Why Thorium is About to Change the WorldMatt FerrellUndecided – Renewable Energy and Sustainable Technology

   Thorium reactors are like the solid-state batteries of nuclear power: vastly superior, yet forever on the horizon. But now, a Danish company is closer than ever to turning them into a reality. Its reactors are designed to deliver on thorium’s decades-old promise: a self-sustaining chain reaction that breeds fresh fuel, making energy cheaper and more abundant than ever. These aren’t just thorium reactors — they’re small modular reactors, designed to function as shipping container-sized, self-maintaining batteries that operate reliably for years. So, how is this company tackling such an ambitious engineering challenge? When might commercial thorium reactors finally arrive? And […]
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Thorium reactors are like the solid-state batteries of nuclear power: vastly superior, yet forever on the horizon.

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But now, a Danish company is closer than ever to turning them into a reality. Its reactors are designed to deliver on thorium’s decades-old promise: a self-sustaining chain reaction that breeds fresh fuel, making energy cheaper and more abundant than ever. These aren’t just thorium reactors — they’re small modular reactors, designed to function as shipping container-sized, self-maintaining batteries that operate reliably for years.

So, how is this company tackling such an ambitious engineering challenge? When might commercial thorium reactors finally arrive? And is thorium the missing piece in the clean energy puzzle?

In the 1950s and 60s, the Atomic Energy Commission sought to transform nuclear energy from a source of fear into a source of energy “too cheap to meter.”¹² Under director Alvin Weinberg, scientists at the Oak Ridge National Laboratory in Tennessee developed a radically different type of reactor and fuel cycle: one that promised to breed more fuel than it burned, potentially powering civilization for thousands of years.³⁴ Their design, a molten salt reactor running on fuel bred from thorium, ran successfully for 15,000 hours from 1965 to 1969, producing a steady 8 MW of heat.⁴

So… why don’t we have this limitless thorium energy today?

Because the project was defunded after the Atomic Energy Commission shifted its research focus to a different kind of breeder reactor, which too was plagued by technical challenges until its own cancellation.⁵⁶ As for Weinberg, he was fired for continuing to champion thorium. He wrote wistfully in his memoir:⁷⁸

“I hope that in a second nuclear era, the molten-salt technology will be resurrected.”

Weinberg’s wish just might come true thanks to a Danish company. No, not the kind with icing and almonds, though that’s a pretty sweet solution to a different kind of energy slump.

Copenhagen Atomics is a startup out of Denmark reviving Weinberg’s hopes for a so-called breeder reactor, one that creates more fissile fuel than it burns. And they’re one of a handful of companies answering the call for cheaper nuclear power with small modular reactors.

When I say small, I mean Copenhagen Atomics’ reactor is designed to fit inside a standard 40-foot shipping container.⁹¹⁰ And by modular, I mean they aim to mass-produce one reactor per day on an assembly line, then truck as many as needed to a plant site to hit the desired power output.

Shrinking nuclear reactors like this could make them cheaper to build and easier to scale with rising energy demand. And because they’re building the same design over and over, they can work out the kinks in the manufacturing process, which means delivering reactors at a predictable cost instead of the massive overruns that have plagued large, custom-built nuclear power plants.

I’m gonna let CEO and co-founder Thomas Jam Pedersen explain how they’re fitting a nuclear reactor into a box. But, first, a refresher on nuclear chain reactions:

You probably know uranium as the go-to fuel in conventional reactors. When struck by a neutron, uranium-235 splits, releasing a huge amount of energy and a few neutrons that go on to split more uranium-235. If there’s enough fuel and a moderator like water to slow those neutrons down, the chain reaction sustains itself, producing heat to make steam. Then, that steam spins a turbine to generate electricity.¹¹

