Small Modular Reactors (SMR): What they are and how they work
What is a Small Modular Reactor (SMR)? Definition and key features
Decarbonisation Energy efficiency
Small Modular Reactors are a new generation of fission reactors designed to be more versatile, flexible, scalable and efficient. Their modular nature enables components to be manufactured industrially and subsequently assembled on site, reducing costs, timescales and construction uncertainties.
Small Modular Reactors, known as SMRs, represent the technological vanguard of nuclear fission energy. Their development aims to broaden the possibilities of this technology by incorporating smaller, more standardised designs and new applications beyond conventional power generation, driving growing global interest.
Unlike the traditional approach of large power stations designed to produce high volumes of electricity, SMRs are a new, more compact nuclear reactor design that incorporates the advantages of portability and ease of construction. This reduces costs and allows them to be transported to various hard-to-reach locations. In a context marked by rising decentralised energy demand – driven primarily by the growth of data centres – and the need to move towards low-carbon economies, SMRs are emerging as a technology to watch closely on the path towards electrification, reducing reliance on fossil fuels and strengthening energy security.
What is a Small Modular Reactor (SMR)? Definition and key features
Small Modular Reactors, or SMRs, represent an evolution in nuclear technology designed to respond to new energy challenges. Their smaller size, factory manufacturing capability and modular transport could help shorten on-site construction times, improve manufacturing quality and facilitate more scalable deployments.
These reactors generally have an electrical output of up to 300 megawatts electric (MWe) per unit – though advanced designs reach up to 470 MWe – representing roughly a third of the generation capacity of traditional reactors. Their modularity reduces construction times and costs, facilitating progressive deployment. This category also includes microreactors with even lower capacities, generally under 10 MW, designed for specific applications or remote off-grid locations.
In addition to generating electricity, the innovative design of SMRs will enable them to play an important role in complementary industrial and energy applications. Potential uses include industrial heat production, hydrogen production, water desalination, district heating, thermal energy storage and energy supply to remote locations.
Overall, modular nuclear reactors offer greater adaptability to different energy needs. Their goal is not to replace large nuclear power stations, but to complement the existing fleet, expanding the reach of nuclear energy into new demand segments and energy applications.
How do Small Modular Reactors work?
Although the concept is innovative, the operating principle of a small modular reactor is based on known and proven technology. As with other modular nuclear power stations, electricity generation is based on nuclear fission.
Nuclear fission and passive safety systems
In a nuclear reactor, electricity is generated from the nuclear fission of uranium, a process in which atomic nuclei split, releasing a large amount of energy in the form of heat. This heat is used to generate steam, which drives a turbine connected to an electrical generator.
SMRs possess a high level of inherent safety. For example, they use passive safety systems based on natural phenomena, such as coolant circulation by gravity or heat transfer by convection. This enables indefinite cooling without operator action and with high independence from external power or water makeup. Some SMR designs feature underground reactor placement, providing greater safety against potential accidents caused by external events and reduced risk from malicious actions. Furthermore, some SMRs offer the possibility of using spent fuel from conventional reactors, reducing radioactive waste and increasing overall cycle efficiency.
Core technologies: water, molten salt and helium gas reactors
Not all small modular reactors are the same. Some use water as a coolant, following the model of traditional reactors. However, more disruptive designs are exploring alternatives such as cooling with molten salt, liquid metal or high-temperature gas.
Molten salt reactors, for instance, use high-temperature salt mixtures that allow operation at higher temperatures with greater thermal efficiency. High-temperature gas-cooled reactors, such as those using helium, offer advantages in stability and process heat supply. Liquid metal-cooled SMRs stand out for their excellent heat transfer and low-pressure operation. This technological diversity expands the potential applications of nuclear energy in both power generation and industrial uses.
Main advantages and drivers for SMRs
SMRs are gaining prominence worldwide due to a range of advantages that address current challenges in the energy sector:
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Enhanced nuclear safety and risk reduction
One of the main attractions of the small modular reactor is its safety-oriented design. The combination of smaller size, passive cooling systems and simplified configurations, along with advanced fuel designs, increases facility robustness and strengthens operational safety margins
Their lower capacity also means a smaller radioactive inventory than conventional reactors, helping limit the potential impact of incidents and facilitating the progressive decommissioning of facilities at the end of their operational life.
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Cost-effectiveness and reduced construction times
Mass manufacturing of modules optimises industrial processes, generates learning economies and reduces costs. Unlike large power stations, whose construction timescales in the West can exceed a decade, SMRs aim to be completed in significantly shorter periods, not exceeding five years. This represents a paradigm shift from traditional "economies of scale" to "series economies" based on standardisation and repetitive production.
These advantages stem primarily from smaller plant sizes, simpler and more standardised designs, and the assembly of prefabricated modules, reducing the complexity and timeframe of on-site construction activities. They also allow for gradual capacity deployment through the progressive addition of modules, reducing initial investment requirements, lowering financial risk and adapting more flexibly to energy demand growth.
