|Fig. 1: Early thorium-based molten salt nuclear reactor at Oak Ridge National Laboratory in the 1960s. (Source: Wikimedia Commons)|
After World War II when the United State's Oak Ridge National Laboratory was developing nuclear energy, uranium was identified as a source for nuclear fuel to produce electricity, so the United States built uranium-based nuclear reactors to generate electricity.  During this period of nuclear energy research, it was also discovered that thorium can be used as a source for nuclear reactors. However, in 1973 the United States government shut down all thorium related nuclear research due to the success of the uranium reactor to produce energy, so the vast majority of the nuclear reactors that exists today use enriched uranium (U-235) or reprocessed plutonium (Pu-239) as their source of energy.  By 2008, there has been a renewed interest in a wide variety of countries and institutions in using thorium instead of uranium as nuclear fuel to generate nuclear power due to the advantages it potentially has. In particular, there is a large interest research and development of thorium from India and China due to the substantial reserves of thorium-bearing material and the limited amount of uranium in their respective countries.  This paper briefly goes over the background on using thorium as a source for nuclear reactors and discusses the major benefits and drawbacks of using thorium as an energy source for nuclear power.
Thorium has properties like uranium which allows it to fuel a nuclear chain reaction. But unlike uranium which splits and releases energy, thorium goes through a series of nuclear reactions when exposed to neutrons until it emerges as an isotope of uranium called U-233. This isotope will readily split and release energy next time it absorbs a neutron. The type of reactor that can handle the thorium-urnaium fuel cycle would be a class of molten salt reactors called liquid fluoride thorium reactors (Fig. 1), which is the type of reactor the Oak Ridge National Laboratory researched on. These types of reactors dissolve the fuel into a vat a liquid salt as opposed to being cast into pellets. The chain reaction of the thorium into uranium heats the salt, which convects through a heat exchanger and into a turbine, thus producing electricity.
Thorium is 3 times as abundant as uranium, almost matching the amount of lead and gallium in the Earth's crust (Fig 2.). Thorium composes 0.0006% of the earth's crust whereas Uranium composes 0.00018% of earth's crust, where a substantial amount of Uranium is found in dissolved sea water. The Thorium Energy Alliance estimates "there is enough thorium in the United States alone to power the country at its current energy level for over 1,000 years." 
Thorium is safer and more efficient to mine than uranium, thus making it more environmentally friendly.  The percentage of thorium found in its ore is generally greater than the percentage of uranium found in its ore, so it is more cost-efficient. Thorium mines has an open pit which does not require ventilation, whereas uranium mines is closed off where the radon level reach potentially dangerous levels.
It is estimated that one ton of thorium can produce as much energy as 35 tons of uranium in a liquid fluoride thorium reactor. Conventional reactors utilizes less than one percent of uranium, whereas a well working reprocessing reactor can utilize 99% of its thorium fuel.  However, when comparing breeder reactors for Uranium and Thorium, the energy per kilogram would be similar.
There is up to two orders of magnitude less of nuclear waste in the liquid fluoride thorium reactor, eliminating the need for large scale and long term storage for the waste.  This is because the Thorium-Uranium fuel cycle does not irradiate U-238, so it does not produce atoms bigger than uranium. Furthermore it takes a couple hundred of years for the radioactivity of the waste to drop to safe levels, whereas it take tens of thousands of years for current nuclear waste to drop to safe level.
The liquid fluoride thorium reactors has the potential to have extremely attractive safety features due to its negative coefficient of reactivity, be proliferation resistant, and utilize its resources efficiently. 
When examining the current reserves of economically extractable material, there is approximately twice as much Uranium as there are Thorium.  There are approximately 5.4 million tons of Uranium and there is approximately 2.6 million tons of Thorium. However, the reserves of extractable Thorium can increase since currently there is low demand.
Thorium would make uranium-232 in reactors when being irradiated, which emits dangerous levels of gamma rays.  The thorium-uraninium cycle produces uranium-232, which decays to Tl-208, which has a 2.6 MeV gamma ray decay mode. This process is therefore require higher costs since it needs more expensive fuel handling and reprocessing since the gamma rays are extremely difficult to shield.
People do not experience operating thorium reactors. The experiments and research done with using and operating thorium reactors were conducted over 40 years ago. With the large up front cost to build a reactor and train people to operate it, Thorium reactors requires significant start up costs and investments, along with additional unknown costs to maintain it.
© Jason Ting. 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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