Why Blykalla picked lead for SEALER SMR

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Swedish startup Blykalla has filed to build two reactor parks with up to 14 small reactors cooled by molten lead instead of water.

There is a small nuclear reactor cooled by the same metal we use in fishing weights and car batteries. Lead.

A Swedish company called Blykalla wants to build six of these at an old industrial port two hours north of Stockholm. Each reactor vessel is about six metres across, and it sits in roughly 800 tonnes of molten lead.

Bly is Swedish for lead, and kalla means cold. Until 2023 the company went by LeadCold.

Here’s the thing though. Lead is heavy, it freezes at 327 degrees Celsius, and when it’s hot it attacks steel. So why would anyone choose it?

To get there, we first need to know what an SMR is, and why everyone suddenly wants one.

The International Atomic Energy Agency’s working definition is a reactor producing up to 300 megawatts of electricity. A typical large reactor today is around a thousand megawatts or more.

The idea is to build the reactor in a factory, the way you’d build an aircraft, ship it to site and install it, instead of pouring a one-off concrete megaproject for a decade.

And that one-off model has hurt. Vogtle units 3 and 4 in Georgia came in about seven years late and cost over $30 billion. Flamanville 3 in France took 17 years from first concrete to grid.

Add AI data centres hungry for clean power around the clock, and small module reactors went from a side project to a crowded race.

So, over a hundred SMR designs exist on paper. Only a handful actually run.

Russia has operated the floating plant Akademik Lomonosov since 2020. China connected its gas-cooled HTR-PM to the grid in 2021, and its 125 megawatt Linglong One on Hainan island is due to start up by the end of 2026.

In the West, most designs stick with water. GE Vernova Hitachi’s BWRX-300 is under construction at Darlington in Canada, targeting the grid by end of 2030. NuScale’s 77 megawatt module got its US design approval in 2025. And Rolls-Royce is building a larger 470 megawatt unit in the UK.

Then there are the ones swapping out water entirely.

TerraPower’s Natrium uses liquid sodium and won its US construction permit in March 2026. X-energy’s Xe-100 uses helium gas.

And a small club is betting on lead. Russia is building the 300 megawatt BREST-OD-300 in Seversk. Newcleo is working on designs in France and Italy. And in Sweden, there’s Blykalla.

Blykalla spun out of KTH Royal Institute of Technology in Stockholm in 2013, but the lead research there goes back to 1996.

The reactor is called SEALER, short for Swedish Advanced Lead Reactor. The first version, published in 2017, was a tiny 3 to 10 megawatt unit meant for remote mining towns in the Canadian Arctic that run on diesel.

Today’s commercial version, SEALER-55, makes up to 150 megawatts of heat and turns that into 55 megawatts of electricity.

In May 2026 Blykalla applied to build six of them at Norrsundet, 330 megawatts in total, and became the first company to apply under Sweden’s new state financing model. In August it filed for a second park at Tierp, up to eight more reactors and 440 megawatts. Both target the first half of the 2030s.

The backers are serious too. A $50 million round in December 2025 was co-led by US reactor developer Oklo, with CoreWeave’s co-founders also investing.

ABB is working on the electrical and control systems, and this month Blykalla announced a Microsoft collaboration using AI to speed up licensing paperwork.

To see why lead, start with the problem in a normal reactor. Water boils at 100 degrees.

A pressurised water reactor wants its water at around 300 degrees, so it squeezes it to roughly 150 times atmospheric pressure to stop it boiling.

That’s why those reactors need steel pressure vessels 15 to 20 centimetres thick, and giant containment buildings sized for high pressure steam if a pipe ever breaks.

Lead doesn’t boil until around 1,750 degrees. SEALER’s lead enters the core at 420 degrees and leaves at 550, at normal atmospheric pressure. That leaves more than a thousand degrees of margin before boiling.

With nothing to hold under pressure, Blykalla says the vessel wall can be about 3 centimetres thick.

Sodium reactors like Natrium also run at low pressure, but sodium burns in air and reacts violently with water. Lead does neither. If it leaks, it cools and solidifies. It also blocks gamma radiation by itself, so the coolant doubles as shielding.

And because lead is so heavy, it barely slows neutrons down. That makes SEALER a fast reactor, which uses fuel more efficiently. Blykalla says a compact core can even breed more fuel than it burns.

When the reactor shuts down, hot lead rises, cools, sinks and circulates on its own, carrying away leftover heat. Blykalla says no electricity, pumps or operator action are needed for emergency cooling.

So then that begs the question — why isn’t everyone using lead?

Well, corrosion. At these temperatures molten lead dissolves elements like nickel straight out of ordinary steel, and it slowly eats the plumbing.

The Soviet Navy ran lead-bismuth cooled reactors in its Alfa class submarines, and keeping the coolant from freezing and from attacking the metal was a constant struggle.

Blykalla’s core answer is metallurgy. Its patented steels contain aluminium, which withstands the corrosive nature of liquid lead.

The aluminium reacts with the tiny amount of oxygen dissolved in the lead and grows a thin film of aluminium oxide on the surface, the same compound sapphire is made of. The company describes that layer as self-healing.

There are three of these steels. An alumina forming steel for the fuel cladding, an austenitic steel for the vessels, and a martensitic steel for the pump impellers, which have to spin inside those 800 tonnes of molten lead.

Swedish steel and alloy makers including Kanthal, Alleima and Höganäs are in the supply chain.

Freezing is the other risk.

Lead turns solid at 327 degrees, so a 420 degree inlet leaves about 90 degrees of headroom. The plant has to stay hot, always.

The long-term fuel plan is uranium nitride, enriched to about 12 percent.

Blykalla says it packs 40% more uranium than standard oxide fuel and conducts heat about seven times better, so the fuel runs cooler.

3 months ago

Energy

Why Blykalla picked lead for SEALER SMR

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