The Nuclear Energy Agency estimated that worldwide uranium reserves will only power us for another 200 years at current consumption rates.¹² More recent estimates emphasize the importance of new exploration and processing techniques that will be necessary to expand its usage in the future.¹³ And as I shared in a recent episode on the revival of old nuclear power plants to power data centers, nuclear energy is set to expand worldwide as we reach for low-emissions solutions to today’s booming energy demands.¹⁴

However, there’s a solution to uranium scarcity, and that’s another element: thorium. It’s not only three to four times more common in Earth’s crust, but because only 0.7% of natural uranium is the chain-reaction-forming 235 variety, thorium is actually nearly 500 times more abundant than the uranium we rely on for nuclear energy today.⁴

Thorium can’t form chain reactions like uranium-235. But if thorium is seeded with a little uranium-235 or plutonium-239, which are both fissile materials that release neutrons when they split, then thorium-232 can absorb a neutron and become thorium-233, which beta decays into protactinium-233, and then into uranium-233.¹⁵

And this the clever bit. Because uranium-233 is fissile, it splits and releases neutrons that sustain a chain reaction. Whereas chain-reacting uranium-235 in an old-school reactor just eats up more fuel, the neutrons split out from uranium-233 create more fuel from thorium.¹⁵ How?

“For this uranium-233, you get 2.35 neutrons if you create your reactor correctly…You need one of those neutrons to split the next uranium atom in the chain reaction. And you need another… neutron to upgrade your thorium to uranium. But then you have this 0.35 extra so you can generate a little bit more fuel than you consume all the time.”

So instead of burning up an ever-dwindling supply of nuclear fuel, or even burning up as much fuel as it makes, the thorium fuel cycle lets us breed more nuclear fuel than it consumes.⁴¹⁶ Once optimized, each reactor could generate enough fuel to start another every ten to twenty years — and those new reactors could do the same. That means doubling nuclear capacity every few decades, without burning through the world’s uranium reserves.⁹

“If you really want to have 10 or 20 or a 100 times more energy on this planet, then we need what is called breeder reactors.”

This is the thorium dream, and Copenhagen Atomics’ ultimate goal: to build a breeder reactor. But to encourage a fundamentally different reaction, they need a fundamentally different reactor: the molten salt reactor, or MSR.

Unlike traditional reactors powered with solid fuel rods, MSRs are powered by uranium or thorium dissolved into molten salts. Because the fuel is liquid, waste products from nuclear fission can be extracted with the reactor still running. This continuous clean-up keeps the thorium-to-uranium reaction humming along by preserving neutrons for fuel production instead of letting them get soaked up by waste.¹⁷

Liquid fuel salt acts as a coolant, too, circulating out of and back into the reactor core to deliver heat for power generation. In Copenhagen Atomics’ layered “Onion Core” MSR, these hot salts are flanked by heavy water that slows down neutrons so they can be better absorbed by thorium in the salt. And the outer layer of the onion will be a “breeding blanket” where additional thorium salts will absorb neutrons produced by the chain reaction within the reactor core, breeding uranium-233.

In the final design, this uranium will be transferred from the blanket to the molten salt fuel in the core.

“When you have liquids, you can actually separate these things out, and it’s a mix of chemistry and electrochemistry to do that. But it’s a completely autonomous system that runs inside the reactor while the reactor is operating.”

Another massive advantage of MSRs is that, unlike traditional water-cooled reactors that operate under high pressure to keep that water from boiling, molten salt reactors run at atmospheric pressure. That means there’s no risk of high-pressure steam explosions, and no sudden loss of coolant or rapid overheating if a breach occurs. The molten salt simply cools and solidifies, trapping radioactive material and minimizing contamination.

Many designs, like Copenhagen Atomics’, even include frozen plugs at the bottom of the reactor vessel. If the system overheats, the plug melts, and the molten salt drains by gravity into a shielded dump tank, passively shutting down the reaction and preventing a meltdown.⁴

“That’s called walkaway safety. That means that if all the electricity is gone, if all the people run away, it’s still safe. It’s safe, safe by itself because it has passive decay heat removal.”

