In the search for new and carbon-neutral energy sources nuclear power is once again considered an option. [1,2] Currently, 16% of world electricity is generated by nuclear reactors using the Uranium fuel cycle.  However, concerns over world Uranium supplies has led to a revival in research on the Thorium fuel cycle. The IAEA predicts problems with meeting Uranium demand as early as 2025.  Thorium is four times more abundant than Uranium and can therefore help prolong the lifespan of nuclear power significantly. The possibility of a Thorium nuclear power plant has been known since the 1940s, but the technology was never commercialized. This may have been due to the technical challenges of a Thorium fuel cycle combined with Uranium being more abundant than expected.  The difficulty in making nuclear weapons utilizing a Thorium fuel cycle may also have contributed to the lack of interest during the cold war.  Today, the inherent proliferation resistance of a Thorium fuel cycle is considered a great advantage. Additionally, it is possible to utilize a mixed fuel of Thorium and weaponized Plutonium to reduce current nuclear arsenals. 
Thorium exists in nature in the form of the 232Th isotope, which is a so-called fertile isotope. A fertile isotope can be converted to fissile material - 232Th is transmuted to fissile 233U by one neutron absorption and two beta decays. In a traditional nuclear reactor, controlled nuclear fission cannot begin with fertile material alone, fissile material must therefore be supplemented to achieve a Thorium fuel cycle. The fissile material can be 235U, 239Pu or 233U produced by exposing 232Th to a neutron source. It has been proposed to use weaponized Plutonium in Thorium reactors, thus consuming Plutonium and fulfilling nuclear disarmament treaties while generating electricity. Generation of 233U is currently being pursued in India's three stage nuclear power program. India has vast Thorium reserves but small and low-grade Uranium supplies, so in order to achieve self-sufficiency of nuclear fuel a working Thorium fuel cycle is required. [3,7] A technically elegant alternative to traditional nuclear reactors running at criticality is to use an accelerator driven nuclear reactor, where fast neutrons are supplied by bombarding a lead alloy with highly energetic protons. The neutrons can then hit a sub-critical nuclear reactor, causing fission. [3,6,7] The accelerator driven nuclear reactor has superior waste characteristics to traditional reactors and has no chance of meltdown, but the large investment cost of the accelerator may prevent it from ever being built.
The fissile 233U can potentially be used to create a nuclear weapon. However, in a Thorium fuel cycle, some 232U is produced from 233U, 232Th and 233Pa. 232U decays to elements such as 212Bi and 208Tl that are high energy gamma emitters with short half-lives. These gamma-rays are easily detected and also necessitate remote handling of the fissile material.  235U and 239Pu do not have these problems and are therefore the isotopes of choice for nuclear weapons. Another barrier to creating a 232U-based nuclear weapon is that there is no prior experience. The process would therefore require tests, illegal by the testing moratorium currently in place and also easy to detect. The use of a Thorium fuel cycle can thereby improve control over nuclear proliferation and increase the barrier to creating nuclear weapons in nations that wish to use nuclear power.
The Thorium fuel cycle produces less long-lived nuclear waste than the Uranium fuel cycle. Long-lived nuclear waste from Uranium fueled reactors consists mainly of the so-called minor actinides, which include Neptunium, Americium, Curium and Californium . These isotopes are generated through absorption of thermal neutrons in the reactor core. The fissile material in a Thorium core, 233U, has a higher fission to capture ratio than both Uranium and Plutonium and does therefore not generate as many minor actinides. The main waste concern of a Thorium fuel cycle is 231Pa which is produced when 233Th absorbs one neutron and emits two neutrons, a (n,2n) reaction. Fortunately, the rate of (n,2n) reactions is very low in thermal neutron reactors, thus giving the Thorium fuel cycle very favorable waste characteristics .
The penetrating gamma radiation present in spent Thorium fuel necessitates automated, remote reprocessing of the fuel. The technology for reprocessing and refabricating Thorium fuel is still largely experimental and has never been done on a large scale. The equivalent process for the Uranium fuel cycle is well known and has been in use for decades. Additionally, Thorium fuels require a one year cooldown period before reprocessing due to the presence of the highly radioactive 234Pa (~74 days half-life) in spent fuel.  Reprocessing of Thorium fuel will therefore cost substantially more than Uranium fuel reprocessing.
The Thorium fuel cycle has the potential to bring the world cleaner and more sustainable nuclear energy, but there remain significant technical and economic barrier to widespread use. Currently, the only country with a large scale research program on Thorium nuclear power is India. In the rest of the world, the fate of Thorium nuclear reactors depends on whether the Thorium will ever become economically viable compared to Uranium, while solar power may be sailing up as the energy source of the future.
© Nils Johan Engelsen. 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.
 Matthew L. Wald, "U.S. Supports New Nuclear Reactors in Georgia ," New York Times, 16 Feb 10.
 Judy Dempsey, "Germany Extends Nuclear Plants' Life," New York Times, 6 Sep 10.
 "Thorium Fuel Cycle - Potential Benefits and Challenges," International Atomic Energy Agency, IAEA-TECDOC-1450, May 2005.
 A. Jameson, "Uranium Shortage Poses Threat," The Times of London, 15 Aug 05.
 D. Clark, "Thorium Nuclear Power," The Guardian, 13 Jul 09.
 T. Dean, "New Age Nuclear," Cosmos Magazine, April 2006.
 L. Pham, "Considering an Alternative Fuel for Nuclear Energy," New York Times, 19 Oct 09.
 R. Brissot et al., "Nuclear Energy With (Almost) No Radioactive Waste?," July 2001.
 J. S. Friedman, "More Power to Thorium?", Bulletin of the Atomic Scientists 53, No. 5, 19 (1997).