Nuclear Power Plant projects are seen as big, expensive, and slow for a good reason, these megaprojects are massive undertakings requiring close collaboration among hundreds of contractors and thousands of individuals.
Enter Small Modular Reactors (SMRs), a class of nuclear reactors that promise lower capital needs, faster deployment, utilization of factory pre-fabrication, compact plant size, inherent safety features, and improved load-following capabilities. Governments, NGOs, and companies have raised SMRs as a crucial energy source to fuel and decarbonize our society. Thus, SMRs have been highly visible in headlines and in wider media. However, it is important to clarify, that even though SMRs are talked about unanimously, their designs vary significantly. So, let’s take a look at the SMR landscape which consists of over 100 unique SMR designs.
Generation III vs. Generation IV SMRs: What’s the Difference?
Gen III:
- Light Water Reactors (LWRs):
LWRs use water (H2O) as a coolant and moderator. Gen III LWR SMRs aim for easier deployment, modular construction, enhanced safety, and improved controllability allowing increased load-following capabilities. In short, the design is similar, scaled down, and simplified when compared to current conventional larger nuclear power plants, making licensing, construction, and operation more straightforward. The fuel is widely used Low-Enriched Uranium (LEU), and primary circuit temperatures vary from 300°C to 350°C with typical pressures ranging from 7-16MPa, however, there are heat-only reactors which operate at much lower temperatures and pressures, such as LDR-50, which operates at c.150°C and <10bar (<1MPa).
Pros: Proven and mature technology, reduced regulatory risk, operators are familiar with the technology, supply chains are well established, and fuel is available from multiple different suppliers.
Cons: Use cases limited by temperature (electricity, steam & hot water generation), high pressures cause mechanical stress and require heavier containment structures, heat-only reactors cannot produce electricity due to low efficiencies.
Gen IV:
- High-Temperature Gas-Cooled Reactors (HTGRs)
HTGRs typically use uranium as fuel, graphite as a moderator and helium as a coolant, where the Gas-Cooled name originates from. These reactors can reach significantly higher operating temperatures of 700°C-900°C, which is ideal for industrial heat.
Pros: High operating temperatures enable better electricity generation efficiency and high-temperature industrial heat applications.
Cons: Reactor design requires advanced materials to handle high temperatures, comparatively low quantity of global operational experience lead to lengthy licensing processes, limited availability of fuels (TRISO), size-wise they are larger than other SMRs (graphite is a worse moderator compared to e.g. light water), and large heat exchangers are needed due to gas being less effective heat transfer medium than liquids.
- Molten Salt Reactors (MSRs)
MSRs use molten salts as a coolant. The reactor can be either a burning or a breeding reactor, which allows the use of thorium as a fuel source in addition to uranium. The operational temperature is below 700°C, however, some concepts target higher temperatures.
Pros: A number of reactor design features, such as low operating pressures provide inherent passive safety features. In addition to uranium, MSRs can also utilize thorium as a fuel source which decreases proliferation risks and allows the use of otherwise underused thorium reserves.
Cons: Molten salts are highly corrosive, which increases with temperature, reactor chemistry is highly complex, and limited operational experience lengthens the licensing process. The fuel cycle for thorium is underdeveloped when compared to uranium causing notable lack of capacity for thorium nuclear fuels.
- Liquid Metal Reactor (LMRs)
LMRs use liquid metals, such as lead or sodium, as a coolant. LMRs can be adapted to be breeder reactors. The reactor operates at high temperatures; sodium at 500°C-550°C and lead at 480°C-800°C.
Pros: High thermal conductivity allows compact designs with good thermal margins, near atmospheric pressures reduces risks of pressure-related failures, fast neutron spectrum allows for breeding fuel, and high operating temperatures allow better efficiency in electricity generation and high-temperature industrial heat applications
Cons: Chemical reactivity and complexity (sodium), material challenges (lead and sodium are corrosive at high temperatures), limited availability of fuels (HALEU), limited long-term operational experience and lengthened licensing processes, as well as long development timelines.
Generation IV International Forum (GIF) also includes Supercritical-water-cooled reactor (SCWR) as a Gen IV design. In SCWR water is heated and pressurized above its critical point (374°C, 22.1MPa) allowing higher efficiency electricity generation, however, higher pressures also mean higher strain and requirements for the materials and components.
