Thorium – The Future of Energy
- Page ID
- 418942
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This Exemplar will teach the following concept(s) from the ACS Examinations Institute General Chemistry ACCM:
I.A.2.b. Atoms of one element have the same number of protons but can have differing numbers of neutrons. These are called isotopes.
I.E.2.a. Quantitative conversions with the concept of the moles are important.
I.F.2.a. Balancing nuclear reactions is based on the simultaneous conservation of both atomic and mass numbers.
I.F.3.c. In a mixture, the amount of a radioactive isotope can be determined via quantitative measurement of radioactive events.
Nuclear Energy
Nuclear energy is often represented in modern-day media as an ill-understood, dangerous practice that can cause disasters like Chernobyl or give people radioactive superpowers like in Marvel comics. However, looking past the stereotypes and oversimplifications, nuclear energy can be described as a useful source of renewable energy that gets safer and more efficient through current advancements in chemical research.
Figure 1. Molten Salt Reactor (from Touran, 2022)
In this figure, we can see the Molten Salt Reactor process in which Thorium 232 is treated to produce energy, starting with a liquid salt that differs from most reactors with solid fuel. Here, a mix of Thorium, Uranium, and fluoride salt pass through circuits to generate heat to produce electricity. This method allows the fluoride forms to be isolated to stop the decay at crucial times.
One topic of current chemical research is that of Thorium-based nuclear reactors, as opposed to the more widely used Uranium-based reactors. One of the most widely talked about reactors for Thorium are Molten Salt Reactors (MSRs) which have been operating since the 1960s (WNA, 2021). There are many advantages to using this type of reactor that may signal a cleaner and safer version of nuclear energy.
Advantages to Thorium
Sustainability Benefits
One of the primary benefits of such an advanced reactor is its ability to utilize more fuel than conventional reactors (which are only capable of burning 1% of the Earth’s Uranium). The liquid fuel provides a more efficient way in which to pull out the fission products quickly to prevent decay into unstable or uncontrollable elements. In addition, once Pa-233 is formed from Thorium, it can be withdrawn to decay to U-233 without further loss of neutrons that spoil the yield. Efficiency is maximized when neutron loss is reduced – this is achieved by using MSRs to cut out common energy loss through cladding, fuel ducts, and grid spacers in typical reactors (Touran, 2022).
Economic Benefits
Predicted economic benefits in MSR systems are a result of the efficiency of reactions achieved through continuous cycles that do not have to be interrupted as often. This process allows the adding of materials for high-capacity throughput. MSRs can handle high temperatures reducing loss of energy when converting heat to electricity and allows for the reaction to occur without pressurized cooling agents. Despite the complexities associated with this model, it is actually a much cheaper design because of its “vat” setup (Touran, 2022).
This system provides a more sustainable and economic approach to nuclear energy, especially the improved safety. Safety is crucial to prevent nuclear catastrophes ensured by lower pressure, being less reactive with air and water, having less of a need to overload and be at risk for reactions with natural disasters, and also a freeze plug protected drain tank mechanism (Figure 1) that is unique to the fluid fuel. There are certainly setbacks to this method including diversity of elements degrading equipment, maintenance of highly radioactive components, as well as the escaping of Tritium. Overall, the chemistry and practicality of MSR ensure more energy for less of an environmental cost, as well as demonstrating that nuclear chemistry is multidimensional and can be modified at an atomic level to provide more benefits than ever before. There is a lot of research and work to be done on design before full commercial use but it is on the right track (Touran, 2022 and WNA, 2021).
Thorium-233: How it Works
To begin with, the atom in question is not just Thorium but specifically Thorium-233. This is an isotope, an atom that has the same number of protons as Thorium but with a different number of neutrons. The number denoted after the dash is the weight of the atom– protons and neutrons combined. Although isotopes may share the same element name, different isotopes can have wildly different properties. For example, the half-life, or time it takes for the amount of the isotope to decay by half, is 68.9 years for the Uranium-232 byproduct, but billions of years for Uranium-238 (Harack, 2010). This is a point in favor of Thorium reactors, which produce the former as radioactive waste that decays fairly quickly. Other reactors that produce U-238 have to deal with dangerous radioactive waste that can seemingly last for an eternity.
Uranium-232 is one of many byproducts from the reaction that is used to turn Thorium-232 into Uranium-233. An advantage of using Thorium for the reaction in the MSR is that more of its byproducts have relatively short half-lives to decay into non-radioactive safe waste compared to using Uranium as a primary reactant. U-232 has a half-life of 68.9 years.
How many years will it take for a sample of U-232 to decay to 1% of its original content?
- Solution
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First, determine how many half-lives have occured. Treat 1 as 100%, so 1% would be .01
\[1(\frac{1}{2})^x=.01\nonumber\]
\[x=log_{\frac{1}{2}}(.01)=6.64 \nonumber\]
Then, multiply the number of half-lives by the number of years in each half-life to get the total number of years.
\[6.64(68.9)=457\nonumber\]
Another property of Thorium reactors is that the isotope in question is considered fertile, as opposed to fissile. This means that it will not do anything on its own– it must first be bombarded with neutrons in order to become the fissile isotope Uranium-233, which is finally able to produce energy. This is a useful safety measure compared to reactors that only use fissile material– if things go wrong, the reactor can be shut off and the fertile Thorium reactants will be much more safe compared to other possible materials.
Figure 2. Thorium Fuel Cycle (from Energy Education)
Th-232 treatment to become fissile U-233. First, a neutron is added to Th-232 to become Th-233, then beta decay is undergone twice to become U-233.