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Siting flexibility and infrastructure requirements
Thanks to their small size, the land and water requirements of SMRs are far lower than those of conventional plants, making them attractive for regions with limited water resources or small distribution networks – even for remote locations without access to the main grid or facilities requiring a highly stable supply, such as military bases or industrial complexes.
Some countries are evaluating the reuse of former coal plant sites for SMR deployment, leveraging existing infrastructure to accelerate the transition towards lower-emission energy systems.
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Integration with renewable energy and new industrial uses
Another major advantage of SMRs is their operational flexibility. Their ability to modulate electrical output allows them to effectively complement renewable energy, supporting its integration into variable generation systems with a high presence of renewables such as solar and wind power. In addition, some SMR designs integrate thermal storage using molten salt, allowing energy to be stored for several hours during the day and electricity to be generated at other times.
Beyond power generation, some designs are also intended for industrial heat applications, such as hydrogen production, seawater desalination or process heat supply for industry.
Challenges facing SMRs
Although small modular reactors offer significant advantages, their large-scale deployment still faces several technical, regulatory and economic challenges.
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Developing an adequate supply chain
Mass production of standardised SMR modules requires factories capable of producing and testing these modules to the nuclear industry's high quality and safety standards – an entirely novel requirement.
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Complex licensing processes
Many designs incorporate innovative technologies that must be evaluated and approved by regulators in each country before implementation, requiring changes to current legal and regulatory frameworks that can lengthen licensing timelines.
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High initial costs
Developing and certifying a new SMR design requires large investments and can take over a decade of work before completion.
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Availability of advanced nuclear fuel
A significant proportion of advanced SMRs use higher-enrichment fuel than current reactors. The industrial capacity available to supply this type of fuel currently represents one of the main challenges to widespread commercial deployment.
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Economic uncertainty
While SMRs promise to reduce costs as the technology matures and achieves economies of series, there is currently uncertainty regarding their actual construction and operating costs. This is because many designs are still in early development phases, with prototypes only beginning construction in a few countries.
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Reliance on public financial support
Given the high investments needed to develop and bring these technologies to market, SMR deployment currently relies heavily on government support and dedicated funding mechanisms to drive development and commercialisation.
Current global status: Operational SMRs and projects
Globally, small modular reactors (SMRs) have evolved from a largely conceptual technology into a phase of demonstration and initial commercial projects, driven by the need to decarbonise the economy, reinforce energy security and meet growing electricity demand from sectors such as data centres and artificial intelligence. Countries including the United States, the United Kingdom, Canada, China and Russia currently have numerous projects underway at various stages of development, from licensing new designs and constructing initial prototypes to bringing the most advanced designs into operation.
The world's first operational small modular reactor was the Akademik Lomonosov floating nuclear power plant, which began supplying electricity to the grid in 2019 in Chukotka, a remote region in eastern Russia. The Chinese HTR-PM reactor, the first advanced high-temperature modular reactor connected to the electricity network, has also been in commercial operation since 2023.
Organisations such as the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency (NEA) are driving international cooperation, knowledge sharing and technical analysis of this technology. They also promote monitoring tools and initiatives such as the NEA Digital Small Modular Reactor Dashboard
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In Europe, this effort is complemented by the European SMR Alliance, an initiative backed by the European Commission that brings together companies, research bodies, regulators and potential users to accelerate SMR development and deployment, strengthen the European supply chain and foster regulatory framework harmonisation. For its part, the European Union considers SMRs a strategic technology to diversify the energy mix and strengthen energy security and autonomy, as well as to pursue net-zero decarbonisation goals.
The future of SMRs: Sustainable development and the net zero pathway
In a context marked by the electrification of the economy and rising electricity demand, nuclear energy acts as a complementary technology to renewables and a key ally in the energy transition. Nuclear power stations produce electricity continuously without emitting greenhouse gases, bringing stability and security of supply to the electricity system. This capacity makes them an ideal complement to renewable sources such as solar and wind power, whose output depends on weather conditions.
This role is especially relevant given the growth of data centres, driven by the expansion of artificial intelligence and digital services. These facilities require a reliable, 24-hour, abundant electricity supply (accounting for around 1% of current global electricity demand), renewing interest in nuclear energy. Reflecting this trend, tech companies such as Google, Meta, Microsoft and Amazon have already signed long-term power purchase agreements and partnerships with the nuclear sector to secure part of their future supply.
In this scenario, small modular reactors can play a key role thanks to their ability to deploy progressively and adapt to various energy needs. Although their commercial development must still overcome significant technological, regulatory and economic challenges, SMRs are positioned as one of the key technologies expected to contribute to a cleaner, more flexible and more resilient energy system.