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What are Small Modular Reactors (SMRs)?


Small Modular Reactors (SMRs) are a type of nuclear fission reactor. Used for power generation, they are smaller than conventional nuclear reactors.


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SMRs are manufactured off-site and then delivered to location for final assembly. This results in less on-site construction, heightened nuclear security and increased containment efficiency. They are seen as a preferable approach to overcome the financial barriers that can inhibit the production of conventional nuclear reactors, and SMRs can offer a carbon free, clean energy alternative to fossil fuels, just as with conventional nuclear reactors. 

According to the UK government, there is a large variety of potential technologies within the nuclear sector. Owing to this variety, it is believed that the term SMR, as often understood and used, is too narrow a term to encompass all next generation nuclear opportunities. Instead, the UK government considers the term ‘Advanced Nuclear Technologies’ to be all encompassing.

All images are courtesy of Rolls Royce, lead partners in the Rolls-Royce SMR Consortium.

Small Modular Reactor Concept

Figure 1: Artist landscape impression of the UK Small Modular Reactor concept

Advanced Nuclear Technologies share common attributes:

  • smaller than conventional nuclear reactors
  • designed so that the majority of the reactor can be built at a factory and then transported to location for final assembly, reducing costs and risk

Advanced Nuclear Technologies generally fall in to one of two categories:

  • Generation 3 water-cooled Small Modular Reactor's; these are similar to existing reactors, but on a smaller scale
  • Generation 4 and beyond Advanced Modular Reactors (AMRs); these use innovative cooling systems or fuels to offer new functionality, such as higher operating temperatures, and step change reduction in costs

The UK government sees the advanced nuclear sector as playing an important part in the UK’s industrial growth strategy. This strategy builds on existing experience, economic strengths and competitive advantages. This will enable the UK to lead new advanced nuclear markets while contributing to tackling the Clean Growth Grand Challenge.

How Does a Small Modular Reactor Work (Fundamentals)?

These types of reactors currently use nuclear fission as the basis for producing energy. Nuclear fission is the process by which the nucleus of an atom splits into two or more smaller, lighter nuclei. The split atom releases large amounts of energy in the form of heat and radiation. This causes a chain reaction, which needs to be sustained to generate nuclear power.

Designs include thermal-neutron reactors and fast-neutron reactors. The difference between the two is the speed in which the neutrons flow. Thermal-neutron reactors rely on a moderator to slow the travelling speed of the neutrons and primarily use uranium as the fissile material. Fast-neutron reactors do not use moderators and rely on the nuclear fuel being able to absorb neutrons travelling at higher speeds. Typically, fast-neutron reactors use plutonium as fissile material. To date, most operating nuclear reactors use the thermal-neutron approach.



Figure 2: Reactor coolant system

Like conventional nuclear reactors, small modular reactors harness thermal energy to generate electrical power. For example, the thermal energy heats water into steam, which then powers a turbine, generating electrical power.


Figure 3: Concept of UK design 

The Reactor Island houses all of the nuclear systems. This includes the reactor core, steam generators and associated safety systems, all of which are within a separate containment structure.

What are the Advantages and Disadvantages?

The main advantage of SMRs is that they can be manufactured off-site. This can contribute to reduced build costs and increased containment efficiency. These are particularly useful for power generation in remote locations. SMRs, by design, need fewer staff for location assembly, maintenance and operation. Moreover, remote locations often have variable power generation needs. Large conventional reactors are generally inflexible in their power generation capabilities. In contrast, SMRs have greater control, generating lower amounts of electricity when demand is reduced.

The World Nuclear Association lists the unique features which include:

  • Small power and compact architecture and usually (at least for nuclear steam supply system and associated safety systems) employment of passive concepts. Therefore there is less reliance on active safety systems and additional pumps, as well as AC power for accident mitigation.
  • The compact architecture enables modularity of fabrication (in-factory), which can also facilitate implementation of higher quality standards.
  • Lower power leading to reduction of the source term as well as smaller radioactive inventory in a reactor (smaller reactors).
  • Potential for sub-grade (underground or underwater) location of the reactor unit providing more protection from natural (e.g. seismic or tsunami according to the location) or man-made (e.g. aircraft impact) hazards.
  • The modular design and small size lends itself to having multiple units on the same site.
  • Lower requirement for access to cooling water – therefore suitable for remote regions and for specific applications such as mining or desalination.
  • Ability to remove reactor module or in-situ decommissioning at the end of the lifetime.

Increasing Demand for Careers in the Nuclear Industry

SMRs potentially offer large opportunities for the manufacturing sector. The UK government believes that the UK can take an international lead in SMR technology development and deployment, with the domestic supply chain producing for a global market.

A report by the National Nuclear Laboratory (NNL) predicts a global SMR market of up to 85GWe with a potential value of £400 billion by 2035. By 2050, a full UK programme of up to 16 domestic SMRs could create up to 40,000 jobs and £52 billion of value to the UK economy. There is also the potential for the Rolls-Royce SMR export market to reach £250 billion.

