Future nuclear energy systems could profit significantly from molten salt reactors (MSRs) as they offer significant advantages in terms of efficiency and safety. The near-term implementation of this novel technology may be made feasible by advanced research, technological development, and licensing across various nations.
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Molten Salt Reactors
Nuclear reactors known as molten salt reactors (MSRs) employ fluid fuel, such as extremely hot fluoride or chloride salt, as opposed to the solid fuel utilized in conventional reactors. Since the fuel salt is a liquid, it may serve as both a fuel and a coolant, generating heat and transferring it to the power plant, respectively.
MSRs work on the same basic concept as modern nuclear power reactors, utilizing controlled fission to generate steam to power turbines that generate electricity. However, there is a crucial difference: molten salts play an essential role in the reactor core, primarily as a coolant instead of water, as most currently operational reactors do. In addition, rather than using fuel rods, most MSR systems use nuclear fuel dispersed in the coolant.
Why Molten Salt?
The nuclear industry acknowledges molten salt as an appropriate working fluid for reactor cooling, power transmission, fuelling, and fission product absorption. Many of the salts under consideration are low-cost, non-toxic, and readily transportable.
Molten salt offers a range of advantages in its role as a coolant and nuclear fuel, encompassing security, efficiency, and versatility. One particularly intriguing aspect of molten salt fuel is its inherent safety feature: if the salt overheats, it undergoes expansion, diminishing the efficiency of the fission process and causing the reactor to shut down automatically.
The Molten Salt Reactor (MSR) core can adapt its power level in response to heat loss, enabling the precise generation of electricity to meet customer demand.
Another significant benefit is the flexibility of fuel options. Salts derived from uranium, thorium, and plutonium can all serve as viable MSR fuels. As the salt remains liquid at reactor-operating temperatures, fresh fuel can be introduced, and existing fuel can be cleaned, purified, and managed. At the same time, the reactor continues to operate, eliminating the necessity for fuel shutdowns.
Limitations of Conventional Nuclear Reactors
Water is injected under high pressure into the reactor's core in current nuclear reactors, where fuel pellets are enclosed in metal rods for fission. This raises the temperature of the water to around 600 Fahrenheit, but the high pressure prohibits it from boiling off.
The super-hot liquid water is then pushed through a chamber filled with more liquid. Its heat causes the water to boil, which produces the steam required to turn the turbines. The cooled water then returns to the fuel chamber to be warmed, allowing the cycle to continue.
The high pressure required to preserve super-hot water as a liquid raises the possibility of a leak. If water penetrates, the fuel can heat up, melting the safety rods and potentially spilling radioactive substances into the water and surroundings. To avoid this, reactors require several backup systems and layoffs, increasing their cost and complexity.
Advantages of Molten Salt Reactor
Molten salt reactors employ molten salt, solid at ambient temperature but liquid at high temperatures, as the medium transporting heat and keeping the fuel at a constant temperature instead of water.
The salt proposed for these reactors stays liquid at temperatures as high as 2,500 Fahrenheit, even without pressurization. The increased temperature improves the efficiency of the reactor, resulting in additional energy, and the absence of pressurization lowers the likelihood of a leak.
Molten salt reactors also have a security feature known as a "freeze valve" or "freeze plug." The molten salt above is separated from a storage tank below by this plug. If the system becomes too hot, the valve melts, and the molten salt drops into the tank under gravity, preventing a tragedy even if all backup mechanisms fail.
Challenges Associated with Molten Salt Reactors
In 1965, researchers at Oak Ridge National Laboratory developed the Molten Salt Reactor Experiment (MSRE), the first demonstration of the concept of a molten salt reactor capable of self-sustaining fission. However, it was abruptly shut down 167 times during the following four years, primarily due to technical issues involving various components, and it was permanently shut down in 1969. Even today, no material can operate adequately in a molten salt reactor's high-radiation, high-temperature, and corrosive environment.
While advancement in MSRs has proceeded in several countries in the past few years, commercial installations have remained distant. This is due to an array of factors, including regulatory constraints such as a lack of MSR licensing regulations, as well as supply chain challenges in acquiring specialized components.
Recent Research and Innovations in Molten Salt Reactors
The first fuel-bearing molten chloride salt irradiation experiment has been planned at a Laboratory Directed Research and Development initiative at INL. This experiment inserts encapsulated fuel salt into an active reactor to comprehend how the characteristics of chloride fuel salt change during irradiation.
Another project, the Molten Salt Thermophysical Examination Capability, is a cutting-edge facility where researchers will handle and closely study irradiated fuel salt using specialized equipment within a protected glovebox. By measuring density, heat capacity, and thickness, researchers seek to learn how materials will react under operational circumstances.
Several MSR designs, including those in the United States and Canada and thorium-based MSRs in China, are reaching deployment readiness. The latter uses thorium-uranium fuel to generate fissile uranium-233 from the thorium in the reactor core. Transmuted uranium-233 is then used as fuel. Some MSRs can be powered by reactor-grade plutonium recovered from SNF stocks, which has the potential to significantly reduce the burden of maintaining SNF, some of which can be radioactive for thousands of years.
Employing a liquid fuel/coolant conjunction in MSRs helps sustain the fundamental stability of the reactor core, allowing for simple operation at decreased power while maintaining safety. Such reactors might help with electrical grid balancing, necessary to accommodate the growing percentage of renewable energy. They might also be utilized to minimize the waste produced by existing reactors.
If researchers could develop a molten salt reactor that can survive corrosion and solve the technological problems that the MSRE faced, the device could increase the supply of electrical power generated by nuclear fission and advance us closer to a clean energy future.
References and Further Reading
Arave, A. (2023, June 19). How molten salt could be the lifeblood of tomorrow’s nuclear energy. Idaho National Laboratory. Available at: https://inl.gov/molten-salt-reactors/how-molten-salt-could-be-the-lifeblood-of-tomorrows-nuclear-energy/
Molten salt reactors could save nuclear power. (2022, October 30). Freethink. Available at: https://www.freethink.com/environment/molten-salt-reactor-52913
Roper, R., Harkema, M., Sabharwall, P., Riddle, C., Chisholm, B., Day, B., & Marotta, P. (2022). Molten salt for advanced energy applications: A review. Annals of Nuclear Energy, 169, 108924. https://www.sciencedirect.com/science/article/abs/pii/S030645492100801X
Wu, J., Chen, J., Cai, X., Zou, C., Yu, C., Cui, Y., & Zhao, H. (2022). A Review of Molten Salt Reactor Multi-Physics Coupling Models and Development Prospects. Energies, 15(21), 8296. https://www.mdpi.com/1996-1073/15/21/8296