Why Switch from Uranium to Thorium?
Traditional nuclear reactors predominantly use uranium as fuel. However, uranium has some drawbacks. It requires costly enrichment to increase its fissile U-235 content and produces radioactive waste that remains hazardous for thousands of years. In contrast, thorium is more abundant and generates far less long-lived transuranic waste. With its unique properties, thorium is a compelling alternative fuel source that could power greener, safer nuclear energy.
Abundance of Thorium Resources
Thorium Reactor is approximately three to four times more abundant in nature than uranium. Known reserves of thorium are estimated to be 3-4 times larger than reserves of uranium. India, for instance, has the world's largest thorium reserves at around 360,000 tons. In comparison, estimated reserves of uranium are around 16 million tons globally. The greater natural abundance of thorium indicates that it could support global nuclear energy production for much longer periods of time.
No Uranium Enrichment Needed
While uranium fuel typically contains only around 0.7% of the fissile isotope U-235, thorium is 100% fertile Th-232 isotope that can be converted to the fissile isotope U-233 in a reactor. This means thorium reactors do not require expensive and energy intensive enrichment of uranium fuel before use. Thorium fuel manufacturing is thus simpler and cheaper than the current nuclear fuel cycle.
Produces Less Long-Lived Waste
Conventional nuclear power plants produce different types of radioactive waste such as used reactor fuel, transuranic actinides and fission products. Of these, transuranic actinides like plutonium have very long half-lives ranging from thousands to millions of years. They dominate the long-term radioactivity and heat load of spent nuclear fuel. Thorium fuel cycles produce far less transuranic actinides and the waste they do produce has much shorter half-lives ranging from only a few decades to a few hundred years. Within a few centuries, the radiotoxicity of such waste would drop to levels comparable to natural uranium ore.
Increased Proliferation Resistance
Thorium-232 cannot sustain a nuclear chain reaction by itself and must be converted to fissile U-233 for energy production. This conversion requires a reactor or other nuclear facilities not easily weaponized. Extracting U-233 from spent thorium fuel is tedious and the U-233 produced contains contaminants like U-232, whose decay daughters emit highly penetrative gamma radiation. Together these factors significantly increase the technical challenge of surreptitiously using thorium technology for nuclear weapons purposes compared to the plutonium in uranium fuel cycles.
Molten Salt Reactors - Ideal for Thorium
Different reactor designs are being researched to optimize the use of thorium as fuel. One particularly promising concept is the molten salt reactor (MSR). In an MSR, the fuel is dissolved or suspended in a fluoride salt coolant that remains liquid at high temperatures. This allows online removal of fission products and easy control/adjustment of reactivity. MSRs offer higher burnup of fuel, greater thermal efficiency, inherent safety and the ability to directly produce industrial process heat or hydrogen. They are well-suited to fully exploit the benefits of thorium in a simple, proliferation-resistant fuel cycle.
Ongoing Research and Development Projects
Several nations including the United States, China, India and Russia are actively conducting research on thorium fuel cycles and molten salt reactors.
India has one of the most advanced thorium programs. It operates a full-scale prototype fast breeder reactor that has bred more nuclear fuel than it has consumed, demonstrating thorium's breeding capabilities. India also plans to build several 300 MWe molten salt thorium reactors in the coming decades.
China has built a experimental 300 MWth pebble bed molten salt reactor to test thorium fuel in recent years. It aims to demonstrate thorium fuel recycling and transmutation technology on an industrial scale.
The US had extensive MSR research until government funding cuts in the 1970s. However, interest has revived and several private companies are now developing experimental MSR designs. A key test will be the planned Oak Ridge Molten Salt Reactor Experiment in 2025.
While challenges remain in developing mature thorium technologies, ongoing research worldwide shows that thorium could play a significant role in expanding sustainable nuclear power and energy security in the future. With further progress on MSRs and fuel cycles, thorium could help transition the world from fossil fuels to an abundant supply of clean, safe nuclear energy for centuries to come.
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