Types of Small Modular Reactors (SMRs)

Small Modular Reactors (SMRs) represent a new generation of nuclear reactors that are smaller in size, designed for modular construction, and offer flexibility in deployment. Below are the main types of SMRs, categorized by their design and cooling technologies:

1. Pressurized Water Reactors (PWRs)

Pressurized Water Reactors (PWRs) are one of the most common types of SMRs and are based on the design of conventional large reactors. These reactors use ordinary water as both a coolant and a moderator. The water is kept under high pressure to prevent it from boiling, allowing for efficient heat transfer to a secondary loop where steam is generated to drive turbines.

  • Examples:
    • NuScale Power Module: A well-known example of a PWR-based SMR. It’s designed to be scalable and uses a passive safety system.
    • SMART (System-integrated Modular Advanced Reactor): Developed in South Korea, it uses advanced passive safety features and is designed for power generation, desalination, and industrial heat.

Key Features:

  • High familiarity: Builds on decades of PWR operational experience.
  • Proven technology: Most conventional nuclear reactors are based on this design.
  • Applications: Power generation, desalination, and industrial use.

2. Molten Salt Reactors (MSRs)

Molten Salt Reactors (MSRs) use molten salt as both the coolant and the fuel medium. In some designs, the nuclear fuel is dissolved directly in the molten salt, allowing for continuous fuel cycling. MSRs are known for their high thermal efficiency and can operate at much higher temperatures compared to traditional water-cooled reactors.

  • Examples:
    • Terrestrial Energy’s IMSR (Integral Molten Salt Reactor): Designed for high efficiency and operational flexibility.
    • ThorCon: A Thorium-based MSR designed for mass production and deployment in developing nations.

Key Features:

  • Fuel Flexibility: Can operate on a variety of fuels, including Thorium.
  • High Efficiency: Capable of operating at much higher temperatures, which improves power generation efficiency.
  • Inherent Safety: Passive safety features, including the ability to drain the molten salt into a safe configuration in the event of overheating.

3. Fast Neutron Reactors (FNRs)

Fast Neutron Reactors (FNRs), also known as Fast Breeder Reactors, do not require a moderator to slow down neutrons, allowing them to use fast neutrons to sustain the nuclear reaction. FNRs can breed fuel by converting fertile isotopes like Uranium-238 into fissile materials like Plutonium-239, making them highly efficient in fuel usage.

  • Examples:
    • PRISM (Power Reactor Innovative Small Module): Developed by GE Hitachi, designed to recycle nuclear waste by using fast neutrons to burn actinides.
    • BREST-OD-300: A Russian lead-cooled fast reactor designed to close the nuclear fuel cycle by breeding fuel from depleted Uranium.

Key Features:

  • Fuel Recycling: Capable of breeding more fissile material than they consume.
  • Long-Term Sustainability: Can use depleted Uranium and spent fuel, reducing nuclear waste.
  • High Efficiency: Fast neutrons allow for a more efficient nuclear reaction and higher fuel utilization.

4. High-Temperature Gas-Cooled Reactors (HTGRs)

High-Temperature Gas-Cooled Reactors (HTGRs) use a gas, typically helium, as the coolant and graphite as the moderator. HTGRs can operate at extremely high temperatures (up to 1,000°C), making them ideal for both electricity generation and industrial applications that require high-temperature heat, such as hydrogen production.

  • Examples:
    • X-Energy’s Xe-100: A pebble-bed HTGR designed for modular deployment and capable of producing electricity and process heat.
    • HTR-PM: A Chinese demonstration project based on the pebble-bed design, aimed at both electricity production and industrial applications.

Key Features:

  • High Operating Temperatures: Suitable for industrial applications like hydrogen production or desalination.
  • Passive Safety: Helium is chemically inert, and the reactor design includes natural heat dissipation mechanisms.
  • Fuel Form: Uses TRISO fuel, which encases uranium particles in multiple layers of ceramic material, providing a high level of containment and safety.