But while thorium reactors are walk-away safe, they’re not exactly walk-up safe. A small amount of uranium-232 forms during the production of uranium-233, and gives off intense gamma rays powerful enough to pass through several centimeters of lead.⁴ That makes uranium-233, which could otherwise be an ideal bomb material, a logistical nightmare to steal or weaponize.¹⁸ However, it also means Copenhagen Atomics will have to go to great lengths to keep that radiation contained.

The plan is to avoid sending humans anywhere near the reactor containers. Each unit will be sealed in a steel box about a half-meter (or 20 inches) thick that’ll weigh in at 1,000 tons, ship in 40-ton sections, and be welded together on site.¹⁰¹⁹ Copenhagen Atomics will house 25 reactors in a single building. Remotely operated cranes and forklifts will handle the heavy lifting, keeping the process as hands-off as possible to cut radiation exposure and red tape, all in one go.

While gamma radiation makes thorium reactors trickier to handle, nuclear waste from a thorium reactor is actually much easier to manage. That’s because fission products from traditional uranium-235 reactors include plutonium and other elements that stay dangerously radioactive for tens of thousands of years.²⁰ The waste from uranium-233 bred from thorium, on the other hand, decays much faster.¹⁵ It’s kind of like choosing paper or plastic at the checkout lane, because instead of needing to store spent uranium fuel underground for tens of thousands or even a hundred thousand years, thorium reactor waste can often be safely stored above ground for just a few hundred years.⁴

It gets better. The bit of fissile material needed to start a thorium chain reaction can come from the waste of traditional uranium reactors, allowing thorium systems to actually burn small amounts of legacy nuclear waste.⁴²¹ Copenhagen Atomics plans to build this into their design, making their thorium reactor not just a fuel breeder, but a waste burner.

So, with all the advantages, how and when will Copenhagen Atomics commercialize their reactors? Well, thorium reactors demand more than just new fuel. They also need pumps, pipes, and control systems that can survive a constant bath of corrosive salt as hot as lava.²² But it’s not just the heat or corrosion — it’s the neutron bombardment from the core that slowly wears down materials at an atomic level.⁹ And because the core also blasts out intense gamma radiation, these systems can’t be serviced once the reactor is up and running. They have to work flawlessly for years, more like a sealed battery pack than a conventional machine.

Here’s how Copenhagen Atomics’ engineers are rising to the challenge. They’ve developed a molten salt pump they say can run literally red hot, circulating a molten cocktail of fluoride, lithium, sodium, and potassium, also known as FLiNaK. This salt mix exits the core at 700°C, or 1300°F, and loops back in at around 600°C, or 1100°F.¹⁰ To avoid the wear and tear that dooms traditional pumps, this pump levitates its rotating parts on magnetic bearings, keeping the system humming for years.²³¹⁰

In preparation for “going nuclear,” the company is running molten FLiNaK through dedicated test loops to shake out problems and fine-tune the performance of the pump, sensors, heat exchangers, and even the custom steel alloy used in the reactor. That steel can endure corrosive salts and neutron bombardment for at least five years, after which the reactor module is shut down and swapped out for a fresh one.¹⁰⁹

That’s right… five years. In traditional solid-fuel reactors, corrosion and radiation mostly wear down the fuel rods and their cladding, which are components designed to be swapped out during routine refueling.²⁴ But in a molten salt reactor, the fuel is dissolved into the circulating salt, which flows through nearly every part of the system. That means the entire reactor module — core, pumps, pipes — takes the hit. So instead of just replacing fuel rods, you eventually have to replace the whole unit.⁹ Ironically, the fuel and water moderator get saved.

“The fuel salt and the heavy water is just the same being used over and over and over again. And that’s also the expensive part.”

