Last Energy

Small modular reactors (SMRs): A beginner’s guide

The role of SMRs in a sustainable energy mix

Olivia Columbus

Despite their size – or, rather, because of their size – small modular reactors (SMRs) have a big role to play in the transition to clean energy.  

To fully understand the significance of this technology, we need to distinguish SMRs from the reactors that came before, as well as newer designs that both expand and complicate the future of nuclear energy.

SMRs: The First Ingredient in a Sustainable Energy Mix

SMRs serve a specific and powerful purpose in the energy industry.  

“Small” and “micro” describe a nuclear power plant’s size in terms of its power capacity. (Provincial Energy Strategy, p. 55). SMRs are smaller than traditional nuclear reactors, which means they’re easier to construct and, in theory, should be less expensive – but in the global energy system, they’re no less powerful.

Like other nuclear reactors, SMRs harness nuclear to generate heat that produces energy. Generally, SMRs have a power output of up to 300 MWe per plant: about one-third of the generating capacity of larger, conventional builds, which can produce around 1,000 MWe.  

Given that 500 MWe can power a small city, energy suppliers can use multiple SMRs to meet the needs of an energy customer, be it a community, industrial site, or country. The Last Energy reactor alone – a 20-megawatt SMR – can power 20,000 homes, and with far less carbon, construction, and land requirements than traditional reactors.

Nuclear Reactor Sizes Based on Output

A Moment of Modularity

Unlike the reactors before them, some SMRs, like Last Energy’s PWR-20, are truly modular, meaning that all of their systems and components are factory-assembled and easily transported as units to other locations for installation. With an SMR, you’re not building a reactor from the ground up: you’re simply assembling the existing pieces.

Compared to preconstructed reactors and partially modular SMRs, the PWR-20 can be assembled quickly on-site. The speed to delivery is less than 24 months, while traditional, non-modular reactors can take over five years to build – and sometimes, decades. Case in point: in the U.S., construction on the Vogtle nuclear plant began in 2012 with projected startup dates of 2016 and 2017. Yet in 2023, it’s still under construction and at least $16 billion over budget.  

...And a Better Baseload

SMRs reduce costs and construction timelines, and they also provide a carbon-free, always-on baseload energy: the minimum amount of electric power delivered or required over a given period of time at a steady rate. And compared to renewable energy sources, SMRs are consistent and dependable: they supply power around the clock, regardless of weather conditions.  

Naturally, energy derived from wind, sun, and water is dependent on the weather. While these renewable energy sources are intermittent and variable, they can pair with nuclear for a distributed energy mix which increases grid reliability.

When a renewable energy goes down, some SMRs can even perform “load following,” meaning they increase or decrease their power output to balance changes in the electricity demand. Currently, nuclear plants are usually operated in baseload mode and less frequently in load following mode, but SMRs present an opportunity to introduce more flexibility and stability – and far less carbon – to our electric grids.  

Smaller Than Small: Microreactors

Microreactors are smaller than SMRs, and they typically produce less than 50 MWe: enough to supply power for some smaller communities in remote locations. In the 1950s, microreactors were first developed for military submarines, which relied on nuclear propulsion.  

Beyond their aquatic applications, microreactors present a crucial opportunity to deploy energy quickly and affordably to remote communities, and to increase grid resiliency and safety in those locations. Microreactors can be created and distributed even more quickly than SMRs: within weeks, designers can deploy them to military bases, communities affected by natural disasters, and other remote locations or urgent situations.  

While microreactors are still regarded as a form of emerging technology, their design is evolving to meet the growing demand for stable, low-carbon energy. Globally, cyberattacks and natural disasters threaten the stability of electric grids; but microreactors are compact, fully-assembled, and ready for transportation to energy-deficient areas, especially in the event of a blackout or another urgent disruption.

The Landscape of Nuclear  

Proven and Operational

Advanced reactors are subject to a mix of science, economics, and politics, but two categories of light water reactors (LWRs) are widely tested and commercially proven: pressurized water reactors (PWRs) and boiling water reactors (BWRs).

In simplest terms, a PWR pressurizes water so that it heats, but doesn’t boil. The pressurized water carries heat to the steam generator, and the production of steam generates electricity by turning the turbine generator inside the PWR.

Compared to PWRs, BWRs take the extra step and boil the water. That water is converted to steam, then recycled back into water by the “condenser” component, to be used again in the heating process. BWRs are simpler than PWRs, with only a single circuit that pressurizes water until it boils. These reactors are common in the U.S., Japan, Sweden, and Taiwan, and makes up approximately 15% of the global supply of active reactors.

