear weapon-material proliferation than uranium dioxide.  Uranium-thorium dioxide fuels improve proliferation-resistance of LWRs partially because uranium-thorium dioxide is a high burnup fuel. The plutonium isotopes in high burnup LWR fuel make design of a nuclear weapon difficult as it releases large amounts of heat along with spontaneous neutrons.  Furthermore, research from Herring et al. states that there will be much less separable weapons material produced, and the refueling periods will be less frequent. 
Research has also been conducted on thorium-plutonium fuels (Th-Pu fuels) for LWRs. Thorium-plutonium mixture has been proposed as an alternative fuel to destroy stockpile plutonium. Alhaj et.al. have concentrated on adding U-238 to Th-Pu fuel in an effort to increase the proliferation-resistance of the fuel. Moreover, Kryuchkov et al. have proposed adding more isotopes to Th-Pu fuels to create barriers against producing nuclear explosive devices.
Ultimately, following the nuclear arms race and the bombing of Hiroshima and Nagasaki, nuclear weapons have been classified as highly-dangerous weapons that should not be taken lightly. And they have been taken seriously - nuclear non-proliferation along with disarmament have been discussed for years on the intercontinental political scale. The effects of these discussions can be seen in the United Nations Resolution 3472, which defined nuclear-weapon-free zones in 1975. But more and more nations have been stockpiling weapons-grade plutonium and accumulating nuclear weapons, leading to potential danger on a global scale. As a result, research on nuclear fuels that are proliferation-resistant is on the rise. Incorporating thorium, a proposed proliferation-resistant fuel, into primarily uranium dioxide based fuels for light water reactors is a movement against malicious use of these fuels. Hopefully, by combining these fuels, we will move one more step towards a more nuclear proliferation-resistant and therefore secure world.
© Colleen Dai. 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.
 M. B. Schaffer. "Abundant Thorium as an Alternative Nuclear Fuel: Important Waste Disposal and Weapon Proliferation Advantages," Energy Policy 60, 4 (2013).
 B. M. Testa, "Thunder on the Horizon: Thorium-Based Reactors Could Begin a New Era of Nuclear Power," ASME 2676352a, Mech. Eng.d 139, No. 2, 38 (2017).
 M. Y. Alhaj et al., "Towards Proliferation-Resistant Thorium Fuels," Ann. Nucl. Energy 101, 586 (2017).
 J. Herring et al., "Low Cost, Proliferation Resistant, Uranium-Thorium Dioxide Fuels for Light Water Reactors." Nucl. Eng. Des. 203, 65 (2001).