Back in 2015, I wrote a post titled What the Heck Is Thorium?.
I was intrigued by the possibility that thorium could overcome some of the biggest objections to conventional nuclear power: safety, radioactive waste, fuel availability and cost.
I returned to the subject in my 2017 Thorium Nuclear Update.
The promise was enormous. But there was always one rather inconvenient problem.
Nobody had managed to turn that promise into an economically viable commercial power station.
Now, nearly a decade later, a Danish company called Copenhagen Atomics may be getting us considerably closer.
I recently watched a fascinating video examining its technology, including a visit to the company's workshops and a detailed explanation of the engineering problems it has been tackling.
What impressed me was not another grand prediction about nuclear energy. It was the machinery, the prototypes and the practical solutions being developed.
For the first time in years, I think thorium deserves another serious look.
Why thorium was so exciting in the first place
Thorium is a naturally occurring radioactive element, roughly three times as abundant in the Earth's crust as uranium.
It has extraordinary energy potential.
But thorium has a peculiar characteristic: it cannot directly sustain the nuclear chain reaction used to produce electricity.
Instead, thorium-232 must absorb a neutron and undergo a series of transformations to become uranium-233, which is an excellent nuclear fuel.
That means a suitably designed reactor can potentially manufacture much of its own fuel while operating.
Imagine a power station that starts with a modest quantity of conventional nuclear fuel, then gradually produces the fuel it needs from a plentiful material that might otherwise be discarded as a mining byproduct.
That is the attraction of a thorium breeder reactor.
And unlike conventional nuclear plants, which generally use solid uranium fuel rods and highly pressurised water, thorium can be used in a molten-salt reactor operating at relatively low pressure.
The potential advantages are substantial:
Safety: Low-pressure operation and passive shutdown systems can reduce certain accident risks.
Fuel efficiency: A successful breeder could extract far more energy from its fuel resources.
Waste: The thorium fuel cycle offers opportunities to reduce some long-lived radioactive waste, although it cannot eliminate radioactive waste.
Abundance: Thorium is widely available, including as a byproduct of rare-earth mining.
Cost: Compact, factory-manufactured reactors could avoid some of the enormous construction costs associated with conventional nuclear projects.
The physics has been understood for decades.
The difficulty has always been making the engineering work reliably and economically.
Enter Copenhagen Atomics
Copenhagen Atomics was founded in 2014 and has spent more than a decade developing a compact molten-salt reactor.
Its proposed commercial unit is designed to produce 100 megawatts of thermal energy, equivalent to approximately 42 megawatts of electricity when connected to suitable generating equipment.
The reactor module itself is designed around the dimensions of a standard shipping container.
That is a remarkable departure from the huge, individually constructed nuclear power stations we are accustomed to seeing.
The company calls its reactor design the Onion Core.
The name comes from its concentric layers, each performing a different function.
At the centre is a molten-salt fuel channel where nuclear fission produces heat and neutrons.
Surrounding it are layers of heavy water that slow the neutrons, helping maintain the chain reaction.
An outer blanket containing thorium captures other neutrons, gradually converting thorium into uranium-233.
The cleverness lies in making the most of every neutron.
A reactor must produce enough neutrons to maintain its own operation while also converting thorium into replacement fuel.
Lose too many neutrons to the surrounding structure or unwanted chemical reactions, and the breeding process fails.
Copenhagen Atomics has designed its geometry around this problem.
The company hopes eventually to develop reactors that can sustain themselves predominantly through thorium breeding.
There is an important qualification: its first planned commercial reactors will still require low-enriched uranium. The company describes them as approaching fuel self-sufficiency rather than achieving full breeding from the outset.
That is an intermediate step, but potentially an important one.
Solving the problems that defeated earlier attempts
The most interesting part of the video concerns two engineering challenges that have frustrated molten-salt reactor developers for decades.
First, corrosion.
Molten fluoride salts operate at temperatures around 600°C or higher. Unfortunately, impurities in those salts can attack the metal containing them.
Earlier researchers addressed this problem by developing specialised nickel-based alloys, including Hastelloy N.
These materials work, but they are expensive and introduce additional complications.
