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| Fig. 1: Proposed Thorium fuel cycle. [5] (Courtesy of the DOE.) |
The discovery of Thorium in 1829 by Jöns Jacob Berzelius, sparked by a rock specimen found near Brevig, Sweden, set the stage for a new era of exploration. This specimen, later identified as Thorium silicate or thorite, marked the initial foray into understanding this intriguing element. [1]
In 1898, the early pioneers of radioactivity, Gerhard Schmidt and Marie Curie, unveiled Thorium's radioactive properties. Schmidt's meticulous experiments, coupled with Curie's ingenious electrometer measurements, revealed faint electrical currents emanating from Thorium, akin to its radioactive counterpart, Uranium. [2]
In the pursuit of unlocking the energy potential of Thorium, scientists embarked on a journey of innovation. One notable milestone in this journey is the Thorium High-Temperature Reactor (THTR-300) in Germany. Operational from 1983 to 1989, this remarkable feat showcased Thorium's viability as a nuclear fuel source, affirming its potential for widespread adoption. [3] However, the THTR-300 was prematurely shut down and decommissioned due to political reasons rather than technical or operational issues. Despite its initial aim to serve as a prototype demonstration plant for future High Temperature Reactors (HTRs), the project faced challenges and controversy, leading to its closure. The decision to shut down the THTR-300 was influenced by changing political dynamics and public perceptions surrounding nuclear power in Germany during that time. [4]
Exploring various reactor designs, researchers sought to maximize Thorium's thermal breeding capabilities, aiming to produce more U-233 than it consumes. Among these designs is the concept of a "fertile matrix," incorporating Plutonium as a fissile catalyst. Unlike Uranium-based reactors, this approach does not yield new Plutonium but marks a significant step towards sustainable nuclear energy. [3] Embracing a heterogeneous fuel arrangement, where the "seed" of high fissile material interacts with the "blanket" of low-power Thorium, researchers orchestrate a delicate dance of nuclear transmutation. This intricate design, known as a heterogeneous fuel arrangement, serves as the linchpin of all Thorium-capable reactor systems.
Thorium finds application in various reactor architectures, each offering distinct advantages. Notable among these is the Heavy Water Reactor (PHWR), renowned for its efficient neutron consumption and adaptable refueling capabilities. [2] They also have flexible refueling capabilities as well as a faster average neutron energy which is more favorable for producing U-233. [2] This technology has already been well established and deployed commercially making it one of the more promising methods for effective Thorium powered nuclear energy. Additionally, Molten Salt Reactors (MSRs), still in their infancy, show promise with their fluidic embrace of Thorium and Uranium. [3] Specifically either U-233 or U-235 can be used in combination with fluorides as well as Thorium to form a salt mixture with a melting range of around 400-700°C. This molten salt mixture serves as both the heat transfer fluid as well a matrix in which the fissions take place. [3] The designs of MSRs can be specifically altered to enhance the amount of U-233 produced, making MSRs a promising potential option for nuclear power generation. Fig. 1 describes the recycling process for a Molten Salt Breeder Reactor (MSBR) from Oak Ridge National Laboratory in 1950. [5] In this process Thorium fuel is dissolved in a molten fluoride salt mixture, serving both as fuel and coolant. As the reactor operates, Thorium undergoes nuclear reactions, producing energy and accumulating fission products within the molten salt. Continuous online reprocessing in molten salt reactors involves chemically separating fission products from the salt fuel mixture during reactor operation. [3] This process typically utilizes sophisticated chemical separation techniques, such as solvent extraction or ion exchange, integrated directly into the reactor system. [3] By continuously removing fission products from the fuel mixture, the reactor can maintain optimal performance and prolong fuel cycle longevity, contributing to the efficiency and sustainability of the nuclear energy production process. [3] Valuable materials such as Thorium and Uranium-233 are recovered and recycled back into the reactor, sustaining the nuclear reactions. The remaining waste is managed within the reprocessing facility. This closed-loop recycling process allows the MSBR to operate continuously, maximizing fuel utilization, minimizing waste generation, and offering a promising pathway for sustainable nuclear energy production.
Thorium's appeal lies in its abundant presence, offering a potential reservoir of sustainable energy. With threefold abundance over uranium, Thorium beckons as a beacon of sustainability, promising a glimpse into a cleaner energy future. Thorium reactors emit fewer greenhouse gases and produce nuclear waste with shorter half-lives, signaling a shift towards cleaner energy production. [6] However, challenges remain, including operational complexities, preparatory hurdles, and the presence of U-232, casting shadows on Thorium's ascendancy.
The legacy of THTR-300 in Germany serves as a testament to Thorium's potential in shaping our energy landscape. Germany's triumph with THTR-300, operational from 1983 to 1989, underscores the practicality and promise of Thorium reactors, paving the way for further exploration and adoption. However, despite the promise of thorium there are still issues with safety as well as As the global demand for sustainable energy escalates and uranium reserves dwindle, the allure of Thorium grows stronger. In India, the minimum energy use per capita is 2500 kgoe (kilograms of oil equivalent). [6] To support the population increase in India, the only long term solutions that are viable are solar and Thorium. [6] India leads the world for having the largest Thorium deposits. However, due to the fertile nature of Thorium, India is still far away with being able to utilize Thorium as its main nuclear energy source. However, continued innovation and investments are being made in Thorium nuclear power potentially making it the future of nuclear energy.
© Beck Jurasius. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J.-P. Revol et al., Eds., Thorium Energy for the World (Springer, 2016).
[2] M. Curie, Radio-Active Substances (Legare Street Press, 2022) [A Translation from the French of Mme. Curie's Thesis Presented to the Faculty of Science of Paris].
[3] T. Kamei, "Recent Research of Thorium Molten-Salt Reactor from a Sustainability Viewpoint," Sustainability 4, 2399 (2012).
[4] M. Esch et al., "State of the Art Helium Heat Exchanger Development for Future HTR-Projects," Proc. 4th Intl. Topical Meeting on High Temperature Reactor Technologies, ASME HTR2008-58146, 28 Sep 08.
[5] J. M. Dukert, Thorium and the Third Fuel (U.S. Atomic Energy Commission, 1970).
[6] A. K. Nayak and B. R. Sehgal, Thorium - Energy for the Future (Springer, 2019).