Small modular reactors, explained
Small modular reactors promise cheaper, faster nuclear power built in factories rather than on site, but the technology still has significant hurdles to clear before it can deliver on that promise.
What makes a reactor “small” and “modular”
A small modular reactor, or SMR, is a nuclear power plant designed to generate a fraction of the electricity produced by a conventional large reactor, typically in the range of tens to a few hundred megawatts rather than the gigawatt-scale output of stations such as Hinkley Point C. The “modular” part refers to how they are built: instead of pouring concrete and welding steel largely on site over many years, the core components are manufactured in a factory as standardised modules, then transported and assembled at the power station location.
The logic is similar to prefabricated construction in other industries. Building repeatable units in a controlled factory environment should, in theory, reduce the construction delays, cost overruns and bespoke engineering problems that have plagued large nuclear projects worldwide. Standard designs can also be replicated across multiple sites, spreading the cost of design and regulatory approval rather than reinventing the plant each time.
SMRs are not a single technology. Some designs are essentially scaled-down versions of the pressurised water reactors used in existing nuclear fleets and submarines, using well-understood physics and materials. Others are more experimental, including designs cooled by molten salt, liquid metal or gas rather than water, which their developers argue could offer improved safety characteristics or the ability to run at higher temperatures for industrial heat applications.
Why governments and investors are interested
The appeal of SMRs rests on several linked arguments. Because the reactors are smaller, the upfront capital required for a single unit is lower than for a giant plant, which can make projects easier to finance and reduce the financial risk if something goes wrong during construction. Factory-based manufacturing should, in principle, make build times shorter and more predictable, addressing one of the biggest criticisms of nuclear power: that large projects routinely run years over schedule and far over budget.
Smaller reactors can also be sited in locations that would struggle to accommodate a gigawatt-scale plant, including former coal or industrial sites with existing grid connections, or alongside energy-intensive facilities such as data centres or hydrogen production plants that need a reliable, low-carbon power source. Some designs are being explored for use in heavy industry, providing heat as well as electricity, or in remote locations not well served by the wider grid.
For countries trying to decarbonise while maintaining energy security, nuclear offers a low-carbon source of power that, unlike wind and solar, does not depend on the weather. SMRs are seen by many governments as a way to keep nuclear in the energy mix without committing to the enormous single projects that have proven difficult to finance and deliver in recent decades.
The obstacles that remain
Despite the promise, no SMR design has yet been deployed commercially at scale in the way its advocates envisage, and the technology faces genuine uncertainties. Regulators in most countries, including the UK’s independent nuclear regulator, must assess each design against rigorous safety standards, a process that takes years regardless of the reactor’s size. Because SMRs use a range of different technologies, some regulators face the challenge of assessing designs that fall outside the established body of experience with conventional water-cooled reactors.
The economics are also unproven in practice. The theoretical savings from factory production only materialise if a design is built repeatedly, spreading fixed costs across many units. The first few reactors of any given design are likely to be more expensive, since manufacturers have not yet achieved the economies of scale that make the model attractive. Supply chains for specialist nuclear-grade components and fuel, along with a skilled workforce, will also need to be built up or expanded.
Finally, questions remain about waste management and decommissioning, which still apply to smaller reactors even if the volumes involved differ from large plants. Public and political support, planning consent, and grid connection all add further layers of complexity that no reactor design can bypass.
Where the UK fits in
The UK has identified SMRs as part of its strategy to expand nuclear capacity and support energy security, with government-backed competitions and support schemes aimed at helping designs progress through regulatory approval and towards construction. Several UK and international developers are pursuing different reactor concepts, reflecting the broader global picture: the technology is advancing, but the first commercially operating SMRs, and the evidence of whether they truly deliver on cost and schedule, are still some years away.