Copenhagen Atomics has taken a different approach.
Rather than relying entirely on exotic metals, it has developed methods of purifying the molten salts to extremely high standards.
The company reports that ordinary stainless steel exposed to its purified salts showed minimal corrosion after 3,000 hours of testing.
If those results hold up over years of operation inside an actual nuclear reactor, the implications for manufacturing costs could be substantial.
Second, the lithium problem.
The molten salts contain lithium, but not all lithium atoms behave alike.
Lithium-7 is useful because it absorbs relatively few neutrons.
Lithium-6, however, absorbs neutrons readily, robbing the reactor of particles needed to sustain the reaction and breed new fuel.
For a thorium breeder, that can be fatal to the economics of the design.
Copenhagen Atomics has therefore been developing highly enriched lithium-7 for its reactor salts.
These may sound like obscure technical details.
But they are precisely the kinds of problems that separate an attractive scientific theory from a commercially useful machine.
And that is why this project interests me.
A different approach to nuclear safety
Another important feature is the reactor's proposed passive safety system.
In a conventional light-water reactor, the combination of high pressure, hot water and the need for continuous cooling creates particular engineering challenges.
Copenhagen Atomics proposes a low-pressure molten-salt system in which a serious loss of power or pumping capability would cause the fuel salt to drain by gravity into specially designed tanks.
There, the nuclear chain reaction would stop and the material could cool.
This does not make the reactor risk-free. Radioactive materials still require containment, shielding and long-term management.
But it potentially eliminates some of the accident mechanisms associated with conventional reactors.
The company also plans to replace the reactor's internal module approximately every five years, retaining the valuable fuel salts for reuse.
Rather than attempting to make every component last for decades, the idea is to manufacture replaceable units economically.
It is an interesting engineering philosophy: make the reactor compact, standardised and serviceable.
Could nuclear power really become this cheap?
Here is where the story becomes particularly interesting.
Copenhagen Atomics is proposing a price of approximately US$50 million for each 100-megawatt thermal reactor module, plus an annual fuel and maintenance fee of around US$2 million.
Its target cost for the nuclear heat and reactor service is approximately US$10 per megawatt-hour.
Once electricity generation equipment, site costs and other infrastructure are included, the company is targeting approximately US$30–40 per megawatt-hour of electricity.
If achieved, that would be competitive with many established forms of electricity generation and dramatically cheaper than numerous recent conventional nuclear projects.
It could also provide reliable electricity around the clock, without depending on sunshine or wind.
But these figures are targets based on the company's engineering and commercial models.
They are not yet prices demonstrated by an operating commercial power station.
The real test will come when the technology operates continuously, meets regulatory requirements and can be manufactured repeatedly at the predicted cost.
Still, the possibility is sufficiently important that it deserves serious attention.
The next test is in Switzerland
Copenhagen Atomics has built two full-scale non-nuclear prototypes, with a third under development. The next milestone is nuclear testing through the BALDER project, in collaboration with Switzerland's Paul Scherrer Institute.
Initial testing is now expected no earlier than 2028, with commercial reactors targeted for the early 2030s.
These are ambitious timelines, and significant engineering and regulatory hurdles remain. But this is no longer simply a theoretical exercise.
What about Australia?
Australia has substantial thorium resources and an obvious interest in reliable, affordable, low-emission electricity.
Our nuclear debate tends to focus on conventional reactors and today's costs. Yet the technology is evolving.
If Copenhagen Atomics delivers on its promises, compact thorium reactors could change that debate considerably.
We should not commit to unproven technology. Equally, we should not make fifty-year energy decisions assuming that today's technology is the last word.
Is thorium's moment finally arriving?
When I first wrote about thorium in 2015, the promise was extraordinary. The engineering wasn't ready.
Eleven years later, Copenhagen Atomics has made tangible progress towards solving some of the problems that held it back.
There is still no commercially proven thorium breeder reactor. But if the Danish team succeeds, we could see a new generation of compact, safer and potentially much cheaper nuclear power.
After more than half a century of promises, thorium may finally be moving from scientific possibility to commercial reality.
That is a development worth watching.
Grappy
Further reading and viewing
