{"id":73852,"date":"2025-12-01T13:04:51","date_gmt":"2025-12-01T11:04:51","guid":{"rendered":"https:\/\/www.sweco.fi\/?post_type=showroom_cpt&p=73852"},"modified":"2025-12-01T13:30:39","modified_gmt":"2025-12-01T11:30:39","slug":"an-underground-plant-concept-was-created-for-steady-energys-small-nuclear-power-plants","status":"publish","type":"showroom_cpt","link":"https:\/\/www.sweco.fi\/en\/projects\/an-underground-plant-concept-was-created-for-steady-energys-small-nuclear-power-plants\/","title":{"rendered":"An underground plant concept was created for Steady Energy\u2019s small nuclear power plants"},"content":{"rendered":"
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An underground plant concept was created for Steady Energy\u2019s small nuclear power plants<\/h1>\n

Published 26.11.2025<\/em><\/p>\n

A Finnish company, Steady Energy<\/a>, is developing small nuclear power technology that can deliver fossil-free heat to district heating networks. Sweco is supporting the company\u2019s pioneering work with concept, layout and rock-structure design for the small nuclear plant.<\/p>\n

Steady Energy\u2019s small nuclear plant concept<\/h2>\n

Over the past couple of years Steady Energy has been developing a small nuclear power plant concept together with Sweco\u2019s experts<\/a>. The underground plant concept enables small nuclear plants that supply heat to district heating networks in cities across Europe.<\/p>\n

\u201cThe potential is large, because nearly 15 percent of Europe is heated by district heating”,\u00a0 says Antti Ter\u00e4svirta<\/strong>, Steady Energy\u2019s project manager. According to him, around 90 percent of European district heat is produced using fossil fuels or by burning biomass. \u201cThere are therefore still major opportunities to reduce emissions in heating energy.\u201d<\/p>\n

Steady Energy has completed a small modular reactor concept<\/a>, and Sweco\u2019s task was to create a space concept and layout for a district-heat-producing small nuclear plant without a specific site.<\/p>\n

\u201cFirst we wanted a rough-level CAPEX estimate to support decision-making, but the plant and layout design evolved round by round,\u201d Ter\u00e4svirta says. \u201cWe have consistently arrived at more cost\u2011efficient solutions, and I believe the design will continue to develop.\u201d<\/p>\n\t\t<\/div>\n\t<\/div>\n<\/div>\n<\/section>\n\n\n

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Project Overview<\/h2>\n

Client:<\/strong> Steady Energy<\/p>\n

Services included in the assignment:<\/strong>
\nPreliminary design and concept description, including layout, piping and rock engineering design, 3D modeling of structures and systems, as well as preparation of piping and instrumentation diagrams.<\/p>\n

Implementation period:<\/strong> 2024\u20132025<\/p>\n

Assignment objective:<\/strong>
\nProducing district heating with small nuclear reactors without using fossil fuels or burning wood mass; cost-efficient underground placement of small nuclear reactors.<\/p>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t

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Steady Energy and Sweco combine their nuclear engineering expertise<\/h2>\n

Topias Siren<\/strong>, Sweco\u2019s leading expert in rock-structure design, says collaboration with the client has been smooth throughout the process thanks to Steady Energy\u2019s extensive in-house expertise in nuclear engineering and plant design.
\n\u201cWe\u2019ve been asked truly challenging questions that have put even our best specialists to the test\u2014in a good way.\u201d<\/em><\/p>\n

Siren himself brings over 14 years of experience in demanding rock mechanics. He wrote his doctoral thesis on the final disposal of nuclear waste and contributed to the rock-structure design for the ONKALO<\/a> facility. He also served as Professor of Practice at Aalto University (2022\u20132025).<\/p>\n

\u201cAlthough our original task was layout design for a district heating plant, the nuclear sector has its own terminology and regulations. Designing the plant concept required experience in nuclear engineering<\/a>.\u201d<\/em><\/p>\n

Ter\u00e4svirta describes the collaboration as flexible and the designers as highly committed. He continues to be impressed by Sweco\u2019s breadth of expertise: specialists in traditional design disciplines as well as niche areas like rock engineering.
\n\u201cWe\u2019ve also received support in mapping different application areas for the technology we\u2019ve developed. For example, studies have explored using our technology for salt removal.\u201d<\/em><\/p>\n

Pipe connections for the small reactor were 3D-modelled<\/h2>\n

Steady Energy\u2019s small nuclear reactors are designed to be placed in an underground hall. The plant\u2019s size affects both its carbon footprint and costs.
\n\u201cBecause the space is enclosed, the layout must be carefully planned in advance”, <\/em>\u00a0Ter\u00e4svirta explains. \u201cYou can\u2019t easily add extra wings underground.\u201d<\/em><\/p>\n

The overall design must be compact for cost efficiency, with pipe runs kept as short as possible. Designers also ensure that safety-critical systems operate independently, with several overlapping and mutually supporting systems planned for the facility.<\/p>\n

\u201cFor nuclear safety reasons, pipe connections have been multiplied, and that takes up a lot of space”, <\/em>Siren notes. Space use is optimized through 3D modeling<\/a>.
\n\u201cOur electrical, piping, and fire-safety designers can quickly identify intersections by modeling.\u201d<\/em><\/p>\n

Natural vaults in Finland\u2019s bedrock are ideal for small reactors<\/h3>\n

Modeling is also used in designing the underground spaces. According to Siren, rock-structure design is reminiscent of building a medieval church:
\n\u201cThe shape of the rock vault is designed to be naturally self-supporting so that as little steel as possible is needed.\u201d<\/em><\/p>\n

A balance is sought between the natural vaulted arch and the masses suspended from it with steel bolts. Sweco simulates the compressive behavior of the rock using 2D and 3D rock-mechanical modeling.<\/p>\n

Sweco\u2019s rock-construction designers have also compared Finnish granite with softer ground conditions elsewhere in Europe.
\n\u201cIt showed that underground construction in Finland is extremely cost-effective. Our old bedrock naturally forms a roof”, <\/em>Siren explains.<\/p>\n

Licensing process makes history<\/h3>\n

Steady Energy\u2019s plant concept is still evolving, and history is being made in terms of licensing. In the new draft Nuclear Energy Act, small reactors would no longer be licensed only for a specific site; the proposal includes a concept evaluation similar to type approval. This will simplify licensing by phasing the process.<\/p>\n

\u201cSite licensing will follow once the location is confirmed, but a pre-approved plant concept will make siting a small reactor easier and faster for energy companies”, <\/em>\u00a0Siren emphasizes.<\/p>\n

Did you know?<\/strong> Sweco also acts as Steady Energy\u2019s EPCM partner at the SMR test facility in Helsinki\u2019s Salmisaari<\/a>.<\/p>\n

Image: Steady Energy<\/em><\/p>\n\t\t<\/div>\n\t<\/div>\n<\/div>\n<\/section>\n\n\n

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