Micro Modular Reactors

Micro Modular Reactors (MMR) are compact nuclear reactor systems classified as a subclass of Small Modular Reactors (SMR), typically capable of generating 1–10 megawatts electric (MWe) of energy or an equivalent amount of thermal power. The term “micro” refers not only to their low power capacity but also to their portability, scalability, and compact design features. Unlike conventional large-scale nuclear power plants, MMRs are designed to be deployed close to the point of energy demand. As a result, they are preferred for providing low-carbon, continuous, and reliable energy in remote areas without access to the electrical grid, mining sites, small isolated communities, military bases, critical infrastructure facilities, and university campuses.

The design philosophy of these reactors is based on flexibility and safety. Manufactured in factories and assembled on-site in modular form, they aim to reduce both construction time and capital requirements. Most designs are produced in standard container sizes suitable for transport and can be easily moved by road, sea, or rail. This feature enables rapid deployment of energy infrastructure even in remote and hard-to-reach regions.

The range of applications for MMRs is not limited to electricity generation. Their ability to provide high-temperature thermal energy enables their use in diverse sectors such as industrial steam production, district heating, desalination plants, and hydrogen production. In addition, their compatibility with microgrids allows the creation of hybrid systems with renewable energy sources, enhancing energy supply reliability.

Despite being significantly smaller than conventional nuclear facilities, MMRs offer advanced safety solutions. Designs based on passive safety systems can ensure the reactor shuts down safely without requiring human intervention. In this regard, MMRs are viewed as an advantageous option not only for low-carbon energy production but also for safety and environmental impact.

The MMR concept emerged in the first quarter of the 21st century as a result of growing interest in small and modular reactors. Challenges associated with conventional nuclear power plants—including high costs, long construction times, and complex safety requirements—have driven focus toward smaller, portable, and more flexible solutions. The 2011 Fukushima Daiichi nuclear accident accelerated research into small-scale reactor designs featuring passive safety systems and greater resilience to accident scenarios. In this context, companies such as Ultra Safe Nuclear Corporation (USNC), Westinghouse, Terrestrial Energy, U-Battery, and LeadCold have become leading actors in micro and small modular reactor technologies.

Canada is one of the countries drawing attention for its development of MMR projects. The MMR project led by Global First Power (GFP) at the Chalk River site stands out as one of the world’s first initiatives aiming for licensing of a microreactor. This project is significant not only for electricity generation but also for providing process heat and reducing dependence on fossil fuels in remote areas. In parallel, McMaster University, in collaboration with USNC and GFP, initiated a feasibility study evaluating the potential use of MMR technology for education and research, public awareness, and addressing community energy needs. These efforts demonstrate that MMR technology can play a role not only in energy provision but also in knowledge generation, public awareness, and industry-academia collaboration.

MMRs are systems designed to be fully or largely assembled in factories and rapidly commissioned on-site. This approach distinguishes them from the prolonged, high-cost, and complex construction processes typical of conventional large-scale nuclear power plants. The reactor core, cooling systems, power conversion units, and auxiliary components are prepared as standardized modules, often housed in ISO-compliant containers. These modules can be transported by road, sea, or rail, providing flexibility in logistics and field deployment.

This manufacturing and installation method offers several key advantages:

• Short Installation Time: Site preparations are typically limited to basic infrastructure work. Since reactor modules are delivered ready-made from the factory, on-site assembly time is greatly reduced, allowing systems to be operational within weeks or months.
• Cost Control: Serial production in a factory environment ensures consistent quality standards and repeatability in the manufacturing process. This supports more predictable project costs and reduces investment risks.
• Flexibility: Standard modular designs can be scaled to suit different geographical conditions. This enables reactors to be deployed not only in major urban centers but also in mountainous areas, islands, or polar regions that are difficult to access.
• Transportability: Reactor components packaged in container sizes can be easily transported using existing infrastructure. This provides significant logistical advantages for remote communities, isolated settlements, and regions with limited energy access.

In conclusion, MMRs differ from large-scale conventional nuclear power plants not only in their energy production method but also in their economic, operational, and logistical efficiency. These characteristics make them an alternative low-carbon energy solution for both developed and developing countries.

This class is among the most common and mature technologies in MMR designs.

• Coolant: Helium, due to its inert nature, is chemically non-reactive and functions as a reliable heat transfer medium even at high temperatures.
• Moderator: Graphite is used to slow down neutrons, contributing to the reactor’s long-term stable operation.
• Fuel: TRISO (Tri-structural Isotropic) or FCM (Fully Ceramic Microencapsulated) fuel is used. These fuel types offer a critical safety advantage by securely encapsulating fission products within multi-layered ceramic coatings.
• Features: They provide high outlet temperatures in the range of 500–700 °C, enabling high efficiency in electricity generation and delivering high-quality heat suitable for industrial processes. Fuel cycles are long, reducing the need for frequent refueling.

An example is the MMR concept developed by Ultra Safe Nuclear Corporation (USNC) and the planned project at the Chalk River site in Canada.

This design draws attention for its passive safety and low maintenance requirements.

• Cooling: Heat generated in the core is directly transferred via closed-loop heat pipes. This system does not require pumps or pressurized coolants.
• Advantages: The absence of water cooling, natural flow without mechanical assistance, and elimination of dependence on active systems enhance reliability. Consequently, minimal human intervention is required.

