Nuclear power renaissance
A retro wonder gleaming white in the sun, propelled by six rear-facing rotors and four jet engines affixed to the longest wings ever produced for a combat aircraft, the Convair B-36 Peacemaker looks like it flew right out of a 1950s science fiction magazine.
One of these bombers, which flew over the American Southwest from 1955 to 1957, was unique. It bore the fan-like symbol for ionizing radiation on its tail. The NB-36H prototype was designed to be powered by a molten salt nuclear reactor — a lightweight alternative to a water-cooled reactor.
Nuclear-propelled aircraft like the NB-36H were intended to fly for weeks or months without stopping, landing only when the crew ran short of food and supplies. So what happened? Why weren’t the skies filled with these fantastical aircraft?
“The problem was that nuclear-powered airplanes are absolutely crazy,” says Per F. Peterson, the William S. Floyd and Jean McCallum Floyd Chair in Nuclear Engineering. “The program was canceled, but the large thermal power to low-weight ratio in molten salt reactors is the reason that they remain interesting today.”
Because of numerous concerns, including possible radioactive contamination in the event of a crash, the idea of nuclear-powered aircraft never took off. But nuclear submarines, using water as coolant, completely replaced their combustion-powered predecessors. Civilian reactors were built on the success of submarine systems, and as a result, most nuclear reactors today are cooled with water.
While most water-cooled reactors can safely and reliably generate carbon-free electricity for decades, they do present numerous challenges in terms of upfront cost and efficiency. Molten salt reactors, like those first designed for nuclear-powered aircraft, address many of the inherent challenges with water-cooled reactors. The high-temperature reaction of such reactors could potentially generate much more energy than water-cooled reactors, hastening efforts to phase out fossil fuels.
Now, at the Department of Nuclear Engineering, multiple researchers, including Peterson, are working to revisit and reinvent molten salt technologies, paving the way for advanced nuclear energy systems that are safer, more efficient and cost-effective — and may be a key for realizing a carbon-free future.
Smaller, safer reactors
In the basement of Etcheverry Hall, there’s a two-inch-thick steel door that looks like it might belong on a bank vault. These days, the door is mostly left open, but for two decades it was the portal between the university and the Berkeley Research Reactor, used mainly for training. In 1966, the reactor first achieved a steady-state of nuclear fission.
Fission occurs when the nucleus of an atom absorbs a neutron and breaks apart, transforming itself into lighter elements. Radioactive elements like uranium naturally release neutrons, and a nuclear reactor harnesses that process. Concentrated radioactive elements interact with neutrons, splitting themselves apart, shooting more neutrons around and splitting more atoms. This self-sustaining chain reaction releases immense amounts of energy in the form of radiation and heat. The heat is transferred to water that propels steam turbines that generate electricity.
The reactor in Etcheverry Hall is long gone, but the gymnasium-sized room now houses experiments designed to test cooling and control systems for molten salt reactors. Peterson demonstrated one of these experiments in August. The Compact Integral Effects Test (CIET) is a 30-foot-tall steel tower packed with twisting pipes. The apparatus uses heat transfer oil to model the circulation of molten salt coolant between a reactor core and its heat exchange system. CIET is contributing extensively to the development of passive safety systems for nuclear reactors. After a fission reaction is shut down, such systems allow for the removal of residual heat caused by radioactive decay of fission products without any electrical power — one of the main safety features of molten salt reactors.
“Molten salts, because they can’t boil away, are intrinsically appealing, which is why they’re emerging as one of the most important technologies in the field of nuclear energy.”
— Per Peterson
The first molten salt reactor tested at Oak Ridge National Laboratory in the 1950s was small enough to fit in an airplane, and the new designs being developed today are not much larger. Conventional water-cooled reactors are comparatively immense — the energy-generating portion of the Diablo Canyon Power Plant in San Luis Obispo County occupies approximately 12 acres, and containment of feedwater is not the only reason why. The core temperature in this type of reactor is usually kept at some 300 degrees Celsius, which requires 140 atmospheres of pressure to keep the water liquid. This need to pressurize the coolant means that the reactor must be built with robust, thick-walled materials, increasing both size and cost. Molten salts don’t require pressurization because they boil at much higher temperatures.
In conventional reactors, water coolant can boil away in an accident, potentially causing the nuclear fuel to meltdown and damage the reactor. Because the boiling point of molten salts are higher than the operational temperature of the reactor, meltdowns are extremely unlikely. Even in the event of an accident, the molten salt would continue to remove heat without any need for electrical power to cycle the coolant — a requirement in conventional reactors.
“Molten salts, because they can’t boil away, are intrinsically appealing, which is why they’re emerging as one of the most important technologies in the field of nuclear energy,” says Peterson.
The big prize: efficiency
To fully grasp the potential benefits of molten salts, one has to visit the labs of the SALT Research Group. Raluca O. Scarlat, assistant professor of nuclear engineering, is the principal investigator for the group’s many molten salt studies. Scarlat’s lab is filled with transparent gloveboxes filled with argon gas. Inside these gloveboxes, Scarlat works with many types of molten salts, including FLiBe, a mixture of beryllium and lithium fluoride. Her team aims to understand exactly how this variety of salt might be altered by exposure to a nuclear reactor core. On the same day that Peterson demonstrated the CIET test, researchers in the SALT lab were investigating how much tritium (a byproduct of fission) beryllium fluoride could absorb.
