What Is a Small Modular Reactor (SMR)?

Last updated: October 26, 2026

A small modular reactor (SMR) is an advanced nuclear reactor design that produces up to 300 megawatts of electricity (MWe) per module, built largely in a factory and shipped to site for assembly rather than constructed piece-by-piece in place. SMRs can be deployed as a single unit or combined into multi-module plants, and they are designed to use the smaller footprint, faster build schedule and standardized manufacturing to cut the cost overruns that have plagued large reactor projects.

Definition: what counts as “small” and “modular”

The International Atomic Energy Agency (IAEA) defines SMRs as reactors with an electrical output of up to 300 MWe per module. Historically the IAEA grouped reactors up to 300 MWe as “small” and those between 300 and 700 MWe as “medium,” often lumping both together under the SMR label in practice — which is why some vendor designs marketed as SMRs, such as Rolls-Royce SMR at 470 MWe, sit above the strict 300 MWe line. A further subcategory, microreactors, covers units of roughly 1–20 MWe aimed at remote sites, military bases and industrial facilities rather than grid-scale power.

“Modular” refers to the manufacturing approach, not just the size: major reactor components are fabricated in a centralized factory under controlled conditions, then transported by truck, rail or barge to the plant site for assembly. This is the opposite of how large reactors such as Vogtle Units 3–4 in Georgia or Hinkley Point C in the UK are built, where most major components are fabricated and assembled on site over many years — a process that has repeatedly run billions of dollars over budget and years behind schedule industry-wide.

How an SMR works

Most SMR designs rely on the same fission physics as conventional reactors — a pressurized water reactor (PWR) or boiling water reactor (BWR) core generates heat that is converted to steam to drive a turbine. What changes is the engineering: many SMRs use an “integral” layout that houses the reactor core, steam generators and pressurizer inside a single vessel, reducing the piping where a major loss-of-coolant accident could start. Most designs also lean on passive safety systems — natural circulation, gravity-fed water and convection-driven cooling — that are intended to shut the reactor down and keep the core cool without electric pumps or operator action for an extended period, addressing a key lesson from the 2011 Fukushima Daiichi accident, where a station blackout disabled active cooling.

Because modules are standardized and factory-built, a vendor can in principle produce several units on an assembly line rather than engineering every plant from scratch, which is the main argument for lower cost growth as a fleet matures — though no Western SMR has yet completed construction to test that claim at scale.

Why SMRs matter

Three trends are driving interest in SMRs. First, large reactor projects have become difficult to finance: multi-billion-dollar price tags and construction timelines stretching past a decade expose utilities and governments to heavy cost and schedule risk. Second, SMRs’ smaller output fits grids, islands and industrial sites that cannot absorb a 1,000+ MWe unit, and several designs are aimed at replacing retiring coal plants on the same site, reusing existing grid connections and, in some proposals, the workforce. Third, the sharp rise in electricity demand from data centers and AI computing has pushed technology companies and utilities to look at SMRs as a source of firm, round-the-clock, carbon-free power that could be sited near load centers.

Where SMRs are being built or operated

Sources vary on exact capacities and dates, and several of these projects have slipped their original schedules — treat “targeted” dates as vendor or government projections, not commitments.

Key characteristics

• Output: up to 300 MWe per module by the IAEA’s definition; some vendor designs marketed as SMRs exceed that threshold
• Construction method: factory-fabricated major components, shipped to site, shorter on-site assembly time
• Safety approach: mostly passive, gravity- or convection-driven cooling rather than reliance on active pumps and AC power
• Siting flexibility: smaller footprint and lower cooling-water needs than large reactors, enabling use at retiring coal sites, industrial parks or off-grid locations
• Economics: lower absolute capital cost per unit than a large reactor, but higher cost per kilowatt of capacity until a design reaches production-line scale — a tradeoff still unproven commercially in Western markets

Related technologies

SMRs are one of several options for firm, low-carbon power alongside carbon capture and storage (CCS) fitted to gas or coal plants and green hydrogen production. Comparing an SMR’s cost against gas, coal, solar or wind generation requires the levelized cost of electricity (LCOE), though LCOE alone does not capture the value of the round-the-clock, dispatchable output that nuclear plants — including SMRs — can provide.

FAQ

What is an SMR in simple terms?
It is a nuclear reactor small enough (up to roughly 300 MWe) to be substantially built in a factory and shipped to its site, instead of being constructed piece by piece in place like a conventional 1,000+ MWe reactor.

Is there an SMR operating today?
Yes. Russia’s Akademik Lomonosov, a floating plant with two 35 MWe KLT-40S reactors, has been in commercial operation since May 2020, making it the only operating SMR as of late 2026. Several other designs are under construction or in licensing.

How is an SMR different from a microreactor?
Microreactors are a smaller subcategory, typically producing roughly 1–20 MWe, aimed at remote sites, military installations or industrial facilities rather than grid-scale supply. SMRs cover the wider range up to 300 MWe (or more, in some vendor usage).

Why are SMRs considered safer than large reactors?
Most SMR designs rely on passive safety systems — natural circulation and gravity-fed cooling — that can keep the core cool without electric pumps or operator action, a response to lessons from the 2011 Fukushima accident, where a station blackout disabled active cooling systems.

Are SMRs cheaper than large nuclear reactors?
Not yet proven. The theory is that factory production should lower costs once a design reaches fleet scale, but no Western SMR has completed construction to test that claim, and early-unit costs — such as the reported roughly $7.7 billion price tag for the first BWRX-300 at Darlington — remain high on a per-unit basis.

Related The Daily Energy pages

• Barakah Nuclear Power Plant
• LCOE (Levelized Cost of Electricity)
• Carbon Capture and Storage (CCS)
• Green Hydrogen
• United States Energy Profile

Sources

• IAEA — Small Modular Reactors (topic page)
• World Nuclear Association — Small Modular Reactors
• World Nuclear Association Reactor Database — Akademik Lomonosov
• American Nuclear Society / Nuclear News — BWRX-300 Darlington construction approval
• American Nuclear Society / Nuclear News — Rolls-Royce SMR UK competition win

Illustrative image. Photo: Nuclear Regulatory Commission, Public domain, via Wikimedia Commons — source