Chimney Top Calculators

What is a small modular reactor (SMR)? Size, modularity and passive safety

A small modular reactor is not a new kind of physics. It is a different way of building and operating reactors: smaller units, made in factories, with simpler safety systems.

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Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

Small, and modular

SMRs are generally defined as reactors of up to about 300 megawatts-electric per unit; today's large US reactors are typically about 1,000 MWe or more. The US Department of Energy describes the SMRs under development as ranging from tens to hundreds of megawatts, for electricity, process heat, desalination and other industrial uses. Microreactors are smaller still: DOE says most designs would produce about 1 to 20 megawatts-thermal and could be moved by truck.

"Modular" refers to two ideas. Major components are fabricated as modules in a factory and shipped to the site, which should improve quality control and shorten on-site construction. And a plant can be built up in steps by adding modules as demand grows. DOE cites smaller footprints, reduced capital investment, siting flexibility and incremental power additions as advantages.

Passive safety and integral designs

Many SMRs rely on passive safety: gravity, natural circulation and heat conduction to a large pool or to the air instead of pumps, AC power and operator action. Natural circulation works because hot coolant is less dense and rises while cooled coolant sinks, so placing the heat sink above the core keeps coolant moving without pumps.

Light-water SMRs are often integral PWRs: the core, steam generators and pressurizer sit inside one vessel. With no large external primary pipes, a large-break loss-of-coolant accident is no longer a design concern. The first SMR design certified by the NRC, NuScale's (final rule January 2023), uses 50 MWe modules, with up to 12 in one reactor building.

Planning zones

Large US reactors have a plume exposure emergency planning zone of about 10 miles and an ingestion pathway zone of about 50 miles. The NRC's alternative emergency preparedness requirements for SMRs and other new technologies let an applicant size its planning zone from the credible accident consequences of its own design, which can be much smaller. Emergency planning is still required.

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Frequently asked questions

How big is a small modular reactor?
Up to about 300 MWe per unit by the common definition. Designs under development in the US range from tens to hundreds of megawatts-electric; microreactors are about 1 to 20 MWt.
Are SMRs safer than large reactors?
Many SMR designs aim for passive safety and smaller radioactive inventories, which can reduce the consequences of accidents. Each design still has to demonstrate its safety case to the NRC; size alone does not make a reactor safe.
Has the NRC approved any SMR design?
Yes. The NRC certified the NuScale small modular reactor design by a final rule published on January 19, 2023 (10 CFR Part 52, Appendix G).
Do SMRs produce spent fuel?
Yes. Like all fission reactors, SMRs produce spent fuel and other radioactive waste that must be stored and managed.

Sources

  1. U.S. DOE Office of Nuclear Energy, Advanced Small Modular Reactors (SMRs) www.energy.gov
  2. U.S. DOE Office of Nuclear Energy, What is a Nuclear Microreactor? www.energy.gov
  3. U.S. EIA, Today in Energy: Small modular reactors and microreactors under development in the United States www.eia.gov
  4. U.S. NRC final rule, NuScale Small Modular Reactor Design Certification, Federal Register 2023-00729 www.federalregister.gov
  5. 10 CFR Part 50, Domestic Licensing of Production and Utilization Facilities (U.S. NRC) www.ecfr.gov

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.