Thorium – The Future of Nuclear Energy?

As we all know that climate change is getting worse and there could be serious consequences for our planet in next 20 years. The Energy/electricity that we use is mostly created by burning Fossil fuels such as coal, oil and natural gas which emit greenhouse gas. Further this Greenhouse gases are contributing to global warming because they trap more of the Sun's energy in the Earth's atmosphere. So what the world needs now is Nuclear energy. I know you might be thinking of 'Non-conventional' sources of energy like solar,wind and hydro. But let me tell you, we live in a world that has high demands for electricity, which at least at this moment can't be fulfilled by non-conventional sources of energy. And according to me, Nuclear energy is clean but dangerous and with the use of Thorium it could drop the tag of being dangerous and just remain clean.

Today’s Nuclear Energy

Today's nuclear technology has principally been based on the use of fissile Uranium-235 and Plutonium-239. Nuclear energy comes from splitting atoms in a reactor to heat water into steam, turn a turbine and generate electricity, all without carbon emissions because reactors use uranium, not fossil fuels. At a very basic level, current reactors convert thermal into electrical energy. While nuclear power reactors are being built in large quantities around the world, these reactors are not without drawbacks. Nuclear meltdowns remain a concern which can explode and/or release large amounts of radiation. Also Uranium has some negative effects such as it can have deleterious health effects and can lead to death because of its radioactivity.

So, Thorium is considered by many as a potential substitute for uranium in nuclear power generation facilities, and following negative publicity after the meltdown of the Fukushima power plant in Japan, advocates hope it can save the image of nuclear as a truly safe and environmentally friendly alternative to coal power.

What is Thorium?

Thorium was discovered in 1828 by a Swedish chemist who named the element after the Norse god of thunder, Thor. It is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium exists in nature in a single isotopic form – Th-232 – which decays very slowly its half-life is about three times the age of the Earth. Thorium is available in small amounts in most rocks and soils, and it is three times more abundant than uranium. On average, soils contain six parts per million (ppm) of thorium. The most common source for thorium is monazite, the rare earth phosphate mineral, which normally contains about 12% thorium phosphate. One of the most important factor of Thorium is that, it is fertile rather than fissile, and can only be used as a fuel in conjunction with a fissile material such as recycled plutonium. Thorium fuels can breed fissile uranium-233 and can be used in various kinds of nuclear reactors. The use of thorium as a new primary energy source has been an exciting prospect for many years. So let’s see, how actually Thorium works

Thorium as a Nuclear Fuel

As I mentioned earlier Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233), which is an excellent fissile fuel material. In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All thorium fuel concepts therefore require that Th-232 which is first irradiated in a reactor to provide the necessary neutron dosing to produce protactinium-233. The Pa-233 that is produced can either be chemically separated from the parent thorium fuel and the decay product U-233 then recycled into new fuel, or the U-233 may be usable ‘in-situ’ in the same fuel form, especially in molten salt reactors (MSRs). Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained.

An important principle in the design of thorium fuel systems is that of heterogeneous fuel arrangement in which a high fissile (and therefore higher power) fuel zone called the seed region is physically separated from the fertile (low or zero power) thorium part of the fuel – often called the blanket. Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233, in fact all thermal breeding fuel designs are heterogeneous. This principle applies to all the thorium-capable reactor systems.

There are several reactors which are able to use Thorium as their nuclear fuel. Let me explain some of them to you.

Reactors able to use Thorium as a Fuel

• High-Temperature Gas-Cooled Reactors (HTRs): These are well suited for thorium-based fuels in the form of robust ‘TRISO’ coated particles of thorium mixed with plutonium or enriched uranium, coated with pyrolytic carbon and silicon carbide layers which retain fission gases. The fuel particles are embedded in a graphite matrix that is very stable at high temperatures. Such fuels can be irradiated for very long periods and thus deeply burn their original fissile charge. Thorium fuels can be designed for both ‘pebble bed’ and ‘prismatic’ types of HTR reactors.
• Heavy Water Reactors (PHWRs): These are well suited for thorium fuels due to their combination of: (i) excellent neutron economy (their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U-233), (ii) slightly faster average neutron energy which favors conversion to U-233, (iii) flexible on-line refueling capability. Furthermore, heavy water reactors are well established and widely-deployed commercial technology for which there is extensive licensing experience.
• Molten Salt Reactors (MSRs): These reactors are still at the design stage but are likely to be very well suited for using thorium as a fuel. The unique fluid fuel can incorporate thorium and uranium (U-233 and/or U-235) fluorides as part of a salt mixture that melts in the range 400-700ºC, and this liquid serves as both heat transfer fluid and the matrix for the fissioning fuel. The fluid circulates through a core region and then through a chemical processing circuit that removes various fission products (poisons) and/or the valuable U-233. The level of moderation is given by the amount of graphite built into the core. Certain MSR designs will be designed specifically for thorium fuels to produce useful amounts of U-233. 

