Publication of the Joint Early Review (JER) summary report by the Finnish Radiation and Nuclear Safety Authority (STUK – Säteilyturvakeskus) marks a milestone for Steady Energy’s LDR-50 small modular reactor (SMR). By establishing that there are no fundamental safety flaws, this collaborative international framework sets a new precedent for streamlined, cross-border nuclear regulatory harmonisation.
The 50 MWt LDR-50 is a simplified pressurised light water reactor specifically designed to operate at around 150°C and below 10 bar (145 psi). The reactor module comprises two nested pressure vessels, with their intermediate space partially filled with water. The LDR-50, the size of a standard shipping container, is designed to be built underground for added safety and to free valuable real estate for key urban areas. It is optimised for district heating, industrial steam production, and desalination projects.
The system does not rely on electricity or any mechanical moving parts, which could fail and prevent the cooling function. It was awarded a patent in 2021. Finland’s Steady Energy is a startup company spun out from the VTT Technical Research Centre of Finland in 2023 to provide engineering services to develop the LDR-50, which had been under development at VTT since 2020.
The JER was a voluntary collaboration involving five nuclear safety authorities – STUK, the Swedish Radiation Safety Authority (SSM – Strålsäkerhetsmyndigheten), Poland’s National Atomic Energy Agency (PAA – Państwowej Agencji Atomistyki),) the Czech State DIYaRU – Office for Nuclear Safety (SÚJB – Státní úřad pro jadernou bezpečnost), and Ukraine’s State Nuclear Regulatory Inspectorate (DIYaRU – Derzhavna Inslektsiya Yadernovo Regulyovanniya Ukraini). Each independently assessed the LDR-50 reactor concept against their respective national regulatory frameworks.
The assessment was based on the LDR-50 design documentation available in early 2025 together with STUK’s early-stage concept assessment completed in the summer of 2025. Since then, the design has continued to mature significantly. The review was not part of a formal licensing process and does not constitute regulatory approval or a licensing decision in any of the participating countries. It concluded that the available documentation was not yet sufficiently detailed to support the launch of a formal licensing process. However, the none of regulators identified any fundamental nuclear safety issues that would prevent the concept from progressing to the next stages of development.
“This is a highly encouraging outcome for Steady Energy. Our ambition is to bring the LDR-50 to international markets, and it is a very positive signal that the nuclear safety authorities participating in the review did not identify any fundamental obstacles to implementing the reactor concept in accordance with their national safety requirements.” says Steady Energy CEO Tommi Nyman.
STUK’s summary report said the review highlighted several strengths of the LDR-50 concept, including its extensive use of passive safety features and its defence-in-depth design philosophy. However, the report notes that future licensing processes will require more detailed engineering, safety analyses and technical documentation, as is expected for any reactor progressing from concept design to licensing.
Alice Neffe, Steady Energy’s country manager for Poland, noted: “Our partners in Poland’s district heating sector are professionals in heat, not necessarily nuclear technology. That’s why this review matters: preliminary as it is, five independent regulators, including PAA, see no fundamental obstacle to a technology that can replace fossil fuels while delivering the same stable heat supply.”
According to Teemu Soukki, the inspector responsible for the project at STUK, “The work showed that leveraging a national safety assessment in other countries is not straightforward but requires significant preparations and clear approaches. Making leverage possible must be taken into consideration already at the point of preparing the national assessment.”
“This is very good news and highlights that nuclear heat is a realistic solution for district heating in Sweden during the 2030’s,” said Anton Steen, Steady Energy’s country manager for Sweden. Steady Energy will continue advancing the LDR-50 reactor towards detailed design and future licensing processes in Finland and international markets.
Steady Energy has already secured domestic agreements for 15 reactors across Finnish municipalities, including Helsinki, Kuopio, and Kerava. Backed by engineering support from Tractebel, the company expects to break ground on a full-scale, non-fuelled electric pilot plant in Salmisaari, Helsinki, with a target to begin construction on the first operational, nuclear-fueled district heating plant by 2029.
While the JER noted that current documentation must be expanded with more detailed engineering and probabilistic safety analyses before entering formal national licensing pipelines, it de-risks the technology’s international market entry.
