This is how Generation IV reactors work

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Google uses a lot of energy; training and operating its AI software is particularly energy-intensive. The technology group therefore hopes to purchase energy from the new microreactors from developer Kairos Power from 2030 onwards. What's special about them is that they are cooled not with water, but with a chemically stable mixture of molten lithium and beryllium fluoride salt (FLiBe). In early October, Kairos Power began construction of a fluoride salt production facility in New Mexico, USA. The facility will produce high-purity liquid fluoride salt as a coolant for the demonstration reactor and the commercial reactor KP-FHR.

Research on small reactors will soon be underway in Switzerland too: the Center for Nuclear Technology and Science at the Paul Scherrer Institute (PSI) in Willigan has agreed to collaborate on an experimental reactor with Danish molten salt developer start-up Copenhagen Atom. The Thorium Liquid Salt Experiment is designed to provide experience with molten salt reactor technology. A small fourth-generation modular nuclear reactor, about the size of a shipping container, is expected to be operational in Filligan within three to four years. This is allowed, but requires very strict security requirements.

The two collaborations are examples of a trend: Nuclear energy is now increasingly playing a role again – as a possible alternative to burning fossil fuels – in discussions about what should be done to combat global warming. Before that, there was the Chernobyl disaster in 1986, and the meltdown at Japan's Fukushima plant 13 years earlier brought nuclear power plants into disrepute. Countries such as Germany and Switzerland subsequently decided to phase out nuclear energy.

Almost all nuclear reactors currently used for commercial energy production are light water reactors – many of them from the so-called third generation, the first representatives of which came into operation in 1996. Although third-generation reactors are safer than second-generation reactors, they still pose a risk of meltdown, with potentially devastating consequences.

As announced by the manufacturer, new generation IV reactors now avoid the inherent danger of core meltdowns. A core meltdown occurs when cooling of the fission products no longer works and the fuel elements melt due to overheating and form a clump, which further heats up internally due to the decay of the fission products. The reactor core could then explode. The danger to people and the environment is that highly radioactive material breaks through the power plant's shielding.

The microreactors of Kairos Power and Copenhagen Atomics are so-called small modular reactors (SMRs), which are also molten salt reactors and belong to the fourth generation of nuclear reactors. What is an SMR and how do molten salt reactors work?

small modular reactor

Small modular reactors (SMRs) are compact nuclear reactors, which are smaller and often prefabricated than conventional reactors. Their modular design makes them easier to transport to the assembly site. They should be safer and, most importantly, avoid the financing problems associated with the construction of large nuclear power plants, since the limited number of components can reduce costs.

There are a variety of SMR concepts, using vastly different designs and coolants, from scaled-down versions of traditional nuclear reactors to new designs for fourth-generation nuclear power plants:

Visitors view a model of a small multifunctional modular reactor in Beijing.Image: Imagery

The concept of SMR is not new; it dates back to developments in the 1950s, when attempts were made to use nuclear power as a propulsion technology for military submarines. However, to date, few systems have progressed beyond the conceptual research stage. In 2020, Russia installed two SMR pilot plants on floating platforms. The SMR project planned to be built by NuScale Power in Idaho, USA, was originally scheduled to go online in 2028, but in November last year the company announced the cancellation of the project. Nuclear power buyers have backed out because forecast prices have risen too much.

Schematic of the NuScale power module reactor. The NuScale power module is the first SMR approved for commercial use in the United States.Graphics card: NuScale

Another company moving forward with SMR projects is North Carolina-based General Electric Hitachi Nuclear Energy (GEH). Their SMR design, the BWRX-300, is a boiling water reactor with passive safety, meaning it remains safe even under extreme conditions without the need for an external power source or human intervention. Various projects in Canada, Sweden, Poland and Estonia are in various stages of implementation, with most not expected to be operational until the 2030s.

Schematic diagram of the BWRX-300.Illustration: GO

SMR technology is often promoted as a complement to renewable energy because SMR can be quickly turned on and off based on fluctuations in production and demand. Small and medium-sized reactors should also be able to be easily disassembled and scrapped in factories. However, exactly how expensive it is to produce electricity remains unclear; however, NuScale's example shows that it will have a hard time surviving in the electricity market. According to a report by the independent German Öko-Institut, an average of 3,000 SMRs need to be produced before it is worthwhile to start SMR production. Regardless, the technology comes too late to meet Paris climate goals, even if it helps combat global warming.

liquid salt reactor

The two micro-reactors of Kairos Power and Copenhagen Atomics mentioned at the beginning are molten salt reactors. In this type of reactor, the fission fuel is dissolved in a liquid salt – usually a mixture of fluoride or chloride salts – at temperatures in excess of 600 °C. These salt melts act as both coolant and fuel carrier. The fuel is evenly distributed in the reactor's main circuit, where the fission reaction also occurs. This means no fuel rods require water cooling. Therefore, core meltdown in the classical sense is ruled out.

Additionally, there is no possibility of a steam explosion in the reactor core area because, unlike conventional pressurized or boiling water reactors, these reactors operate at atmospheric pressure. If it overheats, the reactor will cool itself because the fuel expands and fewer fissile atoms are struck by the neutrons, slowing the chain reaction. If a system failure occurs, the salt flows into the containment vessel, where it can cool and crystallize, stopping the chain reaction.

Schematic diagram of a liquid salt reactor.Picture: Wikimedia

Liquid salt reactors have efficient energy conversion because they operate at very high temperatures. Because the salt melt has good heat transfer properties, the reactor can be made much smaller than a gas-cooled reactor with the same output. They can reprocess spent fuel rods during ongoing operations; therefore, they are “feeders”, not only consuming fuel but also producing new fuel. This means they produce less radioactive waste and only need to be stored for 500 years instead of 10,000 years. However, fission products emit intense gamma radiation, which makes them difficult to process.

If this reactor type is used as a breeder reactor, a small amount of uranium or plutonium is enough to start it; it can then be operated using non-fissile nuclides such as thorium. Thorium occurs about three to four times more frequently in nature than uranium. Finally, the design of molten salt reactors may also make it difficult to produce weapons-grade materials. Therefore, they may provide greater security for military use.

The disadvantage of this type is that salt is corrosive and the material corrodes quickly. This requires the development of complex materials, such as special alloys, that can withstand the high temperatures and chemical aggressiveness of liquid salts. So while the concept has actually been known for a long time, the upfront cost of research and development is very high.

There is already a prototype of a thorium-fueled liquid salt reactor. This Chinese experimental reactor, located in the Gobi Desert, has been operational since June 2023; it is expected to maintain a continuous nuclear reaction and generate 2 megawatts of thermal energy from October 2023. Special high-temperature alloys were developed for the reactor that can withstand extreme temperatures, radiation and chemical corrosion.