Why Thorium could be safer for Environment

• The Th-U fuel cycle does not irradiate Uranium-238 and therefore does not produce the Transuranic elements. Transuranic are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other elements. These transuranics are the major health concern of long-term nuclear waste. Thus, Th-U waste will be less toxic on the 10,000+ year time scale.
• In terms of chemical stability and resistance to radioactivity thorium is a safer alternative compared to uranium.
• Thorium nuclear power reactors produce less amounts of waste compared to other nuclear fuels. The advantage is one of the key properties of thorium reactors, as the vast amount of nuclear waste created by power plants can lead to high radiation and raise temperature levels. It also avoids a need to have a huge storage facility. According to some estimates Thorium could also produce 0.6% of the radioactive waste when compared to uranium based nuclear power.
• The Nuclear waste would degrade much faster as compared to uranium based nuclear power.

Additional benefits of Thorium over Uranium

• As I mentioned earlier, Thorium is more highly concentrated ore than uranium meaning it takes less work to extract the same amount of energy generating potential during the mining process.
• There is three times more Thorium in Earth’s crust than Uranium.
• Also, in terms of Energy production, 1 ton (tonne) of Thorium is approximately equal to 35 tons of Uranium and 4M tons of Coal.

Downsides of Thorium

• We don’t have as much experience with Thorium. Due to focus on Uranium, thorium reactor have been neglected. The nuclear industry is quite conservative, and the biggest problem with Thorium is that we are lacking in operational experience with it. When money is at stake, it’s difficult to get people to change from the norm.
• Thorium fuel is a bit harder to prepare. Thorium dioxide melts at 550 degrees higher temperatures than traditional Uranium dioxide, so very high temperatures are required to produce high-quality solid fuel. Additionally, Thorium is quite inert, making it difficult to chemically process. This is irrelevant for fluid-fueled reactors.
• Put simply, thorium-based reactors are still not economically viable for the most part.
• Presence of Uranium-232 in irradiated thorium or thorium based fuels in large amounts is one of the major disadvantages of thorium nuclear power reactors. It can result in significant emissions of gamma rays.

Thorium R&D Worldwide

• India: The Indian reactors Kakrapar-1 and Kakrapar-2 were the first in the world to use thorium on a large scale. Already in 1995, the reactors succeeded to operate 400 days at full strength based on thorium. India’s long-term interest in thorium is not just about its environmental benefits. India has only one percent of the world’s uranium resources, but about 30 percent of the world’s thorium resources. Canada, the U.S., Germany, UK and the Netherlands have also tested thorium as an alternative fuel.
• Germany: In Germany had operated the Atom Versuchs Reaktor (AVR) at Jülich for over 750 weeks between 1967 and 1988. This was a small pebble bed reactor that operated at 15 MWe, mainly with thorium-HEU fuel. About 1360 kg of thorium was used in some 100,000 pebbles. Burn-ups of 150 GWd/t were achieved.
• Canada: In Canada at AECL’s Chalk River Laboratories Thorium-based fuels for the ‘Candu’ PHWR system have been designed and tested for more than 50 years, including the irradiation of ThO2-based fuels to burn-ups to 47 GWd/t. Dozens of test irradiations have been performed on fuels including: mixed ThO2-UO2, (both LEU and HEU), and mixed ThO2-PuO2, (both reactor- and weapons-grade).
• USA: In the 1960s the Oak Ridge National Laboratory (USA) designed and built a demonstration MSR using U-233 as the main fissile driver in its second campaign. The reactor ran over 1965-69 at powers up to 7.4 MWt. The lithium-beryllium salt worked at 600-700ºC and ambient pressure. The R&D program demonstrated the feasibility of this system and highlighted some unique corrosion and safety issues that would need to be addressed if constructing a larger pilot MSR.

Conclusion: Thorium has definitely got potential to take over the current nuclear fuel, in result making it safer for human beings. But it's worth watching how this technology develops in future.

 Hey! Thanks for reading the whole Blog. Until now you might have understood that Thorium could replace Uranium in nuclear technology. I hope the Blog is helpful to you all and if you have any queries related to this topic make sure to write them in the comment section. If you enjoy reading my blogs then make sure to share them to your friends and family members. Bye until next blog!

                                                                            --- Sourabh M.JR             
                                     
  * DM for credits or removal request for images( no copyright intended ) 🔗 All rights and credits reserved to the respective owner.