Small modular reactor
A small modular reactor (SMR) is a type of nuclear fission reactor with a rated electrical power of 300 MWe 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 (PWRs) 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 MWe (electric) and up to 300 MWe per module. SMRs may also be designed purely for desalination or facility heating rather than electricity. These SMRs are measured in megawatts thermal MWt. Many SMR designs rely on a modular system, allowing customers to simply add modules to achieve a desired electrical output.
Similar-sized reactors have been used for military use since the 1950s for nuclear marine propulsion. The thermal output of the largest naval reactor as of 2025 is estimated at 700 MWt (the A1B reactor), similar in capacity to some large SMR designs. Between 1954 and 1977, the US Army experimented with powering military installations with small land-based reactors during the Army Nuclear Power Program, but never applied the modular scalability and advanced safety designs used with SMRs. Still, naval nuclear reactors have an excellent record of safety. According to public information, the US Navy's Naval Reactors program has never succumbed to a meltdown or radioactive release 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.
However, military small reactors are quite different from commercial SMR designs. The military, especially for naval propulsion, has historically relied on highly enriched uranium (HEU) to power their reactors and not the low-enriched uranium (LEU) fuel used by almost all civilian power reactors, including many proposed SMRs. This is because submarine reactors are severely space-constrained and require higher power density than civilian reactors. Many naval reactors also operate for over a decade or more without refueling.
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 (OSU), 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.