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A '''small modular reactor''' ('''SMR''') is a type of [[nuclear fission reactor]] with a rated electrical power of 300 MW<sub>e</sub> 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.<ref>{{cite journal |last1=Hussein |first1=Esam M. A. |title=Emerging small modular nuclear power reactors: A critical review |journal=Physics Open |date=2020 |volume=5 |article-number=100038 |doi=10.1016/j.physo.2020.100038 |doi-access=free}}</ref> The term SMR refers to the size, capacity and [[Modular design|modular]] construction approach. Reactor technology and nuclear processes may vary significantly among designs. Among current SMR designs under development, [[pressurized water reactor]]s (PWRs) represent the most prevalent technology. However, SMR concepts encompass various reactor types including [[generation IV reactor|generation IV]], [[thermal-neutron reactor]]s, [[fast-neutron reactor]]s, [[Molten salt reactor|molten salt]], and [[gas-cooled reactor]] models.<ref name=":2" />
Commercial SMRs have been designed to deliver an [[electrical power]] output as low as 5 [[Watt#MWe|MW<sub>e</sub>]] (electric) and up to 300 MW<sub>e</sub> per module. SMRs may also be designed purely for
Similar military small reactors were first designed in the 1950s to power submarines and ships with nuclear propulsion.<ref name="BASE">{{cite web |author=BASE, the German Federal Office for the Safety of Nuclear Waste Management |date=2023-01-15 |title=Small Modular Reactors (SMR) |url=https://www.base.bund.de/en/nuclear-safety/nuclear-technology/small-modular-reactors/small-modular-reactors.html |access-date=2023-12-12 |website=BASE}}</ref> However, military small reactors are quite different from commercial SMRs in fuel type, design, and safety. The military, historically, relied on highly-enriched uranium (HEU) to power their small plants and not the low-enriched uranium (LEU) fuel type used in 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<sub>t</sub> (the [[A1B reactor]]).<ref>{{Cite web |date=4 February 2025 |title=Nuclear-Powered Ships: Nuclear Propulsion Systems |url=http://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-powered-ships.aspx |publisher=World Nuclear Association}}</ref> Pressure Water Reactor (PWR) SMRs generate much smaller power loads per module, which are used to heat large amounts of freshwater, stored inside the module and surrounding the
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.<ref>{{cite web |title=Small Nuclear Power Reactors |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/small-nuclear-power-reactors |website=world-nuclear.org |publisher=World Nuclear Association |access-date=24 August 2025}}</ref> 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.<ref>{{cite web |title=NASA's organizational and management challenges in the wake of the Columbia disaster |url=https://www.congress.gov/event/108th-congress/house-event/LC15416/text |website=www.congress.gov |quote="our nuclear‑powered ships ... have steamed ... without a reactor accident ... with no measurable negative impact on the environment or human health" |date=29 October 2003}}</ref>
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=== Fast reactors ===
[[Fast-neutron reactor|Fast reactors]] don't use moderators. Instead they rely on
Fast reactors can be [[breeder reactor]]s. These reactors release enough neutrons to [[Nuclear transmutation|transmute]] [[Fertile material|non-fissionable elements]] into fissionable ones. A common use for a breeder reactor is to surround the core by a "blanket" of {{chem|238|U}}, the most easily available isotope. Once the {{chem|238|U}} undergoes a [[neutron absorption]] reaction, it becomes {{chem|239|Pu}}, which can be removed from the reactor during refueling, and subsequently [[Nuclear reprocessing|reprocessed]] and used as fuel.<ref name="world-nuclear1">Carlson, J. [http://www.world-nuclear.org/info/inf98.html "Fast Neutron Reactors"] {{Webarchive|url=https://web.archive.org/web/20130224035726/http://www.world-nuclear.org/info/inf98.html |date=24 February 2013 }}, [http://world-nuclear.org ''World Nuclear Association'']</ref>
