In search of the practical advanced reactor


Danish start-up Seaborg has a new concept in reactor design that promises clean and reliable power in a small package. Bringing it to market requires innovation from a technical team growing under co-founder and CTO Eirik Eide Pettersen.

The story of Seaborg’s technology begins around 60 years ago with experimental reactors at Oak Ridge National Laboratory in the US, which made key achievements that Seaborg wants to scale up and build upon.

“Necessity is the mother of invention,” Eirik laughs, explaining that his technology was born in an unlikely project to create a nuclear-powered aircraft as part of America’s Cold War defence. From the start, Oak Ridge knew their design would have to be small, simple to run and reliable even during aerial manoeuvres as well as take-offs and landings. They developed a 2.5 MWt design in which the uranium fuel was mixed with liquid fluoride salts that could be circulated through a reactor core, where fission reactions would heat it up, and then through heat exchangers which would take the heat away to do useful work. The idea was for its nuclear heat to drive jet engines in place of burning jet fuel.

In the end, such flying power plants were not necessary and for various good reasons never came to be. However, Oak Ridge teams learned enough from the Aircraft Reactor Experiment (ARE) to see potential applications on land and their invention lived on through the 1960s as the Molten-Salt Reactor Experiment (MSRE). This produced three times more heat at 7.4 MWt and it is the research that came with this reactor that forms the basis of Seaborg’s technology.

Molten salt has several advantages over water, which most reactors today use as a coolant. Water turns to steam at temperatures over 100°C if it isn’t kept under pressure, so the physical energy bottled up in water cooled reactor systems means they have to be made of heavier components. The buildings around them have additional safety requirements, too. Molten salt avoids a lot of this heavy engineering because it remains liquid until temperatures as high as 1300°C, which is more than enough for a range of applications. Above 500°C the salt is in a molten state where the uranium fuel can be dissolved into it and it can be circulated continuously in a loop to and from the reactor core. And while a liquid fuel does sound difficult to manage, it reverts to a solid on shutdown. Safety systems include a drain tank so liquid fuel can be rapidly removed to halt the fission reactions if a problem develops with the main circuit.

Although MSRE was small by power station standards, Eirik says, “It was big enough to try out the pumps, heat exchanger, reactor core, rods, the drain tank, and so on.” And regarding the fuel salt, he says, “the chemistry behaved as expected and with no corrosion problems. The reactor responded as predicted.”

“MSRE was deemed ‘quite practical’ as a reactor design by the scientists,” says Eirik, noting that this was pretty high praise from that crowd, all things considered. The team also created an “incredible resource” of more than 1000 technical reports on its technology and the tests carried out in operation, all of which are public. “To take those lessons learned and try to improve on these obstacles,” is Seaborg’s work right now.

As well as success with molten salt fuel, Oak Ridge also uncovered some difficulties. To maintain a chain reaction the neutrons produced by each fission need to be slowed down somewhat. At MSRE this moderation job was done by making the neutrons pass through graphite, which works well but tends to fracture over time due to repeated heating and cooling as well as tiny changes at the atomic level caused by intense radiation. Oak Ridge found that if liquid fuel gets into any cracks it would keep generating heat but would not be cooled by the flow of molten salt anymore, which would eventually lead to unacceptable hotspots. The team at Oak Ridge identified this issue but didn’t propose a solution other than to remove the graphite for replacement. That kind of surgery on the reactor core would never be practical for a commercial power system.

Go liquid

Seaborg’s major innovation is to avoid the problems of graphite entirely by using another moderator material. They are proposing to use another molten salt, sodium hydroxide (NaOH). The result is a reactor concept that has never been built before – a reactor made of metal alloy tubes carrying flowing molten fuel salt, which pass inside a larger tube of molten salt working as a moderator. Seaborg’s Compact Molten Salt Reactor is a unique all-liquid concept which brings another advantage: The weight of a hydrogen atom is about one twelfth that of the carbon in the graphite matrix, which means Seaborg needs much less NaOH to do the moderation job and the entire reactor can be significantly smaller. This could open new applications for nuclear energy. Seaborg is proposing to mount its reactors on floating nuclear power barges and supply power to shore as a direct replacement for coal power plants, starting in south east Asia.

NaOH does bring some concerns of its own. It is used in the home as oven cleaner and demands respect from human operators who could suffer skin burns on contact. It is also corrosive to metals. Oak Ridge was able to control corrosion in their salts by purifying them and controlling the chemistry such that it did not prove a problem in their five years of operation. However, Seaborg needs its reactor to last for 12 years and its reactor is 50 times larger than MSRE, at 250 MWe.

“At Oak Ridge they didn’t get that far in mastering the corrosion”, says Eirik. “They recognised the corrosion obstacles and decided not to pursue it further. They did not look into exactly why that corrosion occurs, and that has been a scientific development since then.” Now, he says, “Our understanding has improved greatly, as has material science, and we can create alloys that are better suited to these environments and there are ways to control and minimise the corrosion.”

There is also another specifically nuclear corrosion issue. The element tellurium is produced by fission and will therefore be present in the fuel salt. It can penetrate metal surfaces and can change positions with other elements in a phenomenon called grain-boundary corrosion. Oak Ridge came up with different solutions involving protecting the surfaces and adjusting the chemistry of the fuel salt, and there have been more developments in the time since, but “controlling corrosion in an industrial application under irradiation is something that hasn’t been looked into. We are the first,” says Eirik. Developments since the days of MSRE “give us confidence that the problem with tellurium can be overcome,” he adds, “The pieces of the puzzle are out there.”


Proving that corrosion can be controlled is Seaborg’s main area of activity at the moment. To do this the company is using a number of test rigs to expose metal test samples to NaOH at operational temperatures and observe the resulting corrosion.

“Seaborg’s core IP is based on corrosion control in the moderator salt, and applying the lessons learned since the 1950s. But it is not just a question of corrosion, it is also how easy it is to put these things together,” says Eirik. “Hands-on experience is important. They need to be welded, tested, inspected, maintained” in a way that builds experience for designing the final reactor.

“We are working towards having perhaps 20 or 30 test loops in Copenhagen, with the experiments designed, set up and executed.” Their results will be passed to a reactor design team that will incorporate them into an evolving design for the reactor core. “The conceptual design has already done; we are now working on the basic design and in that way we are working up towards a full-scale prototype.”

A lot is riding on Seaborg being able to solve these corrosion issues. “That’s the duality of innovation,” says Eirik, “It gives you a unique advantage, but it also introduces risks because you are doing something nobody has thought of doing before.” The company’s backers seem to agree that the advantage is there, and that the risks are worth taking.

In 2020 Seaborg raised a EUR two-digit-million sum from a group of investors lead by Heartland, the investment company of retail billionaire Anders Holch Povisen and Team Europe, the investment company of internet entrepreneur Lukasz Gadowski. Both of them moved again in March to buy all Seaborg shares from early investor Pree Seed Ventures, who invested in 2018.

The base of knowledge built up by ARE and MSRE is invaluable. Oak Ridge left behind a public stock of over 1000 scientific papers on their findings. Building on those and taking molten salt reactors into the 21st Century using advances in materials science, Eirik modestly hopes his team will “end up with a reactor that is ‘quite practical’ – or maybe even ‘practical’.”

Help us grow and achieve your potential at a values-driven business.