Small modular reactor

A small modular reactor is a type of nuclear fission reactor with a rated electrical power of 300 MW_e or less. SMRs are designed to be factory-fabricated and transported to the installation site as prefabricated modules, allowing for streamlined construction, enhanced scalability, and potential integration into multi-unit configurations. The term SMR refers to the size, capacity and modular construction approach. Reactor technology and nuclear processes may vary significantly among designs. Among current SMR designs under development, pressurized water reactors represent the most prevalent technology. However, SMR concepts encompass various reactor types including generation IV, thermal-neutron reactors, fast-neutron reactors, molten salt, and gas-cooled reactor models.
Commercial SMRs are designed to deliver an electrical power output as low as 5 MW_e and up to 300 MW_e per module. SMRs may also be designed purely for desalination or facility heating rather than electricity. These SMRs are measured in megawatts thermal MW_t. Many SMR designs rely on a modular system, allowing customers to simply add modules to achieve a desired electrical output.
Similar military small reactors were first designed in the 1950s to power submarines and ships with nuclear propulsion. However, military small reactors are quite different from commercial SMRs in fuel type, design, and safety. The military, historically, relied on highly enriched uranium to power their small plants and not the low-enriched uranium fuel type planned for use in many SMRs. Power generation requirements are also substantially different. Nuclear-powered naval ships require instantaneous bursts of power and must rely on small, onboard tanks of seawater and freshwater for steam-driven electricity. The thermal output of the largest naval reactor as of 2025 is estimated at 700 MW_t. Pressure Water Reactor SMRs generate much smaller power loads per module, which are used to heat large amounts of freshwater, stored inside the module and surrounding the reactor. SMRs also maintain a fixed power load for up to a decade, with uninterrupted refueling cycles occurring every 2 years on average.
To overcome the substantial space limitations facing naval designers, sacrifices in safety and efficiency systems are required to ensure fitment. Today's SMRs are designed to operate on many acres of rural land, creating near limitless space for radically different storage and safety technology designs. Still, small military reactors have an excellent record of safety. According to public information, the Navy has never succumbed to a meltdown or radioactive release in the United States over its 60 years of service. In 2003, Admiral Frank Bowman backed up the Navy's claim by testifying no such accident has ever occurred.
There has been strong interest from technology corporations in using SMRs to power data centers.
Modular reactors are expected to reduce on-site construction and increase containment efficiency. These reactors are also expected to enhance safety through passive safety systems that operate without external power or human intervention during emergency scenarios, although this is not specific to SMRs but rather a characteristic of most modern reactor designs. SMRs are also claimed to have lower power plant staffing costs, as their operation is fairly simple, and are claimed to have the ability to bypass financial and safety barriers that inhibit the construction of conventional reactors.
Researchers at Oregon State University, headed by José N. Reyes Jr., invented the first commercial SMR in 2007. Their research and design component prototypes formed the basis for NuScale Power's commercial SMR design. NuScale and OSU developed the first full-scale SMR prototype in 2013 and NuScale received the first Nuclear Regulatory Commission Design Certification approval for a commercial SMR in the United States in 2022. In 2025, two more NuScale SMRs, the VOYGR-4 and VOYGR-6, received NRC approval.

Operational SMRs

, only China and Russia have successfully built operational SMRs. Russia has been operating a floating nuclear power plant Akademik Lomonosov, in Russia's Far East, commercially since 2020. China's pebble-bed modular high-temperature gas-cooled reactor HTR-PM was connected to the grid in 2021.
As of 2025, there were 127 modular reactor designs, with seven designs operating or under construction, 51 in the pre-licensing or licensing process, and 85 designers in discussions with potential site owners.

Background

The term was brought to wider use when U.S. Secretary of Energy Steven Chu identified "small modular reactors" as "America's new nuclear option" in a 2010 Wall Street Journal op-ed, where he stated "SMRs would be ready to 'plug and play' upon arrival " and be more affordable. He announced that President Barack Obama had requested $39million for a new SMR design and licensing program.

