Factsheet: Small Modular Reactors (SMR)

What is an SMR?

Small modular reactors (SMRs) are small nuclear power plants, also known as mini NPPs. They are designed to be largely prefabricated in a factory, assembled, and transported to the site ready for use. This process is expected to significantly reduce construction time, complexity, and costs.

• They require significantly less space than conventional nuclear power plants.
• Modularization enables the flexible adaptation of electricity production to actual demand.
• The safety systems and standards are similar to those of conventional NPPs.

How does a Small Modular Reactor work?

An SMR operates similarly to traditional nuclear reactors but on a smaller scale. The core contains nuclear fuel, typically uranium, which releases a significant amount of heat through nuclear fission. This heat converts water into steam, which drives a turbine connected to a generator, thus producing electricity. The technology is essentially the same as that of a conventional nuclear power plant.

How small are SMRs? Can they fit in a backyard?

SMRs are significantly smaller than conventional nuclear reactors. They can be prefabricated as large components in a factory and transported to the site. Their exact size varies depending on the design and power output, but they generally occupy only a fraction of the space required by conventional reactors. Let’s assume an SMR requires only about 0.01 square kilometers, that is less than two soccer pitches. However, they are not small enough to fit in a backyard.

For comparison: A conventional nuclear power plant requires about 1 to 2 square kilometers, that’s approximately 140 to 280 football pitches.

Is the output of an SMR comparable to that of a conventional nuclear power plant?

No, the output is not identical. Most SMR designs generate between 100 and 400 megawatts (MW) of energy. There are also plans for larger SMRs in the 400–500 MW range. For example, Rolls-Royce SMR is developing such a plant in partnership with Siemens Energy. In contrast, conventional nuclear power plants typically have a capacity of about one gigawatt (GW) nowadays, making SMRs about two to five times smaller in output.

Do SMRs produce nuclear waste?

SMRs produce nuclear waste. The amount and type of waste can vary depending on the specific design and technology of the SMR. The question whether SMRs produce less, similar, or more nuclear waste than conventional reactors cannot be fully answered today. Conclusive results will require the construction and operation of several SMRs.

Are there any operational SMRs?

Yes, there are two operational SMRs worldwide: China and Russia have built and commissioned SMRs, one SMR is under construction in Argentina. For example, China connected its pebble-bed modular high-temperature gas-cooled reactor (HTR-PM) to the grid in 2021. Additionally, many countries are researching SMR technology and plan to use SMRs for their future energy supply.

When are the first commercial-scale SMRs expected to be operational?

The European Industrial Alliance for SMRs aims to accelerate the development, demonstration, and deployment of SMRs in Europe by the early 2030s. In early 2026, Rolls-Royce SMR received an order from Great British Energy - Nuclear (GBE-N) to build three SMRs in Wylfa, Wales. In April 2026, the company signed a contract with the ČEZ Group to plan SMRs at the Temelín nuclear power plant in the Czech Republic.

What is a Generation 3+ reactor?

Nuclear reactors are categorized into different generations based on their design, technology, and safety features. Generation III+ reactors improve upon Generation III reactors, which have been in operation since the 1990s, by offering increased safety, efficiency, and operational lifetimes. These reactors have passive systems that function without human intervention or external power in emergencies.

What is Siemens Energy's contribution?

Although Siemens Energy is not directly involved in developing the reactors, its decades of experience in the conventional nuclear sector make it a natural supplier for such power plants. Siemens Energy's portfolio includes steam turbines, generators, and operational instrumentation and control systems that monitor the entire power plant.