Westinghouse’s eVinci™ microreactor is designed for approximately 5 MWe of electricity and 15 MWt of heat. It is expected to operate for eight years or longer on a single fuel load.

This class, still in the research and development phase, represents more advanced technological options.

• Coolants: Materials such as molten salt, liquid sodium, or lead are used. These coolants exhibit stable behavior at high temperatures and enable integration into fast reactor concepts in some designs.
• Features: Their ability to operate at high temperatures offers potential for more efficient electricity generation and direct process heat applications. Reduced need for pumping simplifies design.
• Application Areas: Not yet widely commercialized, they are being researched for long fuel cycles and high-temperature energy applications.
• Example: Lead-cooled designs developed by Sweden-based LeadCold and molten salt-based initiatives by Moltex Energy are examples in this category.

Fuels used in MMRs are designed with safety and long cycle life in mind. Most designs favor low-enriched uranium (LEU). However, next-generation concepts anticipate the use of high-assay low-enriched uranium (HALEU) to enable longer operational periods and higher energy density. HALEU has uranium-235 enrichment up to 19.75%, higher than the ~5% LEU used in conventional commercial reactors but lower than highly enriched uranium (HEU) used in research reactors. Thus, it occupies an intermediate position in terms of both safety and fuel cycle management. The prominent fuel types in MMR technologies include:

• TRISO (Tri-structural Isotropic) Fuel: Uranium fuel particles are coated with multiple layers of ceramic barriers such as carbon and silicon carbide. This structure enhances safety by preventing the release of fission products even at high temperatures.
• FCM (Fully Ceramic Microencapsulated) Fuel: A more advanced design in which TRISO particles are embedded in a dense ceramic matrix. This method increases radiation resistance and strengthens the mechanical durability of the fuel.

Thanks to these fuel types, MMRs can maintain fuel integrity even under potential accident scenarios and minimize the release of fission products into the environment. Fuel cycles are planned to enable uninterrupted operation for 8 to 20 years depending on the design. These long cycle durations eliminate the need for frequent refueling, reducing operational costs and enhancing safety. In most cases, a single fuel load is sufficient for the reactor’s lifetime, after which the fuel is either returned to the manufacturer or directed toward long-term storage.

One of the most important features of MMRs is that their safety architecture is largely based on passive systems. While conventional large-scale nuclear power plants require complex active safety mechanisms—such as pumps, electrical systems, and continuous human intervention—MMRs minimize this dependency. Their low power density and high surface-to-volume ratio facilitate natural heat dissipation from the core. This physical advantage allows heat to be removed passively through natural convection and conduction, enabling the reactor to maintain a stable temperature range without active cooling systems.

The primary passive safety elements used in MMRs include:

• Heat Pipes: Transfer energy generated in the core without pumps, eliminating mechanical failure risks and increasing system reliability.
• Natural Circulation: Fluids move due to density differences without pumps, enabling passive removal of residual heat from the core.
• Gravity-Driven Flows: Coolants circulate within the system under their own weight, supporting safety without additional energy input.
• High Thermal Inertia: The reactor core and surrounding materials have high thermal capacity, which dampens sudden temperature increases and ensures system stability.

Thanks to these design principles, the risk of core meltdown in MMRs is considered extremely low. Even in accident scenarios, the reactor is designed to “walk away safe”—meaning it can shut down safely and continue removing decay heat without operator intervention or external power. This approach contributes significantly to distinguishing MMRs not only in energy production but also in terms of safety culture and public acceptance.

The Chalk River MMR Project, led by Global First Power (GFP), is subject to environmental assessment and licensing procedures under the oversight of the Canadian Nuclear Safety Commission (CNSC). The project targets 15 MWt of process heat and electricity generation and is designed for a 20-year operational life.

McMaster University is conducting a feasibility study with USNC and GFP to evaluate the possibility of deploying an MMR on campus or a related site. The study covers design, safety, public acceptance, and integration with energy systems.

Developed by Westinghouse, eVinci™ is a microreactor based on heat pipe technology with a capacity of 5 MWe/15 MWt. Its lack of water cooling, portability, and an 8+ year fuel cycle make it stand out. It is designed for remote communities, data centers, mining facilities, and military bases.

The UK-based U-Battery project (4 MWe), the Sweden-Canada partnership LeadCold (3–10 MWe), StarCore (10 MWe) in the USA, and various academic initiatives are also working on different micro-scale designs.

MMRs offer lower initial capital investment, shorter installation times, and flexible scalability compared to large-scale nuclear power plants. Unit costs are expected to decrease through serial production. However, supply chains for fuels such as HALEU remain limited, potentially slowing commercial deployment.

MMR operations produce no direct CO₂ emissions. Designs that do not require water cooling offer advantages in arid regions. Their small land footprint can limit environmental pressure. However, historical perceptions of nuclear energy and safety concerns remain significant barriers to public acceptance. Therefore, demonstration projects at universities and research centers, public education initiatives, and transparency are of great importance.

MMRs hold significant potential for hybrid use with renewable energy sources, enhancing the resilience of critical infrastructure, expanding energy access in remote areas, and supporting industrial heat applications. In the coming period, the alignment of regulatory frameworks, development of fuel supply chains, and implementation of cost-reducing serial production strategies will determine whether these reactors can achieve broader integration into energy systems.