Small Modular Reactors (SMRs)

A small modular reactor, or SMR, is a nuclear reactor that is much smaller than a normal nuclear power plant. The International Atomic Energy Agency (IAEA) calls a reactor "small" if it produces up to about 300 MWe of electricity per unit. "Modular" means most of the reactor is made in a factory as ready parts (modules), carried to the site and fitted together, like putting together a prefabricated building. Several SMRs can also be placed side by side to make a bigger plant.

Why do we need small reactors?

A normal, large nuclear plant produces 700 to 1,600 MWe from one reactor. It costs tens of thousands of crores, needs a big area of land and takes many years to build. Many users do not need so much power in one place. A steel plant, a cement factory or a remote town may need only 50 to 300 MWe.

SMRs are meant to give such users clean, round-the-clock power without building a giant plant. They can also replace old coal power plants, using the same site and the same power lines.

Where did the idea come from?

Small reactors are not new. Nuclear submarines and ice-breaker ships have used small reactors for decades. India's own early PHWRs were 220 MWe units, which is "small" by the IAEA definition. What is new is the "modular" factory-built approach, plus new safety designs. In the 2010s, many countries and companies started designing SMRs. Russia was the first to run one commercially.

Its floating plant Akademik Lomonosov, a barge with two KLT-40S reactors of about 35 MWe each, began commercial operation in May 2020 at Pevek in Russia's Arctic region. China's HTR-PM, a high-temperature gas-cooled reactor, started commercial operation in December 2023. China is also building Linglong One (ACP100) at Changjiang in Hainan, designed as a land-based commercial SMR.

How does an SMR work?

It works on the same basic principle as any nuclear reactor:

1. Uranium atoms in the fuel are split. This is called fission, and it releases a lot of heat.
2. The heat is carried away by a coolant (usually water, but some designs use gas, liquid metal or molten salt).
3. The heat turns water into steam.
4. The steam spins a turbine, which turns a generator and makes electricity.

The difference is in size and design. Because the reactor core is small, it holds less fuel and produces less heat. This makes it easier to cool it in an emergency.

Passive safety: the biggest selling point

Most SMR designs use "passive" safety. This means the reactor can cool itself using natural forces like gravity and natural circulation (hot water rises, cool water sinks), without pumps, electricity or a human operator. Think of a pressure cooker that releases steam on its own, instead of needing someone to switch it off. Because of this, regulators may allow a smaller emergency planning zone (the area around a plant where emergency plans must be ready).

In the United States, the regulator has accepted a method under which the zone for some SMR designs could be as small as the plant boundary. A smaller zone means SMRs could be placed nearer to factories and towns.

The main types of SMRs

• Light-water SMRs: Use ordinary water as coolant, like most large reactors today. Example: NuScale's US460 design (77 MWe per module, six modules for 462 MWe), which received a Standard Design Approval from the US Nuclear Regulatory Commission in May 2025.
• High-temperature gas-cooled reactors: Use helium gas as coolant and can give very hot heat for industry or hydrogen production. Example: China's HTR-PM.
• Fast neutron and molten-salt reactors: Advanced designs still mostly under development.
• Floating SMRs: Reactors mounted on a barge, for remote coastal areas. Example: Akademik Lomonosov.
• Microreactors: Very small units, generally up to about 10 MWe, for remote sites or military bases. The IAEA treats them as a sub-group of SMRs.

India's position and plans

India has three strands of work:

• Bharat Small Reactors (BSR): In the Union Budget 2024-25, the government said it would partner with the private sector to set up Bharat Small Reactors. These are India's proven 220 MWe PHWRs, upgraded with features such as a steel liner and better control systems. In December 2024, NPCIL invited proposals from industries (steel, aluminium, copper, cement) to fund and host BSRs for captive use, with NPCIL running them.
• Nuclear Energy Mission: The Union Budget 2025-26 set up a Nuclear Energy Mission with ₹20,000 crore for research and development of SMRs. Its goal is at least five indigenous (Indian-designed) SMRs running by 2033.
• BARC's new designs: The Bhabha Atomic Research Centre (BARC) is designing the Bharat Small Modular Reactor (BSMR-200), a pressurised water reactor of about 200 MWe meant for heavy industry, and the SMR-55, a 55 MWe version. Tarapur in Maharashtra has been approved as the site for the first BSMR-200. As of August 2026, BSMR-200 was reported to have received administrative and financial sanction.
• The law: The SHANTI Act, 2025 now allows private Indian companies to own and run nuclear plants, which makes industry-owned captive SMRs legally possible.

Commonly confused concepts

• SMR vs large reactor: An SMR produces up to about 300 MWe per unit; a large reactor like Kudankulam's units produces about 1,000 MWe. SMRs are mostly factory-built; large reactors are built mostly on site.
• SMR vs Bharat Small Reactor (BSR): A BSR is an upgraded version of India's existing, proven 220 MWe PHWR design. It is "small", but it is not a new factory-built modular design. BSMR-200 and SMR-55 are the new modular designs.
• SMR vs microreactor: A microreactor is a very small SMR (generally up to about 10 MWe). All microreactors are SMRs, but most SMRs are not microreactors.
• MWe vs MWt: MWe is megawatts of electricity. MWt is megawatts of heat (thermal). A reactor's thermal output is about three times its electrical output, because much heat is lost when making electricity.
• "Small modular reactor" vs "small and medium reactor": The IAEA once used SMR for "small and medium-sized reactors" (up to about 700 MWe). Today SMR almost always means "small modular reactor".

Issues, criticism and the way forward

• Cost: Big plants are cheaper per unit of power because of size (economies of scale). Small reactors lose this advantage. SMRs must win it back through factory mass production, but that needs many orders. In November 2023, NuScale and its US partner cancelled the first planned NuScale plant in Idaho after its expected power price rose to about $89 per megawatt-hour and not enough buyers signed up.
• Few working examples: Only a handful of SMRs run commercially anywhere in the world, so real costs and performance are still uncertain.
• Regulation: Regulators must approve new designs, smaller emergency zones and siting near industry. India's regulator must build this capacity, now under the SHANTI Act framework.
• Fuel and waste: Some advanced designs need fuel enriched more than normal (called HALEU, high-assay low-enriched uranium), which is in short supply. Many small units spread across the country also create more sites with spent fuel to manage.
• Security and public trust: More reactors close to towns and factories mean more sites to protect and more need for public acceptance.
• Way forward: Experts suggest standardising a few designs, building in fleets so costs fall, clear licensing rules for industry-owned captive reactors, and steady government support for early units.

Concepts to Know

• Fission: Splitting a heavy atom such as uranium into smaller atoms. This releases a large amount of heat.
• MWe (megawatt electric): A unit of electricity output. 1 GW = 1,000 MW.
• Captive power plant: A power plant that a company builds to supply electricity mainly to its own factory, instead of selling to the grid.
• Coolant: A liquid or gas that carries heat away from the reactor core, like the water in a car radiator.
• Pressurised water reactor (PWR): A reactor that uses ordinary water, kept under very high pressure so it does not boil, to carry heat away from the core.
• Emergency planning zone: The area around a nuclear plant where plans for warning, sheltering or moving people must be ready in case of an accident.
• Economies of scale: The idea that making something in large size or large numbers lowers the cost of each unit.