On the Horizon: Gen IV Nuclear  

In addition to SMRs, some nuclear scientists are developing alternative nuclear technology, often called Gen IV reactors. These reactors require more research, and most are only partially modular; still, this technological generation features some promising projects, including traveling wave reactors (TWRs).  

TWRs, like those designed by TerraPower, are uniquely designed to operate after startup using only natural or depleted uranium, which allows for higher fuel utilization and less uranium mining. In the long-term, this technology could enable even more cost savings and clean electricity production on a larger scale; but in the U.S. alone, the actual implementation of Gen IV reactors is still two to four decades away.

Unproven Nuclear Reactor Designs

Beyond PWRs and BWRs lies an unsteady ground of untested and unproven reactor designs.  

While many companies recognize the potential of nuclear, so-called “advanced” reactors don’t have an operating supply chain to model, and they’re largely based on unproven concepts introduced more than 50 years ago. Many of these designs use sodium or molten salt or gas for cooling, backed by claims of greater safety, affordability, and security compared to the LWRs in predominant use today.  

Rather than contend with the regulatory challenges of commercializing unproven, untested reactors, a 2021 report by the U.S. Union of Concerned Scientists recommended investing more research into the improvement and expansion of LWRs: ideally, ones that are actually small and actually modular.

Last Energy: A Case Study in Small Modularity

In keeping with this recommendation, Last Energy designed the PWR-20: a 20-megawatt SMR. It’s a modular, factory-built nuclear plant, and it employs the existing technology of PWRs, which are the most common type of nuclear reactor. On-site, all modules of the PWR-20 are quickly connected and commissioned, which eliminates on-site construction.  

Using only proven and operational technology, Last Energy relies on existing supply chains to deploy the PWR-20 design with speed and accuracy.  

Crucially, the Last Energy approach to nuclear energy is scalable to larger energy demands, simply by increasing the number of units. And with a power output of 20 MWe, the PWR-20 offers behind-the-meter solutions for industrial facilities as well as distributed baseload for grid applications.  

The size and modular building approach means Last Energy’s PWR-20 is under $100 million per unit, making it sized for private capital markets, and it can be delivered in approximately 24 months, quickly bringing 24/7 clean energy to industrial customers.  

SMRs: Small Potatoes or the Future of Nuclear?

Interest in SMRs is rising as countries recognize the need for a more flexible, powerful energy source with readily available parts and less complicated construction (Provincial Energy Strategy, p. 147). In some regions, such as the Province of Limburg in the Netherlands, scientists advocate for a sustainable energy mix of SMRs and “mini-SMRs,” or microreactors, with a unit power of 20-50 MWe each (p. 8).  

In consideration of the geotechnical conditions of Limburg, the report endorses a combination of SMRs and mini-SMRs based on proven LWR technology. In view of Limburg’s energetic needs and financial constraints, large plants are nearly impossible to fund during the 2030-2035 period, given the geotechnical conditions needed to cool water in large LWRs (p. 8).  

Although this report highlights the possibilities of nuclear in just one region of the world, it has far-reaching implications for the deployment of nuclear energy in other areas. Above all, it highlights the importance of size when assessing nuclear reactors, and how the appropriate combination of plants can sustain a rich, commercially competitive, and reliable mix of carbon-free energy.

Recapping the Rewards of SMRs

In the world of nuclear, smallness reigns supreme for several reasons:

  1. Due to their compact, modular design, the construction of SMRs is fast and efficient. Their components can be factory-built, like Last Energy’s PWR-20, which ultimately reduces cost overruns. This is a common concern when building reactors with larger builds, which often extend beyond the projected construction timeline.
  1. Using existing supply chains, SMRs can be easily distributed. If they’re factory-assembled, they can also be transported as units to various locations for installation.  
  1. Despite their small size, SMRs produce large amounts of low-carbon electricity. Consequently, many countries envision SMRs and microreactors as essential components of a carbon-free future.
  1. SMRs also have reduced fuel requirements: every 3 to 7 years, versus every 1 to 2 years for large, conventional plants. Some SMRs are designed to operate for 30 years without refueling.  
  1. With progressive designs like the PWR-20, SMRs and other commercially proven reactors have the potential to replace our dependence on renewables. In theory, PWR-20 plants can be built close to industrial facilities and supply round-the-clock-carbon-free power, or plugged into the grid when utilities need more clean baseload power.  

Fittingly, SMRs supply a succinct answer to the energy trilemma. They’re small, powerful, and their goals are targeted: reduce costs, support sustainability, and increase the security of our global energy reserves.