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Several molten salt reactors are being developed as SMRs, but they are not a new concept. Operational as research and test plants since the 1950s, molten salt reactors are now being touted as a clean and safe alternative to traditional water-cooled SMRs. One of the earliest molten salt reactor experiments was operated at Tennessee's Oak Ridge for four years, but shut down in 1969 after going critical. Even though the Molten Salt Reactor Experiment did end in a critical event, it was well known and respected throughout the nuclear research community as a success. However, later studies found the reactor only operated around 40 percent of the time, and experienced 171 unplanned shutdowns. These shutdowns were attributed to a number of technical problems, including: chronic pipe plugging, which led to charcoal beds designed to capture and remove radioactive materials; blower failures designed to remove reactor heat; and leaks within the freeze-valve safety system allowing fuel escapes. So far, modern metals have proven incapable of sustaining the natural corrosiveness of a small reactor's molten salt over a 4 year application.<ref>{{cite web |last1=Ramana |first1=M.V. |title=Molten salt reactors were trouble in the 1960s—and they remain trouble today |url=https://thebulletin.org/2022/06/molten-salt-reactors-were-trouble-in-the-1960s-and-they-remain-trouble-today/ |website=thebulletin.org |date=20 June 2022 |publisher=Bulletin of the Atomic Scientists |access-date=26 August 2025}}</ref>
Even Fluoride-Salt-Cooled High-Temperature Reactors (FHR) suffer from internal buildups of fission products, clogging cooling and safety systems. A method of reductive extraction can be used to catch buildups before they occur. This method removes the uranium fuel before the fission products. Unfortunately, the gas produced from the fluoride is highly corrosive and exposes plant metal to damage. As an alternative, nitrogen trifluoride is being proposed. However, ongoing research has not proven this to be a viable alternative and its efficacy is unclear.<ref>{{cite web |last1=Scheele |first1=Randall |title=Flibe Molten Salt Processing |url=https://www.energy.gov/ne/articles/flibe-molten-salt-processing |website=energy.gov |publisher=Flibe Energy, Inc. |access-date=31 August 2025}}</ref>
== Radioactive waste ==
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Light-water reactors designed to run on [[thorium]] offer increased [[Nuclear proliferation|proliferation]] resistance compared to the conventional uranium cycle, though [[molten salt reactors]] have a substantial risk.<ref name="U-232 and the proliferation-resistance of U-233 in spent fuel2">{{Cite journal |last1=Kang |first1=J. |last2=Von Hippel |first2=F. N. |year=2001 |title=U-232 and the proliferation-resistance of U-233 in spent fuel |journal=Science & Global Security |volume=9 |issue=1 |pages=1–32 |bibcode=2001S&GS....9....1K |doi=10.1080/08929880108426485 |s2cid=8033110}} {{cite web |title=Archived copy |url=http://www.torium.se/res/Documents/9_1kang.pdf |url-status=dead |archive-url=https://web.archive.org/web/20141203135336/http://www.torium.se/res/Documents/9_1kang.pdf |archive-date=3 December 2014 |access-date=2 March 2015}}</ref><ref>{{Cite journal |last1=Ashley |first1=Stephen |year=2012 |title=Thorium fuel has risks |journal=Nature |volume=492 |issue=7427 |pages=31–33 |bibcode=2012Natur.492...31A |doi=10.1038/492031a |pmid=23222590 |s2cid=4414368 |doi-access=free}}</ref>
SMRs are transported from the factories without fuel, as they are fueled on the ultimate site, except some [[Nuclear microreactor|microreactors]].<ref>{{cite web |author=Office of Nuclear Energy |url=https://www.energy.gov/ne/articles/what-nuclear-microreactor |title=What is a Nuclear Microreactor? |publisher=Office of Nuclear Energy |date= |access-date=2022-08-18}}</ref> This implies an independent transport of the fuel to the site and therefore increases the risk of nuclear proliferation. At the same time, millions of tons of nuclear waste are being shipped across the nation each year and there is no history of nuclear fuel or waste theft from these deliveries in the US.
== Licensing process ==
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