Most existing nuclear reactors are big – typically ranging in size from 600 to more than 1,500 MW. New technologies in development promise much smaller reactors ranging in size from 2 to 300 MW. These smaller units could be installed as a single small power plant or as installations of multiple modules to obtain larger amounts of capacity. Hence, they are called small modular reactors or SMRs.
SMRs, according to the International Atomic Energy Agency (IAEA), are “advanced reactors that produce electricity of up to 300 MW(e) per module ... these reactors have advanced engineered features, are deployable either as a single or multi-module plant, and are designed to be built in factories and shipped to utilities for installation as demand arises.”
Numerous and diverse SMR technologies are being investigated. The IAEA notes that there are currently about 50 conceptual designs globally. Most designs are still in research and development, but there are three advanced SMR power plants under construction in Argentina, China, and Russia. Other existing small nuclear units use traditional designs rather than advanced technologies.
Key types based on existing technologies include:
- Land-based water-cooled SMR: A reactor similar in concept to existing commercial large nuclear units. Designs may use pressurized water reactor (PWR), boiling water reactor (BWR), or pressurized heavy water reactor (PHWR) concepts. Existing units include power plants in China, India, Pakistan, and Russia. An advanced-technology 25 MW unit is now under construction in Argentina.
- Marine-based water-cooled SMR: A pressurized water reactor similar to a land-based water-cooled SMR but the power plant is located on water, either on a barge or under the water. Russia has adapted the technology from marine propulsion and currently has a two-unit 70 MW power plant under operation. A 60 MW unit is under construction in China.
- High-temperature gas-cooled SMR: A reactor that is cooled by gas such as helium and operates at a temperature as high as 1800°F (1000°C). Such temperatures create high-temperature and high-pressure steam that increases the thermal efficiency of the generation process. Two units were built in the U.S. — the 40 MW Peach Bottom 1 in Pennsylvania operated from 1967 to 1974, and the 300 MW Fort St. Vrain in Colorado operated from 1979 to 1989. A 210 MW unit is now under construction in China.
Numerous other technologies are under conceptual or design development but not construction. As of early 2022, the World Nuclear Association listed the following in the well-developed phase with potential for near-term deployment:
There is strong interest in SMRs for several reasons:
- Nuclear power offers a carbon-free source of electricity without the intermittency of renewables.
- Modularity allows construction to better match load growth – current nuclear units are built in blocks of 1,000 to 2,000 MW, which can often take years for load to grow and utilize all the capacity.
- Modularity could allow units to be built and placed into service more quickly than current nuclear units that typically take five or more years to construct after permits are obtained.
- Modularity promises more cost control – units recently under construction have tended to balloon in cost during the construction phase whereas SMRs will be constructed in a factory with control over costs.
- The smaller size may allow units to be used in remote locations without the need to connect to the larger electric grid.
- Some SMRs are designed to be underground for greater security and safety.
- Some SMRs are designed for passive shutdown, meaning if something goes wrong the unit automatically shuts down into a safe mode without human intervention.
- Some SMRs are designed to allow output to be ramped up and down to match variations in load.
Unfortunately, using existing technology designs in smaller units is currently very costly and will not compete with the economics of other power sources. So future growth of SMR depends on advanced technologies that are still under design and unproven.