This transformation shown in Figure 2 is achieved through radioactive decay, where unstable radioactive elements will release different types of particles such as alpha or beta. In this case, beta particles are emitted twice to change Th-233 into U-233. When the particles are released, the number of protons or neutrons of the original isotope will change correspondingly and the atom may become a different element/isotope (Figure 3).
Figure 3. Decay Transformation Chart (from Radioactivity and Nuclear Chemistry)
Different types of radioactive decay. Atoms and particles are denoted with a superscript
number displaying the mass number and the subscript displaying the number of protons.
The Thorium Cascade produces byproducts including actinium, bismuth, lead, polonium, radium, radon, and thallium.
What type of decay is demonstrated by the following examples from the Thorium Cascade?
\[{}_{92}^{232}U \xrightarrow {}_{90}^{228}Th \]
- Answer
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The loss of 4 in the top number and 2 in the bottom number indicates that a helium, or alpha particle has been lost.
\[Alpha -\ {\alpha} \nonumber\]
\[{}_{82}^{212}Pb \xrightarrow {}_{83}^{212}Bi\]
- Answer
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The gain of 1 in the bottom number indicates that an electron, or beta particle has been lost.
\[Beta - {\beta} \nonumber\]
\[{}_{83}^{212}Bi \xrightarrow {}_{81}^{208}Tl\]
- Answer
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The loss of 4 in the top number and 2 in the bottom number indicates that a helium, or alpha particle has been lost.
\[Alpha - {\alpha} \nonumber\]
\[{}_{81}^{208}Tl \xrightarrow {}_{82}^{208}Pb\]
- Answer
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The gain of 1 in the bottom number indicates that an electron, or beta particle has been lost.
\[Beta - {\beta} \nonumber\]
Radioactive decay is also the process by which energy is mainly generated from radioactive isotopes. The newly produced Uranium-233 fuel, through its radioactive decay into various byproducts, produces huge amounts of energy through this method. When balancing these nuclear equations, one can add up the masses of the particles on both sides of the equation to find the mass defect, the difference between the two sums. This mass is converted to energy through Einstein’s famous equation E= mc2 , where c is the speed of light, m is the mass defect in kg, and E is the energy produced in Joules. The equation is such that with even tiny amounts of mass lost, a large amount of energy is generated.
Thorium-232 is considered a fertile isotope. In order to be used as a nuclear fuel, it must first undergo treatment to become Uranium-233, a fissile isotope corresponding to the reaction shown below:
\[{}_{90}^{232}Th + n \xrightarrow {}_{90}^{233}Th \xrightarrow {}_{91}^{233}Pa + {}_{-1}^0{e} \xrightarrow {}_{92}^{233}U + {}_{-1}^0{e} + {}_{-1}^0{e} \nonumber\]
How much energy (in J/mol Th-232) is produced in this reaction?
mass of neutron: 1.008865 amu
mass of electron: 0.0005486 amu
mass of Th-232: 232.03805 amu
mass of U-233: 233.039 amu
- Solution
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First, calculate the mass defect by finding the difference in masses of the beginning and end terms of the equation:
\[232.03805amu + 1.008665 amu -(233.039 amu + 2(0.0005486 amu)) = .0066178amu \nonumber\]
Then, convert the mass defect from amu into kg per mole. Einstein's equation needs to use mass in kg, and the problem asks for one mole. Multiply by the mass of an atom in kg, then multiply by Avogadro's number (the number of particles in a mole).
\[.0066178amu*\frac{1.66*10^{-27}kg}{atom}*\frac{6.022*10^{23}atoms}{mol}=6.616*10^{-6}kg \nonumber\]
Now, plug in the mass defect per mole into the equation
\[E=m*c^2\nonumber\]
to find the energy produced. Plug in the mass that was just solved for as well as the speed of light.
\[E=(6.616*10^{-6}kg)(299792458m/s)^2\nonumber\]
\[E=5.946*10^{11}J\nonumber\]
References
(1) Touran, N. Molten salt reactors https://whatisnuclear.com/msr.html#:...0power%20plant (accessed Nov 11, 2022).
(2) Molten Salt Reactors. https://world-nuclear.org/informatio...-reactors.aspx (accessed Nov 11, 2022).
(3) Harack, B. Does nuclear waste last millions of years?: Vision of Earth. https://www.visionofearth.org/news/d...20time (accessed Nov 11, 2022).
(4) Thorium fuel cycle. https://energyeducation.ca/encyclope...ium_fuel_cycle (accessed Nov 11, 2022).
(5) Radioactivity and Nuclear Chemistry. https://wou.edu/chemistry/courses/on...radioactivity/ (accessed Nov 11, 2022).
Additional Reading
Ferrell, M. Revisiting thorium energy – the future of nuclear power? - undecided with Matt Ferrell. https://undecidedmf.com/episodes/rev...-nuclear-power (accessed Oct 27, 2022).
Humphrey, U. E.; Khandaker, M. U. Viability of thorium-based nuclear fuel cycle for the Next Generation Nuclear Reactor: Issues and Prospects. Renewable and Sustainable Energy Reviews 2018, 97, 259–275.
Thorium. https://world-nuclear.org/informatio...n/thorium.aspx (accessed Oct 27, 2022).
Wikipedia contributors. Thorium fuel cycle. https://en.wikipedia.org/w/index.php?title=Thorium_fuel_cycle&oldid=1125247121 (accessed Nov 11, 2022).