Small modular reactors (SMRs), explained

What is a small modular reactor?

The World Nuclear Association defines SMRs as nuclear reactors "generally 300 MWe equivalent or less, designed with modular technology using module factory fabrication, pursuing economies of series production and short construction times". For comparison, it notes that traditional reactors produce around 1000 MWe or more per unit.

The U.S. Energy Information Administration uses similar numbers. A large-scale nuclear design typically ranges between 550 MW and 1,500 MW per unit, while SMRs have a capacity of about 300 MW per unit or less. EIA also describes microreactors as a subset of SMRs that generally have a capacity of 20 MW or less. We cover that smaller class on our micro modular reactors page.

How do small modular reactors work?

Every SMR does the same basic job as a large reactor. Uranium atoms split in a controlled chain reaction, the reaction releases heat, a coolant carries that heat away and a power conversion system turns it into electricity, or delivers it as process heat.

What varies is the combination of moderator, coolant and fuel:

• Moderator. Slows the neutrons so they are more likely to cause fission. It is usually ordinary (light) water, but it may be heavy water or graphite.
• Coolant. Carries heat out of the core. DOE lists light water or non-light-water coolants such as a gas, liquid metal, or molten salt.
• Power conversion. Most designs make steam and drive a steam turbine. Some gas-cooled and advanced designs use a gas turbine in a closed cycle, which can remove the need for condenser water.

The main types of SMR

WNA groups the field into four main technology options: light water reactors, fast neutron reactors, graphite-moderated high temperature reactors and various kinds of molten salt reactors.

Where SMRs stand in 2026

The European Commission counts more than 120 SMR designs at different stages of development. Only a few have reached hardware. Some public milestones:

• China, HTR-PM. The demonstration plant at Shidaowan, two pebble-bed reactors driving one 210 MWe turbine, entered commercial operation in December 2023.
• Canada, Darlington. The Canadian Nuclear Safety Commission issued OPG a licence to construct one BWRX-300 in April 2025.
• United States, NuScale. The NRC approved the standard design of the 77 MWe-per-module US460 in May 2025.

For a complete list of projects and their status, see our SMR tracker.

The case for SMRs

DOE lists the attractions as relatively small physical footprints, reduced capital investment, ability to be sited in locations not possible for larger nuclear plants, and provisions for incremental power additions.

For an industrial or data-centre buyer the useful features are concrete:

• Smaller capital steps. Capacity can be added module by module as load grows.
• Siting. Smaller cores and passive features open the possibility of smaller emergency planning zones and sites nearer to the load.
• Firm output. Nuclear runs around the clock, which matters for loads that cannot follow the weather. Our page on nuclear power for data centers covers that use case.

The trade-offs

SMRs have real critics, and a buyer should hear them. The Union of Concerned Scientists argues that small reactors should have lower capital costs and construction times than large reactors, but that cost per kilowatt-hour depends on economies of scale and serial production that no design has yet proven.

Other open questions are fuel supply for HALEU designs, heavy-forging supply chains, used-fuel management on site, and regulator capacity. We set them out in full in disadvantages of small modular reactors.

Fuel choice matters

DOE says HALEU, enriched between 5% and 20% U-235, is required for most U.S. advanced reactors to achieve smaller designs, and that the majority of these designs need a fuel that is not yet available at commercial scale. We look at that constraint in HALEU vs natural uranium.

Power before the reactor arrives

Every SMR timeline sits behind licensing and construction. BridgePower Nuclear's answer is a bridge. A thermal power block from USP&E, which brings 24+ years of experience, 150+ power projects in 45+ countries and 2,000+ MW in stock, grid-tied or behind the meter.