Nuclear energy, once hailed as a source of power “too cheap to meter,” has long been a double-edged sword. While it provides immense, low-carbon power, it’s also plagued by concerns over radioactive waste, safety, and the scarcity of its primary fuel, uranium. For decades, a promising alternative has lingered in the background: thorium reactors. These reactors, which have been called the “solid-state batteries of nuclear power,” offer a vision of abundant, clean, and safe energy. Now, a Danish company, Copenhagen Atomics, is on the cusp of making this vision a reality with its innovative small modular reactors.
These reactors are designed to be shipping container-sized, self-maintaining units that can operate reliably for years. They tackle the engineering challenges of traditional nuclear power by using a liquid fuel design and leveraging the unique properties of thorium to create a self-sustaining chain reaction. This technology could be the key to unlocking a new era of clean energy, but significant regulatory hurdles and technological challenges remain.
The Promise of Thorium: Abundance and Sustainability
The conventional nuclear reactors in use today rely on uranium-235, a rare isotope that makes up only 0.7% of natural uranium. While current reserves are estimated to last for another 200 years at current consumption rates, the need for new exploration and processing techniques highlights the resource’s finite nature. Thorium, on the other hand, is three to four times more abundant than uranium in the Earth’s crust, making it a far more plentiful potential fuel source. In fact, due to the scarcity of the fissile uranium-235 isotope, thorium is nearly 500 times more abundant than the uranium we currently use for nuclear power.
Unlike uranium-235, thorium-232 is not fissile on its own, meaning it cannot sustain a chain reaction. However, it is a “fertile” material that can be converted into fissile fuel. This conversion process is the core of the thorium dream. By seeding a thorium reactor with a small amount of fissile material, such as uranium-235 or plutonium-239, a chain reaction can begin. When thorium-232 absorbs a neutron, it transmutes through a series of steps into uranium-233, which is fissile and can sustain a chain reaction.
This is where the magic happens. While traditional reactors simply burn up their fuel, a thorium breeder reactor creates more fissile fuel than it consumes. The chain reaction of uranium-233 produces neutrons, and these extra neutrons are used to “breed” more uranium-233 from the surrounding thorium. This means that a single thorium reactor could generate enough fuel to start another one every 10 to 20 years, effectively doubling nuclear capacity every few decades without depleting the world’s uranium reserves. This breeding capability is the reason why thorium reactors could provide energy for thousands of years, potentially solving the problem of fuel scarcity for good.
A Different Kind of Reactor: The Molten Salt Reactor (MSR)
To harness the potential of thorium, a fundamentally different reactor design is required: the molten salt reactor (MSR). Unlike traditional reactors that use solid fuel rods, MSRs dissolve their nuclear fuel (uranium or thorium) into a circulating liquid salt. This liquid fuel acts as both the fuel source and the coolant, a major departure from conventional reactor designs.
This unique liquid fuel design offers several key advantages. First, it allows for the continuous removal of waste products. As nuclear fission occurs, waste products can be extracted from the salt while the reactor is still running. This continuous clean-up process is crucial because it prevents waste products from absorbing neutrons, ensuring that the chain reaction remains efficient and that neutrons are available for breeding new fuel from the thorium.
Second, MSRs operate at atmospheric pressure, in stark contrast to the high-pressure environments of water-cooled reactors. This eliminates the risk of high-pressure steam explosions and a sudden loss of coolant. In the event of a breach, the molten salt simply cools and solidifies, trapping radioactive material and preventing a meltdown. Furthermore, many MSR designs incorporate a “frozen plug” at the bottom of the reactor vessel. If the system overheats, this plug melts, allowing the molten salt to drain by gravity into a shielded dump tank. This passive safety feature ensures a “walkaway safety” design, where the reactor shuts down safely on its own, even if all power and human intervention are lost.
Copenhagen Atomics’ MSR design, with its layered “Onion Core,” takes this concept a step further. The hot salts in the core are surrounded by a heavy water moderator that slows down neutrons, making them more easily absorbed by thorium. An outer “breeding blanket” of additional thorium salts then captures these neutrons to breed uranium-233. This continuous, on-site fuel processing is a game-changer, eliminating the need for frequent refueling and allowing for a self-sustaining system.