SMRs Beyond Electricity – Welcome the Nuclear Heat
Gen IV reactors have higher operating temperatures à Hydrogen production via high-temperature electrolysis or thermochemical cycles & industrial process heat & steam for steel, cement, chemical, and petroleum industries.
Gen III reactors, on the other hand, due to lower temperatures can be used for district heating, seawater desalination via evaporation, and industrial process steam for lower temperature use cases: such as food & beverage, pulp & paper, and pharmaceutical applications.
However, long-distance transportation of heat is economically challenging, thus, constructing the nuclear plant near the heat consumption becomes a necessity. SMR’s reduced footprint, improved safety features and renewing regulatory landscape make siting a nuclear plant closer to the demand more viable than before. As such, SMRs have significantly improved potential for utilizing the energy in the nuclear fuel, such as heat which current conventional large nuclear power plants waste by focusing only on electricity generation.
Realistic timelines
In general, Gen III designs are more mature, due to existing widespread usage and experience, and thus, are likely to be first implemented in commercial use. However, Gen IV designs are being developed and demonstrated which will inevitably lead to commercial adoption in hard-to-decarbonize use cases. Even though water-cooled reactors are much more common, it is important to realize that Gen IV coolants (gas, molten salts, and liquid metals) have been researched since the early days of nuclear research from the mid-1940s into the 50s and 60s. However, the fact is that nuclear energy industry is very conventional and risk-averse due to the potential consequences if work is not done properly. For that reason, regulation is one of the greatest contributors to the increasing safety and non-proliferation requirements. As Gen IV designs currently lack a similar widespread adoption that water-cooled reactors benefit from, it is very likely that regulators will place additional emphasis on certifying and checking the designs, suppliers, construction, and operation of such nuclear plants that utilize multiple different First-of-a-Kind technologies together.
Additionally, fuel supply can be an issue for some designs as current market supply for HALEU and TRISO fuels is somewhat limited. Investing in increasing the production of these fuel supplies requires certainty and clarity in market conditions and collaboration between fuel suppliers and fuel users. However, many SMR designs, such as the LDR-50, still utilize the same standardized LEU fuel which current Pressurized Water Reactors use, and thus, are not limited to fuel availability.
FOAKs and Pilot Projects
An important clarification is that few SMRs already exist: a Gen III PWR, KLT-40S in Russia, and a Gen IV Gas-cooled HTR-PM in China. Additionally, there are SMR projects under construction such as a Gen III iPWR CAREEM-25 in Argentina, ACP100 (PWR) in China, BWRX-300 (BWR) in Canada, and Gen IV demonstrator reactors such as BREST-OD-300 (Lead-cooled LMR) in Russia, and Hermes (Fluoride salt cooled MSR) in USA. Additionally, many research reactors at Universities and Research Institutions fulfill the definition of a SMR as do the reactors inside nuclear fueled ships such as submarines and aircraft carriers.
Some nuclear technology startups, such as Steady Energy and Blykalla, have decided to build a non-nuclear pilot plant to validate their simulations and test critical components and safety systems at scale. The data obtained from the pilot tests will be used to validate system codes such as APROS that model the reference facility so that accident and other behavior analysis conducted by the models are credible and properly captured. Additionally, constructing a pilot already requires significant project and supplier management expertise, and as such, can provide important learning experiences before a FOAK plant project.
In Steady Energy’s case, the LDR-e pilot project will be constructed in decommissioned Salmisaari B turbine hall in Helsinki, Finland. The LDR-e is a full-scale reactor vessel, which replaces the nuclear core with a powerful electric heater, which warms up the water inside the reactor vessel necessary for conducting the planned tests.
Conclusions: SMRs and the Future of Clean Energy
SMRs are defined by their smaller size, but there is considerable variety within the SMR designs; they vary by power output, temperature output, technology and fuel cycle.
Unfortunately, there is no single SMR which will solve all problems, thus, it makes sense to pick the best suited solution from the 100+ SMR designs for each own issue be it high temperature industrial heat decarbonization, low temperature district heating decarbonization, or electricity generation decarbonization and grid flexibility services. Therefore, it is more than likely that we will see multiple different SMRs in use in the upcoming decades.
Current trends indicate that selected SMRs will be built, operated, and commercially adopted into fleets. Achieving this future will require collaboration among all stakeholders.