SMRs creating jobs

How Much Does a Small Modular Reactor Cost?

These advanced reactors should be much more affordable to build than conventional power reactors. These types of reactors avoid the huge upfront costs associated with the long-term planning and lead times of conventional reactors. SMRs enable modular build of power generation systems. This allows distribution of build costs over a longer duration. An individual SMR could be built in four or five years. Once operational it will generate revenue to aid funding of additional modular units, if required.

Owing to SMRs being built in larger numbers in factories, manufacturers will be able to better implement processes typically used in industry to drive down costs, such as buying high-value components in bulk, which is not possible for  one-off, location-built large reactors.

Regarding energy output costs, initial cost models suggest that SMRs will not be significantly cheaper per unit of energy produced. A 2014 study led by the National Nuclear Laboratory gives a best estimate of over £80/MWh, which is comparable to the agreed strike price for Hinkley Point C. However, Rolls Royce is targeting a price of £60/MWh for its SMR designs.

Global SMR Developments

There are numerous SMR designs being generated around the globe, with the gross power generation ranging from 0.068 – 500 MWe. The table below details several SMRs, along with their gross power, producer and country of origin:

SMR name

Gross Power (MWe)







Rolls-Royce SMR


Rolls Royce

United Kingdom



OKBM Afrikantov




GE Hitachi Nuclear Energy

United States/Japan



OKBM Afrikantov








Kurchatov Institute


In March 2019 BEIS released a 2016 report on microreactors that defined them as having a capacity up to 100 MWt/30 MWe, and projecting a global market for around 570 units of an average 5 MWe by 2030, total 2850 MWe. It notes that they are generally not water-moderated or water cooled, but "use a compact reactor and heat exchange arrangement, frequently integrated in a single reactor vessel." Most are HTRs.

'Facts and Figures'

A Rolls-Royce SMR would produce 440MWe of electricity, which is enough to power a city the size of Leeds, charge 88,000,000 smartphones, light 40,000,000 lightbulbs, run 8,000,000 large televisions, or charge 62,857 electric cars.

Despite this considerable output, an SMR power station is small enough to fit inside Wembley stadium. With regards to cost, it is hoped that the levelised cost will be just £60 per MWh.

The move towards SMRs will also provide employment opportunities, with 40,000 skilled jobs expected to be created between 2030 and 2050, along with a countrywide £100bn boost to the economy. The SMRs should also tap into the £400bn global export market, which is mostly outside of the EU.

The growth of Rolls-Royce SMRs will reduce the nation's reliance on foreign gas imports while also helping the UK to deliver on the 2050 decarbonisation commitments, delivering carbon and emission-free electricity by 2030.

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UK Consortium Concept

TWI has a long history of assisting the nuclear industry, and is a trusted partner for developing SMRs. TWI recently entered a Rolls-Royce SMR consortium led by Rolls-Royce, with Assystem, BAM Nuttall, Laing O’Rourke, National Nuclear Laboratory (NNL), Atkins, Wood and the Nuclear AMRC. The consortium has received matching funding from UK Research & Innovation (UKRI) to develop domestic small modular nuclear reactors.

The UK consortium collectively brings together a tremendous depth of expertise. A convincing alternative. Rolls-Royce SMR technology envisaged by the Rolls-Royce-led UK consortium can produce nuclear power in a new way anywhere in the world. It solves the conundrum of how to create affordable energy, and more of it, with a lower carbon footprint. The global energy sector is facing increasing pressure to produce more power more quickly and in more places with more certainty of availability cost, capacity and flexibility and lower input costs and a smaller environmental impact. Satisfying both has called for a new approach. Our Rolls-Royce SMR concept is the answer. Good things come in smaller packages, and by re-thinking proven technologies into a different concept our Rolls-Royce SMR approach has been developed from the ground up based on the needs of energy utility companies and operators to provide the best possible support to their customers.


Using recognised nuclear standards and technologies strips out uncertainty from the licencing process – but also builds in certainty about the cost of creating a plant, the time it will take to create, and the cost of the electricity it will generate.


At every point in the development of the Rolls-Royce SMR solution, a modular approach has been taken to drive down the cost of electricity to as low as practically possible, whilst at the same time building in multiple layers of fault prevention and protection to make sure the technology is safe.


Rolls Royce are leading the largest national engineering collaboration the UK has ever seen, uniting some of the most respected and innovating engineering organisations on the planet. Each has a track record for successful delivery of large-scale and complex programmes.


What are SMRs used for?

Advanced SMRs can range in size from tens to hundreds of megawatts, offering them a range of potential uses including power generation, process heat, and desalination alongside other related industrial applications.

What are the Benefits of SMR?

SMRs offer a variety of benefits when compared to larger nuclear power plants. These advantages include enhanced safety, reduced cost of production, simplicity of design and improved flexibility for financing, siting, plant sizing, and end-use applications. In addition, extra modules can be easily added as energy demand increases.

Are SMR Reactors Safe?

SMR reactors are safer than conventional reactors as they contain improved passive safety features and more efficient containment, with fewer concerns over proliferation.

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