|Fig. 1: A fast-breeder reactor at the Kalpakkam Nuclear Complex in India (Source: Wikimedia Commons)|
Thorium has been a part of the research agenda since the start of major nuclear energy research, but uranium is still the dominant nuclear power reactor despite billions spent on thorium research and development. The initial enthusiasm surrounding thorium's potential as an energy source has been tempered by a variety of setbacks and challenges in the last half a century. Still, optimism remains for thorium, especially since its a cheaper and supposedly safer form of nuclear power that produces less nuclear waste. 
In the early days of U.S. nuclear energy research, researchers discovered thorium's potential for energy, finding that Th-232 absorbed neutrons and could convert to U-233, which is the fissile fuel.  This process of fissile fuel production from non-fissile fuel is called breeding.  In the years afterward, the United States Atomic Energy Commission invested billions into thorium fuel research.  Thorium is 3 to 4 times as abundant as uranium in Earth's crust, and researchers thought then that the more abundant thorium could mitigate the exhaustion of uranium supplies in the future. 
In the 1960s, the United States built a thorium-based molten salt breeder reactor at the Oak Ridge National Laboratory and developed thorium-fueled nuclear power projects.  In addition, the Shippingport Atomic Power Station reached criticality in 1957 and was a thermal breeder reactor, eventually transmuting thorium to U-233 as part of its fuel cycle.  A breeder ratio refers to the ratio of new fissile material in spent fuel to fissile material consumed from the fresh fuel. The Shippingport breeding ratio attained 1.01, meaning that the reactor just produced as much fissile material as it uses.  However, after 10 years passed and billions invested, the U.S. Atomic Energy Commission abandoned thorium research, with uranium-fueled nuclear power becoming the standard.  In the 1980s, commercial thorium ventures failed, such as the Indian Point Unit I water reactor near New York City, because of the vast financial costs and fuel and equipment failures.  By the 1990s, the US nuclear power industry had abandoned thorium, partly because thorium's breeding ratio was thought insufficient to produce enough fuel for commercial industries. 
Considerable research has been conducted to investigate the feasibility of thorium, often concluding that current economics favor the uranium fuel cycle.  However, thorium continues to attract much attention, especially since research into this route may allow countries like India, with 61,000 tons in uranium reserves compared to 225,000 tons of thorium reserves, to achieve more energy independence.  Indeed, since the 1980s, India has been developing a prototype fast breeder reactor at Kalpakkam with rods of thorium.  With the neutrons that sustain the atomic chain reaction prevented from thermalizing and reaching higher velocities, these fast breeder reactors are designed to maximize atomic fuel and reach higher breeding ratios, generating more fissile material than they consume and extracting up to 70% more energy than traditional reactors.  Indeed, by 2012 the Kalpakkam fast reactor prototype had reached a breeding ratio of 1.05, higher than the Shippingport breeding ratio.  After years of development, India's Department of Atomic Energy is now reportedly ready to commission this reactor.  A picture of a fast-breeder reactor at Kalpakkam is shown in Fig. 1.
In addition, researchers at the Nuclear Research and Consulting Group, a Dutch nuclear research institute, built a molten salt reactor powered by thorium.  In molten salt reactor designs, the fuel is a liquid that contains isotopes involved in the fission chain reaction and a salt, like molten fluoride.  Thorium is particularly well-suited for this design, proponents argue, because its fluid can achieve stability at higher temperatures than uranium.  However, challenges remain. The testing, analysis, and licensing for thorium fuel will be expensive, necessitating broad support from government.  Uranium is still cheap and abundant, so economic incentives for thorium research are lacking.  Thus, it still remains to be seen whether this effort will result in a breakthrough in a historically challenging field.
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