5. Lead-Cooled Fast Reactors (LFRs)

Lead-Cooled Fast Reactors (LFRs) use liquid lead or lead-bismuth eutectic as the coolant. LFRs are fast reactors, which means they operate using fast neutrons and do not require a moderator. The use of liquid lead as a coolant allows for high operating temperatures and passive safety mechanisms, as the coolant has a high boiling point and provides radiation shielding.

  • Examples:
    • SEALER (Swedish Advanced Lead Reactor): Designed for off-grid communities and for long-term, low-maintenance operation.
    • BREST-OD-300: A lead-cooled fast reactor developed in Russia for fuel breeding and high efficiency.

Key Features:

  • Corrosion Resistance: Lead acts as both a coolant and radiation shield, but reactor materials need to be highly resistant to corrosion from lead.
  • High Efficiency: Can operate at higher temperatures than water-cooled reactors, improving efficiency.
  • Fuel Recycling: Capable of using various fuel types, including nuclear waste and depleted Uranium.

6. Gas-Cooled Fast Reactors (GFRs)

Gas-Cooled Fast Reactors (GFRs) use fast neutrons for the fission process and a gas, typically helium, as the coolant. They are designed to operate at very high temperatures, similar to HTGRs, and are capable of breeding fuel from fertile materials like Uranium-238 or Thorium. GFRs are intended to close the nuclear fuel cycle by recycling spent fuel.

  • Examples:
    • ALLEGRO: A European project aimed at demonstrating the viability of GFR technology for both electricity generation and fuel breeding.

Key Features:

  • High-Temperature Operation: Can achieve high thermal efficiencies and provide process heat for industrial applications.
  • Fuel Breeding: Capable of converting fertile material into fissile fuel, contributing to long-term fuel sustainability.
  • Inherent Safety: Passive safety systems relying on natural circulation and heat dissipation.

7. Micro Modular Reactors (MMRs)

Micro Modular Reactors (MMRs) are small, factory-built reactors designed for off-grid or remote applications. These reactors are compact, simple to deploy, and designed for long-term, low-maintenance operation. MMRs can provide power to isolated communities, mining operations, or military installations where traditional large-scale reactors are impractical.

  • Examples:
    • Ultra Safe Nuclear Corporation (USNC) MMR: A small reactor designed to provide power for remote industrial operations and small communities with minimal maintenance.

Key Features:

  • Small Size: Designed for remote or off-grid applications where traditional large reactors are not feasible.
  • Low Maintenance: Built for extended operation with minimal human intervention, often for 20 years or more.
  • Modular Design: Factory-built and transportable, making deployment quick and scalable.

8. Sodium-Cooled Fast Reactors (SFRs)

Sodium-Cooled Fast Reactors (SFRs) use liquid sodium as a coolant and operate with fast neutrons, without the need for a neutron moderator. SFRs can achieve high levels of fuel efficiency by breeding more fissile material from fertile isotopes like Uranium-238 or Thorium-232.

  • Examples:
    • TerraPower’s Natrium Reactor: A sodium-cooled fast reactor that integrates advanced energy storage technology.
    • BN-800: A Russian fast reactor that uses sodium as a coolant and is designed to close the nuclear fuel cycle by recycling spent fuel.

Key Features:

  • Fuel Recycling: SFRs can breed fuel from depleted Uranium or Thorium, extending fuel supplies.
  • High Operating Temperatures: Sodium allows for high thermal efficiency, but also presents challenges due to its reactivity with water and air.
  • Passive Safety: The high heat capacity of liquid sodium allows for passive heat removal in case of an emergency.

Conclusion

The wide variety of Small Modular Reactors (SMRs) offers the potential to revolutionize the nuclear energy landscape by providing more flexible, scalable, and safer options than traditional large-scale reactors. From Pressurized Water Reactors (PWRs) to Molten Salt Reactors (MSRs) and Lead-Cooled Fast Reactors (LFRs), each design has unique benefits and challenges, making SMRs suitable for a broad range of applications—from large urban power grids to remote and industrial sites. With ongoing research and development, SMRs are set to play a significant role in the future of clean, efficient, and reliable nuclear energy.