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| Fig. 1: Thorium metal. (Source: Wikimedia Commons) |
Nuclear power, generated by the process of sustained fission of molecules with large binding energies, provides 6% of the world's energy usage, with approximately 50% of electricity production originating in France and Japan. [1] Uranium-235, the most commonly utilized element in nuclear fission reactors, produces over a million times more energy than coal, per kilogram. [2] Nonetheless, there are safety related (i.e. Fukushima), practical (Earth's uranium reserves), and political (i.e. nuclear proliferation) issues that continue to present themselves, prompting the search for safer, less morally obtrusive, more durable nuclear energy production mechanisms. The thorium reactor is a prime candidate. [3]
Thorium-232 is a naturally occurring isotope of thorium that, while not fissile itself, can produce uranium-233 by beta decay and neutron capture, which can then be placed into a separate reactor to produce thermal energy. [4] U-233, on the other hand, is fissile and long-lived, prompting the concept of a thorium based breeder reactor. A nuclear breeder reactor produces fuel in the process of consuming it. [5] India has expressed massive interest in developing effective thorium breeder nuclear reactors. [6] In 2008, it stated that by 2050 it planned to meet 30% of its energy demands via thorium reactors.
The question of how much R&D should be invested in thorium based nuclear energy has been a subject of much debate for over 40 years, and oscillates alongside public notions of nuclear energy.
A major benefit to developing thorium breeder reactors is that thorium is far more naturally prevalent that U-235. Furthermore, naturally occurring thorium is 100% useable, whereas uranium obtained from the soil is only 0.7% pure. In addition, fission of U-233 (an integral part of the thorium cycle) liberates 2 neutrons per neutron absorbed over a wider thermal spectrum, allowing for the cycle to operate with a variety of neutron spectra. [4] Also, it is believed that the thorium breeder fuel cycle does not produce nearly as many long-lived radioactive elements as the conventional uranium based reactor, [7] since in theory all actinides produced can be recycled and consumed. [8] Proponents also state that use of Th-232 decreases the risk of nuclear proliferation, since it is not fissile. Also, the chance of an accidental runaway chain reaction is very low, since fission stops unless it is primed, making thorium reactors more stable.
There are, however, numerous rebuttals to these alleged benefits of thorium breeder reactors. First of all, since Th-232 is not fissile, a fissile element such as U-235 or Pu-239 is necessary to initiate the reaction, making any theoretical thorium breeder reactor design far more complex and expensive. [4] Also, despite challenges, a certain amount of actinides can be harvested from breeder reactors, which must in turn be reprocessed, creating further radioactive waste. [9] This also raises doubts as to whether the proliferation risk would actually be significantly diminished by a move to thorium-based breeders.
Another influential factor to be reckoned with is simply monetary, given the current circumstance of the global economy, accompanied by disasters like the nuclear meltdown at Fukushima. [10] It undoubtedly detracts from the credibility of the nuclear industry, without which countries will be far less likely to invest sufficient funds into R&D for future thorium reactors.
Though hailed by many as the "holy grail" of nuclear energy, thorium reactors still seem a distant reality. In the end, the world is looking for renewable energy sources and, though abundant in nature, even thorium is limited, and we should perhaps be pursuing more enduring, less hazardous, energy alternatives.
© Julien De Mori. 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] "Key World Energy Statistics 2007," International Energy Agency, 2007.
[2] J. Bernstein Nuclear Weapons: What You Need to Know (Cambridge U. Press, 2008).
[3] T. Dean, "New Age Nuclear," Cosmos Magazine, April 2006.
[4] "Thorium Fuel Cycle-Potential Benefits and Challenges," International Atomic Energy Agency, IAEA-TECDOC-1450, May 2005.
[5] A. E. Waltar and A. B. Reynolds, Fast Breeder Reactors (Pergamon Press, 1981).
[6] S. Agarwal, "Indian Thorium Based Reactor Design Complete - by 2050 30% of Indian Electricity Will Be Generated with Thorium Based Reactors All over the Nation," India Daily, 18 Feb 08.
[7] "The Use of Thorium as Nuclear Fuel," American Nuclear Society, Position Paper 78, November 2006.
[8] B. L. Cohen, "Breeder Reactors: A Renewable Energy Source," Am. J. Phys. 51, No. 1, 75 (1983).
[9] P. A. Karam, "How Do Fast Breeder Reactors Differ from Regular Nuclear Power Plants?" Scientific American, 17 Jul 06.
[10] J. Paton, "Fukushima Crisis Worse for Atomic Power Than Chernobyl, UBS Says," Bloomberg, 3 Apr 11.