Hope of enhanced safety and reduced costs

Economic factors of scale mean that nuclear reactors tend to be large, to such an extent that size itself becomes a limiting factor. Furthermore, the 1986 Chernobyl disaster caused a major set-back for the nuclear industry, with worldwide suspension of development, cutting down of funding, and closure of reactor plants.
In response, researchers at Oregon State University began development of the first commercial SMR prototypes in the late 1990s and early 2000s. A radically different reactor than the one used by the military, OSU's SMR design decreased fabrication time, advanced operational safety, and reduced the cost of operation. The goal was to make it easier for commercial and public entities to afford a traditionally cost-prohibitive form of energy. Credited as the inventor of the commercial SMR, OSU researchers believed the smaller form factor and modular design would allow manufacturers to swap economies-of-unit-scale for economies-of-unit-mass-production — lowering production costs and improving manufacturing efficiency. NuScale Power partnered with OSU to become the first to apply this manufacturing strategy starting in 2006.
Proponents claim that SMRs would be less expensive due to the application of standardized modules that could be industrially produced off-site in a dedicated factory. SMRs do, however, also have economic disadvantages. Several studies suggest that the overall costs of SMRs are comparable with those of conventional large reactors. Moreover, extremely limited information about SMR modules transportation has been published. Critics say that modular building will only be cost-effective for a high number of the same SMR type, given the still remaining high costs for each SMR. A high market share is thus needed to obtain sufficient orders.

Contribution to the net zero emissions pathways

In February 2024, the European Commission recognized SMR technology as an important contributor to decarbonization as part of EU Green Deal.
In its pathway to reach global net zero emissions by 2050, the International Energy Agency considers that worldwide nuclear power should be multiplied by two between 2020 and 2050. Antonio Vaya Soler, an expert from the Nuclear Energy Agency, agrees that although renewable energy is essential to fight global warming, it will not be sufficient to achieve net zero emissions and nuclear energy capacity should be at least doubled.
To produce the same electrical power as the nuclear power reactors in the world today, BASE, the German Federal Office for the Safety of Nuclear Waste Management, warns that it would be necessary to build several thousands to tens of thousands of SMRs.
Several fleets of SMRs of exactly the same type, industrially manufactured in large numbers, should be rapidly deployed worldwide to significantly reduce emissions of. The Nuclear Energy Agency launched at COP 28 an initiative Accelerating SMRs for Net Zero to foster collaboration between research organizations, nuclear industry, safety authorities, and governments, in order to reduce carbon emissions to net zero before 2050 to limit global surface temperature increase.

Future challenges

Proponents say that nuclear energy with proven technology can be safer; the nuclear industry contends that smaller size will make SMRs even safer than larger conventional plants. This is because the main problem associated with nuclear meltdowns is the decay heat that is present after reactor shutdown, which would be much lower for SMRs because of their lower power output. Critics say that many more small nuclear reactors pose a higher risk, requiring more transportation of nuclear fuel and also increasing the production of radioactive waste. SMRs require new designs with new technology, the safety of which has yet to be proven.
Until 2020, no truly modular SMRs had been commissioned for commercial use. In May 2020, the first prototype of a floating nuclear power plant with two 30 MW_e reactors – the type KLT-40 – started operation in Pevek, Russia. This concept is based on the design of nuclear icebreakers. The operation of the first commercial land-based, 125 MW_e demonstration reactor ACP100 is due to start in China by the end of 2026.

Designs

SMRs are envisioned in multiple designs. Some are simplified versions of current reactors, others involve entirely new technologies. All proposed SMRs use nuclear fission with designs including thermal-neutron reactors and fast-neutron reactors.

Thermal-neutron reactors

Thermal-neutron reactors rely on a moderator to slow neutrons and generally use as fissile material. Most conventional operating reactors are of this type.

Fast reactors

don't use moderators. Instead they rely on Highly Enriched Uranium fuel to absorb fast neutrons. This usually means changing the fuel arrangement within the core, or using different fuels. E.g., is more likely to absorb a fast neutron than.
Fast reactors can be breeder reactors. These reactors release enough neutrons to transmute non-fissionable elements into fissionable ones. A common use for a breeder reactor is to surround the core by a "blanket" of, the most easily available isotope. Once the undergoes a neutron absorption reaction, it becomes, which can be removed from the reactor during refueling, and subsequently reprocessed and used as fuel.