Finland backs full-scale nuclear heat pilot for Helsinki district heating

A full-scale nuclear heat pilot is moving out of the slide deck and into a former coal plant in Helsinki. Steady Energy has secured a €10.5 million loan from Business Finland for the Salmisaari project, and construction has already begun inside the turbine hall of Helen’s decommissioned Salmisaari B power plant. The test plant is scheduled to start operations in spring 2027.

Instead of loading fuel rods, the pilot will use an electric resistor to mimic the decay heat of an actual reactor. That makes the Salmisaari unit a commercialization test, not a conventional reactor loop: Steady Energy said the goal is to prove the safety systems work as designed and to cut schedule risk before commercial delivery. Helen said the finished pilot should produce about six megawatts of heat for its district-heating network, tying the machine to an operating urban energy system rather than an isolated lab setup.

The setting gives the project unusual weight. Helen decommissioned Salmisaari B on 1 April 2025, and the company and Steady Energy signed a lease for the site in May 2025 that runs until 2028. First concrete was poured in January 2026, and the pilot is being built where coal once burned, a practical signal that the Finnish heat market is starting to make room for nuclear hardware. Helen has said it wants to phase out combustion-based energy production by 2040, and nuclear heat is now part of that push.

The broader demand case is hard to miss in Helsinki. Helen says more than 90% of housing companies in the city are heated with district heating, and a separate Helen nuclear-energy page puts Helsinki’s population at about 700,000 with nearly 90% of the building stock connected to district heat. That is why the Salmisaari pilot matters beyond one company’s engineering timetable: it is aimed squarely at a live municipal network with scale.

Steady Energy, founded in 2023 by Tommi Nyman, Hannes Haapalahti and Petteri Tenhunen after spinning out from VTT Technical Research Centre of Finland, has been pitching the LDR-50 as a 50 MW thermal reactor for underground installation close to heat networks. The company completed a €32 million funding round in 2025, and Fortum added strategic support in 2026 with a €2.1 million investment and a framework agreement on operation and maintenance planning. For a sector long dominated by paper concepts, a funded full-scale pilot in Helsinki is a concrete step toward proving that heat-only reactors can be built, connected and eventually commercialized.