Even with frequent reactor swaps, Copenhagen Atomics still sees a path to low-cost power. And Pedersen is quick to note the upside:

“The great thing about that is that, every five years, you are able to upgrade your technology. Whereas now with the classical nuclear reactors, they run for 50 or 60 years and it’s still the old technology from back in whenever they were built. But in our case… even after 50 years, it’s still five-year-old technology. So that’s sort of a plus in terms of keeping on improving the efficiency of the system.”

The company’s next step is trucking a full-scale prototype reactor down to the Paul Scherrer Institute in Switzerland. That’s the country’s premier research campus for nuclear energy, with both a hot lab to handle radiation and an administration to handle regulation.²⁵

This public-private experiment will mark the first time a nuclear chain reaction kicks off inside a Copenhagen Atomics reactor and the first time a thorium reactor is tested in Europe. The reactor will be powered by enriched uranium in the core and contain thorium in the surrounding breeding blanket.¹⁰ The test is slated for 2026 and will run for up to 30 days at just 1% of the reactor’s expected power output but, critically, enough to validate computer models.²⁵

Over the following four years, in partnership with the Paul Scherrer Institute, Pedersen’s team plans to begin transferring uranium-233 bred in the blanket into the core — an essential step towards a self-sustaining reactor that only needs refueling once every 3-5 years. That’s far less often than for traditional reactors, which are refueled every year and a half to two years.¹⁰²⁶²⁷ The team will also optimize systems to clean fission products out of the fuel salt with the reactor still running, keeping the nuclear chain reaction chugging along at a steady clip.

For now, their biggest holdup isn’t technology: it’s regulation. They’ve got to scale a Matterhorn of paperwork before testing can begin in Switzerland. The Paul Scherrer Institute is on board, but there’s no ski lift through nuclear approval, and the pass isn’t cheap.

“One of the most expensive things are these licenses and approvals, but I assume that once we have built more than a hundred of these reactors, then the price of all this licensing and approval will also come down.”

While Copenhagen Atomics gears up, China appears to have crossed the starting line. In October 2024, a state-sponsored molten salt reactor ran at full power for 10 days with thorium and uranium in its fuel mix.²⁸²⁹ It even bred protactinium-233: a clear sign that nuclear breeding was underway. In April 2025, Chinese researchers announced they’d topped the reactor up with fresh fuel without first shutting it down — another early milestone on the road to continuous operation.³⁰

But no one’s claimed the prize just yet. Neither China nor Copenhagen Atomics has demonstrated a true breeder reactor that produces more fissile fuel than it consumes. Copenhagen Atomics believes hitting that milestone, along with producing reactors at factory scale, could drive energy costs as low as $20–40 per MWh, just a quarter of today’s average for nuclear energy.³¹⁹

To hit that price point, the company is re-imagining not just the reactor but the entire business model. It won’t sell its reactors. Instead, it’ll build, site, operate, and eventually decommission each reactor itself, all without taxpayer funding. And in exchange for taking the heat on nuclear installation, its clients will literally take the heat the reactors produce for use directly in industrial applications or for conversion to electricity.²¹²³

By taking full responsibility for everything from site evaluation to nuclear waste management, Copenhagen Atomics could make nuclear power viable even in countries that have never had a nuclear power plant. Indonesia is one example, and that also happens to be the company’s first customer. The plan is to use the energy from the thorium reactors to produce hydrogen and ammonia.³²

Electrification isn’t always the answer, and nuclear reactors provide something solar and wind can’t: ultra-low emissions heat for everything from producing ammonia and smelting aluminum to drying paper.¹⁰ And thanks to thorium, nuclear energy’s ongoing headache of radioactive waste may soon go from a 100,000-year curse to a 300-year chore.

Copenhagen Atomics expects to go commercial within a decade, but the biggest breakthrough — self-sustaining breeding reactors fueled only by cheap and abundant thorium — that’s a milestone with an uncertain timeline.²¹ But it’s also a technology our planet can hardly wait for, because energy unlocks solutions, and thorium might just give us the key.