Small modular reactors (SMRs) are popping up in headlines as the next big thing in clean energy — but what exactly are they, and how are they different from the nuclear power plants we already know?
The first nuclear power plants intended for commercial electricity production were built in the 1950s and 1960s. By today's standards, they can be considered quite small, simple, and fast to construct. Reactors from that era had typically an electrical output of less than 300 MWe and a construction time of 3–5 years.
The rapid increase in nuclear power construction during the 1970s led to larger reactor sizes and more complex plant systems. Safety requirements were also increased and tightened. The increased demands and pursuit of economic efficiency through ever-larger reactors have led to a situation where each new plant project has become a very unique, highly customized entity, requiring numerous design modifications based on the demands of the customer and the local regulator. In addition, the consortium of companies constructing the facility, including subcontractor chains, has been formed individually for each specific plant project. All of this has led to, among other things, quality issues, delays in construction schedules, and additional costs.
Recently, SMRs have emerged as a solution capable of reversing that development trend. In a way, they represent a return to the past, where things were smaller and simpler, but taking advantage of everything, we have learned during the past decades, especially from a nuclear safety point of view.
But what is an SMR, and how does it differ from the more familiar large nuclear power reactors? As the name suggests, an SMR unit is a small, modular reactor unit, but SMR units are not a homogeneous, precisely defined group. The term small is typically interpreted to mean a plant that can generate up to 300 MWe of electricity. But even that isn't set in stone. Although most designs labeled as SMRs fall below this threshold, the Rolls‑Royce SMR — often highlighted in the media — produces 470 MWe, corresponding to nearly 1400 MWth of thermal power. In practice, that puts it close in scale to a Loviisa reactor unit. At the other end of the SMR spectrum there are micro reactors planned to produce electricity with an output of only 10 MWe or even less, or reactors like Steady Energy’s LDR-50 that is designed to produce only heat, e.g. for district heating networks, with thermal power of 50 MWth.
The term modular refers to modular construction. In other words, SMR units are planned to be constructed of modules that are manufactured elsewhere, transported to the site, and installed into place. This means efficient, standardized factory-like construction with less quality issues and uncertainties.
And the term reactor means that it is still a nuclear reactor based on nuclear fission. The first batch of SMR designs are based on light water reactor technology, like most of the commercial nuclear reactors operating today, but the SMR designs currently under development include also many so-called Generation IV designs, like molten salt reactors, which aim to introduce new features such as higher operating temperatures, improved fuel efficiency, and/or enhanced safety characteristics.
But what makes SMRs a game changer. The key concepts here are cost-effectiveness, safety, and versatile applicability.
The ever-increasing size of large plant units has been based on the so-called economy of scale. SMRs will challenge this thinking and demonstrate that with the help of serial production even small reactor units will be economically viable.
Modular, standardized construction means faster construction times and reduced costs. Modularity in itself is nothing new. It has been utilized, e.g. in the shipyard industry, for a long time. Modularity can be utilized also in the construction of large reactor units, but SMR units have greater potential for the benefits because their expected construction volume is higher, which enables a more efficient, assembly line type of production of modules — not to forget the smaller unit size and simpler design, which help here as well. Modularity means also scalability as some SMR designs can accommodate one, two, three or even more reactor modules under the same roof, meaning that the plant output can more easily be adapted to match the customer's needs.
Another cornerstone of the SMR concept is safety: lower reactor power makes it easier to ensure the safety of the nuclear fuel. For example, complex safety systems that rely on electrically operated pumps to cool the fuel can be replaced with simple passive solutions based on natural circulation. Additionally, SMR units can easily be equipped with water reservoirs sufficient to meet the cooling needs for long periods of time, even for several months. This means a strong ability to cope on their own without any external help. The simplicity of the safety systems is also reflected in the operating costs, as there are fewer devices and plant systems to maintain and service.
But, as nuclear safety is based on the principle of defense-in-depth, meaning multiple layers of safety functions that reinforce each other, along with "what-if" thinking, we must also contemplate situations, where the safety of the plant cannot be ensured. When considering the location of a nuclear power plant, one must inevitably ask: what would occur if, despite all safety measures, a severe accident resulted in a radioactive release. What are the consequences? What kind of effects could an accident have on people and the surrounding environment? How can those effects be mitigated?
Again, the smaller size of the SMR proves advantageous. Because the reactor contains less fuel, the potential for radioactive releases is lower, which also means smaller and more manageable consequences. This, in turn, supports the third cornerstone: versatile applicability. Since an SMR unit requires significantly less space than a large facility, and the environmental impact of a severe accident would be minor, SMR plants can be more freely located, for example, near residential or industrial areas, or even underground. This enables new types of applications, such as the production of district heating for cities or industrial heat for factories, or the production of synthetic fuels utilizing CO2 obtained from a nearby industrial facility - enhancing further the economic viability of SMRs.
And all of this isn’t just a beautiful theory on paper. SMR projects are progressing. In China, the world’s first commercial land‑based SMR — the 125 MWe Linglong One — is currently in the commissioning phase. And if we consider North America and Europe, the most advanced SMR project is currently the construction of BWRX-300 units to Ontario, Canada. The Canadian regulator issued a construction licence for the first of the planned four units in April 2025. Several SMR projects with different technologies are also ongoing in the USA. In the UK, Rolls‑Royce SMR won the government’s SMR competition in June 2025 and is progressing toward building three units in Britain. Rolls-Royce has also signed an Early Works Agreement for deployment of SMR units in Czechia. In Sweden, Vattenfall is proceeding with their plans to build 3-5 SMR units to Ringhals. The final competition and selection of the plant design will take place between GE Vernova’s BWRX-300 and Rolls-Royce SMR. Construction of SMR units is also being actively explored in countries such as Estonia, Norway, France, Poland, Netherlands and Romania. In Finland, the use of nuclear technology for district heating is explored e.g. in Helsinki, Kuopio and Kerava.
SMRs are a practical evolution of proven nuclear technology, and they are genuinely moving forward. That’s very good news, because the world and our climate targets need ways to harness nuclear energy quickly, efficiently, and safely.