Small modular reactors could produce not just electricity, but also thermal energy for large-scale district heat networks. Sam Friggens considers the opportunities and challenges

SMRs may have a future in the district heating sector

In government you have to make tough choices,” UK Prime Minister David Cameron told Parliament recently.

In response to a question about the cancellation of the £1 billion ($1.4 billion) competition to demonstrate carbon capture and storage (CCS) in the UK, he said that the government would instead support innovation in other areas – namely energy storage and small modular reactors (SMRs) – to meet the nation’s carbon targets.

Alongside commitments from Chancellor George Osborne to allocate £250 million to nuclear research over the next five years and the launch of a new competition to select the best SMR design for the UK, the government is setting out a new vision for Britain’s future energy system.

SMRs are nuclear reactors with an electrical output below 300 MW. They have characteristics that distinguish them from large reactors, such as modular design with prefabrication in off-site factories and the potential for multiple reactors to be deployed at the same site to create bigger power plants.

Proponents claim they will be faster, cheaper, less risky to build, and safer to operate than large nuclear plants.

Right now there are no operational nuclear plants anywhere in the world that could be considered fully-fledged SMRs. But a number of countries and companies are at different stages in the development of SMR technologies. Two US developers – NuScale and Westinghouse – have recently expressed an interest in the UK market.

Low carbon heat

While SMRs could be focused exclusively on producing electricity for the grid in much the same way as large reactors, they could also operate as combined heat and power plants, providing not just electricity but also thermal energy to large-scale district heat networks.

Generating hot water centrally and supplying it to homes using networks of underground pipes is rare in the UK at present. It would represent a radical change in how the country gets its heat, but is a widely used solution in large cities in northern Europe, such as Copenhagen and Helsinki.

Some studies suggest this will be the most practical solution for achieving the near total elimination of carbon emissions from Britain’s building stock in cities, which is required by 2050 to meet the country’s climate change targets.

Most leading SMR designs are based on light water reactor technology. Large amounts of otherwise waste heat from the steam cycle can be extracted and supplied to district heat mains. This can be achieved with relatively minor changes to SMR plant steam cycles and only modest reductions in power output.

Large nuclear reactors could also perform this role, but for two reasons SMRs will be better placed. First, a district heat network fed by numerous smaller plants is more resilient in the face of outages than a network fed by a single large source. Second, due to their smaller size and lower water requirements, SMRs have the potential to be located by inland waterways within 10-20 km of urban centres.

New infrastructure

The scale of infrastructure required to support city-wide district heating is comparable with modern transport networks. Analysis of heat demand data across Britain shows that 50 cities are large enough to support heat networks fed by SMRs. This equates to roughly 20 GWe (40 GWt) SMR plant capacity, or 400 NuScale-sized modules.

Each city would require many kilometres of tunnels several metres in diameter to pump hot water into city centres and around its neighbourhoods. Buildings would need their boilers replaced with new heat exchangers and control systems.

Is such a vision realistic? The challenge of delivering such a transformation would be huge, both in terms of infrastructure delivery and end-user acceptance. But the alternatives – such as retrofitting millions of Victorian houses to Passivhaus standards of efficiency – are likely to be of comparable magnitude, and would still require additional power plant capacity to be built.

Independent assessments of future energy costs suggest SMRs may be more expensive than large nuclear plants or onshore wind when considering electricity only. But the additional revenue from heat sales could transform this picture.

Heat extraction will require only a modest uplift in plant capital costs. Extra revenues from heat supply could make SMRs competitive with other forms of generation, including other CHP technologies.

Nearly 20 GW of coal capacity supplied one-third of the UK’s electricity in 2015. The government’s current proposal is to close all of these plants within a decade.

Based on an assessment of the time required for detailed design, testing, licensing, first-of-a-kind demonstration and supply chain mobilization, a realistic development programme could see market-leading SMRs ready for mass deployment in the early 2030s, more than five years after the last coal plant closes. To meet these timescales, entering one or more SMR designs into the UK’s nuclear licensing process will need to be a near-term priority.

Modelling by the UK’s Energy Technologies Institute suggests that a fleet of SMRs could sit alongside large reactors and offshore wind farms in the future, contributing around one fifth of the UK’s electricity and one quarter of total space heat production by mid-century.

A global race

It doesn’t necessarily follow from these potential benefits that the UK government should invest in SMR technology development. After all, why not simply import at a later date from the US, China or elsewhere?

One reason would be to ensure that specific requirements such as flexible heat extraction are incorporated into SMR designs. Another would be to ensure the technology is ready for deployment when it is needed in the 2030s.

But for the Chancellor, who aims to “position the UK as a global leader in innovative nuclear technologies”, it is the potential economic gains from manufacture, export and jobs that appeal most.

Technology development is a high-risk endeavour, and there are fundamental challenges that will need to be overcome for gigawatt-scale deployment of SMRs in the UK.

Vendors need to prove their claims relating to cost reductions and safety can be achieved in practice.

Innovation in renewables, power storage, efficiency and smart technologies, driven by fast manufacturing cycles, is yielding rapid cost reductions and improving performance. By the time SMRs are ready for mass deployment in the 2030s, might their market have disappeared?

And, in any case, will SMRs be acceptable to the public? The closest-to-market SMR technologies produce the same waste as current large reactors and will need refueling every few years. New sites closer to demand may be attractive from an energy system perspective, but perhaps not to residents of the cities in question.

Overall, the challenges associated with SMR deployment are likely to be of similar magnitude to those faced by CCS. At the same time, recent work suggests that if these challenges can be overcome, then smaller, flexible nuclear technologies could play an important role in the UK’s future energy system. With the Chancellor’s £250 million to accelerate R&D and the worsening effects of climate change affecting public attitudes, the next five years will be interesting for SMRs.

Sam Friggens is an energy economist at Mott MacDonald and was joint author of the System Requirements for Alternative Nuclear Technologies Report (2015), commissioned by the Energy Technologies Institute.