Overcoming the Challenges: Engineering and Regulation
While the concept of thorium reactors is compelling, bringing them to commercial reality presents significant engineering and regulatory hurdles. The corrosive and high-temperature nature of the molten salt presents a major challenge for the pumps, pipes, and control systems that must be built to withstand a constant bath of salt as hot as lava. Additionally, the constant bombardment of neutrons from the core slowly degrades materials at an atomic level. Since these systems cannot be serviced by humans once the reactor is operational, they must be designed to function flawlessly for years.
Copenhagen Atomics is tackling these challenges with innovative solutions. They have developed a molten salt pump that uses magnetic bearings to levitate its rotating parts, minimizing wear and tear and allowing it to operate for years without maintenance. The company is also using a custom steel alloy that can endure the corrosive salts and neutron bombardment for at least five years. After this period, the entire reactor module—core, pumps, and pipes—is designed to be swapped out for a fresh one. While this may seem counterintuitive, it offers a major advantage: continuous technological upgrades. As CEO Thomas Jam Pedersen notes, “Even after 50 years, it’s still five-year-old technology.” This modular, swappable design allows for constant improvement in efficiency and performance.
The biggest current obstacle, however, is not technology but regulation. The path to nuclear approval is a “Matterhorn of paperwork,” with licensing and approvals being one of the most expensive and time-consuming parts of the process. Copenhagen Atomics plans to test a full-scale prototype at the Paul Scherrer Institute in Switzerland in 2026. This public-private partnership will be the first test of a thorium reactor in Europe and will be crucial for validating their computer models. The test will run at just 1% of the reactor’s expected power output, but its success will be a critical step toward a self-sustaining reactor.
Thorium’s Unique Benefits: Waste and Security
Beyond its potential for limitless energy, the thorium fuel cycle offers significant advantages in managing nuclear waste and ensuring security. The waste from traditional uranium-235 reactors includes plutonium and other elements that remain dangerously radioactive for tens of thousands of years, requiring long-term underground storage. In contrast, the waste from a thorium reactor, which is based on uranium-233, decays much faster and can be safely stored above ground for just a few hundred years. This dramatically reduces the long-term burden of nuclear waste management.
Furthermore, thorium reactors can act as “waste burners.” The initial fissile material needed to start the thorium chain reaction can be sourced from the spent fuel of traditional uranium reactors, allowing thorium systems to consume and reduce existing nuclear waste. Copenhagen Atomics plans to incorporate this feature into its design, making its reactors a solution for both future energy needs and past nuclear liabilities.
In terms of security, the thorium fuel cycle is inherently proliferation-resistant. During the production of uranium-233, a small amount of uranium-232 is also formed. This isotope emits intense gamma rays that are powerful enough to pass through several centimeters of lead, making uranium-233 a “logistical nightmare” to steal or weaponize. This natural radiation signature makes it incredibly difficult to handle and transport, serving as a powerful deterrent to illicit use. Copenhagen Atomics plans to manage this by designing its reactors to be completely hands-off, with thick steel shielding and remote-operated cranes to keep humans away from the contained radiation.
The Road Ahead: Commercialization and a New Business Model
Copenhagen Atomics aims to go commercial within a decade, but reaching that goal requires not just a new reactor but a new business model. Instead of selling its reactors, the company plans to build, site, operate, and eventually decommission each unit itself. In exchange for taking full responsibility for the entire nuclear lifecycle, from site evaluation to waste management, clients will purchase the heat produced by the reactors for industrial applications or for conversion to electricity. This model makes nuclear power viable even in countries like Indonesia, the company’s first customer, which have no prior experience with nuclear power.
The ultimate goal remains the creation of a true breeder reactor that produces more fissile fuel than it consumes. While China has made strides with its own molten salt reactor, no company has yet demonstrated a true breeder capability. Copenhagen Atomics believes that by achieving this milestone and scaling production to a factory level, they can drive energy costs as low as $20–$40 per MWh, a quarter of the current average for nuclear energy.
This technology represents a potential paradigm shift in the energy landscape. Nuclear reactors can provide something solar and wind can’t: a continuous, ultra-low emissions source of heat for industrial processes like producing hydrogen and ammonia, smelting aluminum, and drying paper. The thorium dream, once championed by Alvin Weinberg and then defunded, is being resurrected by companies like Copenhagen Atomics. While the timeline for a fully self-sustaining breeder reactor is uncertain, the pursuit is more vital than ever, as thorium might just be the missing piece in the clean energy puzzle, offering a key to a future of abundant and sustainable power.
What do you think? Is this technology a realistic roadmap to a clean energy future, or is it still too far on the horizon?