Salts are ionic compounds, meaning that they contain elements that have lost electrons and other elements that have gained electrons, resulting in a substance that carries no net electric charge. Ionic compounds are very complex and very stable. They can absorb a large range of radioactive elements. This changes considerations around nuclear waste, especially if the radioactive fuel is dissolved into the molten salt. Waste products could be electrochemically separated from the molten salts, reducing waste volumes and conditioning the waste for geologic disposal. Waste might not even be the proper term for some of these byproducts, as many are useful for other applications — like tritium, which is a fuel for fusion reactors.
Salts can also absorb a lot of heat. FLiBe remains liquid between approximately 460 degrees and 1460 degrees Celsius. The higher operating temperature of molten salt coolant means more steam generation and more electricity, greatly increasing the efficiency of the reactor, and for Scarlat, efficiency is the big prize.
“If we filled the Campanile with coal and burned it to create electricity, a corresponding volume of uranium fuel would be the size of a tennis ball,” says Scarlat. “Having hope that we can decarbonize and decrease some of the geopolitical issues that come from fossil fuel exploration is very exciting.”
“Finding good compromises”
Thermal efficiency refers to the amount of useful energy produced by a system as compared with the heat put into it. A combustion engine achieves about 20% thermal efficiency. A conventional water-cooled nuclear reactor generally achieves about 32%. According to Massimiliano Fratoni, Xenel Distinguished Associate Professor in the Department of Nuclear Engineering, a high-temperature, molten salt reactor might achieve 45% thermal efficiency.
So, with all the potential benefits of molten salt reactors, why weren’t they widely adopted years ago? According to Peter Hosemann, Professor and Ernest S. Kuh Chair in Engineering, there’s a significant challenge inherent in molten salt reactors: identifying materials that can withstand contact with the salt.
“Having hope that we can decarbonize and decrease some of the geopolitical issues that come from fossil fuel exploration is very exciting.”
— Raluca Scarlat
Anyone who’s driven regularly in a region with icy roads has probably seen trucks and cars with ragged holes eaten in the metal around the wheel wells. Salt spread on roads to melt ice is highly corrosive to metal. A small amount of moisture in the salt coolant of a nuclear reactor could cause similar corrosion, and when combined with extreme heat and high radiation, getting the salt’s chemistry right is even more critical.
Hosemann, a materials scientist, uses electron microscopes to magnify metal samples by about a million times. The samples have been corroded and or irradiated, and Hosemann studies how such damage alters their structures and properties. These experiments may help reactor designers estimate how much corrosion to expect every year in a molten salt reactor housing. Hosemann says molten salt reactors present special engineering challenges because the salt coolant freezes well above room-temperatures, meaning that repairs must either be done at high temperatures, or the coolant must first be drained.
Commercially successful molten salt reactors then will have to be very reliable, and that won’t be simple. For example, molten salt reactors with liquid fuel may be appealing in terms of waste management, but they also add impurities into the salt that make it more corrosive. Liquid fuel designs will need to be more robust to counter corrosion, resulting in higher costs, and the radioactive coolant presents further maintenance challenges.
“Good engineering is always a process of finding good compromises. Even the molten salt reactor, as beautiful as it is, has to make compromises,” says Hosemann.
Peterson thinks the compromise is in making molten salt reactors modular. He was the principal investigator on the Department of Energy-funded Integrated Research Project that conducted molten salt reactor experiments from 2012 to 2018. His research was spun off into Kairos Power, which he co-founded with Berkeley Engineering alums Edward Blandford (Ph.D.’10 NE) and Mike Laufer (Ph.D.’13 NE), and where Peterson serves as Chief Nuclear Officer. The U.S. Nuclear Regulatory Commission just completed a review of Kairos Power’s application for a demonstration reactor, Hermes, as a proof of concept. Peterson says that high-temperature parts of Kairos Power’s reactors would likely last for 15 to 25 years before they’d need to be replaced, and because the replacement parts will be lighter than those of conventional reactors, they’ll consume fewer resources.
“As soon as you’re forced to make these high-temperature components replaceable, you’re systematically able to improve them. You’re building improvements, replacing the old parts
and testing the new ones, iteratively getting better and better,” says Peterson.
Lowering energy costs
California is committed to reaching net zero carbon emissions by 2045. It’s tempting to assume that this goal can be reached with renewables alone, but electricity demand doesn’t follow peak energy generating times for renewables. Natural gas power surges in the evenings as renewable energy wanes. Even optimistic studies on swift renewable energy adoption in California still assume that some 10% of energy requirements won’t be achieved with renewables and storage alone. Considering the increasing risks to infrastructure in California from wildfires and intensifying storms, it’s likely that non-renewable energy sources will still be needed to meet the state’s energy needs.
Engineers in the Department of Nuclear Engineering expect that nuclear reactors will make more sense than natural gas for future non-renewable energy needs because they produce carbon-free energy at a lower cost. In 2022, the price of natural gas in the United States fluctuated from around $2 to $9 per million BTUs. Peterson notes that energy from nuclear fuel currently costs about 50 cents per million BTUs. If new reactors can be designed with high intrinsic safety and lower construction and operating costs, nuclear energy might be even more affordable.
Even if molten salt reactors do not end up replacing natural gas, Hosemann says the research will still prove valuable. He points to other large-scale scientific and engineering endeavors like fusion reactors, which in 60 years of development have never been used commercially but have led to other breakthroughs.
“Do I think we’ll have fusion-generated power in our homes in the next five years? Absolutely not. But it’s still valuable because it drives development of superconductors, plasmas and our understanding of materials in extreme environments, which today get used in MRI systems and semiconductor manufacturing,” says Hosemann. “Who knows what we’ll find as we study molten salt reactors?”