Thorium is a slightly radioactive metallic chemical element with symbol Th and atomic number 90. It is denser than lead and remains a solid at room temperature but melts at 1775 °C. Thorium is ubiquitous in the Earth's crust and is about three times more abundant than uranium. Thorium is found in small amounts in most rocks and soils and is about as common as lead in the Earth's crust.

Potential as Nuclear Fuel

Thorium Reactor  has been recognized as a potential nuclear fuel since the 1960s. Thorium cannot undergo nuclear fission on its own but is fertile, meaning it can be transmuted into the fissile uranium isotope U-233. One advantage of thorium fuel cycles over uranium is that thorium-232 absorbs slower neutrons more readily than uranium-238. This property allows the use of thermal neutron reactors without the need for enriched fuel. Thorium can be used to burn up actinide waste from uranium-fueled reactors. Thorium fueled molten salt reactors have been proposed as a way to generate nuclear energy with reduced nuclear waste production and reduced potential for nuclear weapons proliferation. Overall, thorium fuel cycles are often said to offer more intrinsic proliferation resistance and potential safety advantages over uranium.

Molten Salt Reactors

One design well suited to using thorium as fuel is the molten salt reactor (MSR). In an MSR, the thorium (or uranium) is dissolved in a fluoride salt mixture along with the fissile uranium isotope U-233 produced from thorium. This fluoride salt mixture forms a liquid at typical operating temperatures and becomes the primary coolant that circulates through the reactor core. The dissolved nuclear fuel mixture remains in liquid form rather than using solid nuclear fuel elements or rods like conventional nuclear reactors. This makes MSRs theoretically very safe, as it is difficult for a catastrophic core meltdown to occur. If there is a sustained loss of cooling, the fissile material drains harmlessly into a passively cooled dump tank. MSR technology was researched in the United States from the 1950s to 1969, but the program was eventually cancelled in favor of light water reactors. However, renewed interest in thorium-fueled molten salt reactor technologies is ongoing in several countries.

Benefits of Thorium Fuel Cycles

Thorium fuel cycles offer several potential benefits over conventional uranium fuel cycles:

- Abundant Resource - Thorium reserves are 3-4 times more abundant than uranium reserves and are available in many countries. This improved economics and energy security.

- Less Nuclear Waste - Thorium fuel cycles can fully consume previously accumulated actinide waste from uranium reactors. They generate much lower volumes of long-lived radioactive waste. Some designs only generate waste lasting 300 years rather than thousands of years.

- Improved Proliferation Resistance - All the reactors designs preclude the possibility of using the reactor system to create weapons grade plutonium or uranium.

- Higher Burnup - Molten salt and other advanced designs can extract over 99% of the energy from thorium compared to less than 1% from light water uranium reactors currently in use.

- Improved Safety - Some advanced thorium reactor designs have significant safety advantages by operating at high temperatures with liquid fluoride salts that act as passive safety systems.

- Sustainable Energy Source - At projected rates of consumption, proven uranium reserves may last no more than 100 years whereas thorium reserves would last several thousand years.

Research and Development Status

While thorium fuel cycles offer promising advantages, significant research and development remains to be done to demonstrate thorium reactor technologies at commercial scales. Several nations including India, China, the USA, Norway, and Canada are actively conducting research programs. The primary focus has been on developing molten salt reactors.

In India, researchers have been operating a thorium fuel fabrication and testing facility called the Kudankulam Nuclear Power Plant since 2019. Their three stage nuclear program aims to use thorium as the primary fuel. In China, scientists have experimented with pebble bed molten salt reactors and plan to build a demonstration molten salt reactor by 2030.

In the USA, several companies and national labs continue R&D on molten salt reactor designs including Terrestrial Energy, ThorCon, Kairos, and TerraPower. The Oak Ridge National Laboratory has conducted experiments on their Molten Salt Reactor Experiment. In Canada, researchers at Canadian Nuclear Laboratories developed the TMSR molten salt concept. Norway operates a molten salt loop called the Halden reactor for testing nuclear materials in a molten fluoride salt environment.

Overall, while thorium fuel cycles have strong potential advantages, more experiments and demonstration projects are needed before commercial scale thorium fueled molten salt or other reactors can be built and connected to the electrical grid. Significant challenges remain in materials, fuel recycling, and safety testing that will require intense research programs over the coming decades.

In the thorium offers a promising alternative nuclear fuel that addresses several limitations of uranium fuel cycles. Molten salt reactors in particular are well suited to using thorium fuel in a safe and sustainable manner. While challenges remain, ongoing research programs worldwide seek to develop and demonstrate thorium fueled reactor technologies that could someday lead to commercially viable power plants producing clean, low carbon energy. With its abundant resources and potential proliferation resistance and waste reduction advantages, thorium may play an important long term role in meeting global energy needs if technical and economic obstacles can be overcome. Continued research is critical to realizing thorium's potential as a future nuclear fuel.

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