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The Office of Nuclear Energy (NE) seeks to establish a commercial-scale recompression, closed Brayton cycle for optimizing the conversion of heat from a small modular reactor into electricity. Investments toward this goal leverage the most promising capabilities within private industry and the Department of Energy’s national labs to achieve relevant, near-term industry objectives and longer-term NE objectives.

Applications for Nuclear Plants:

The primary cycle of interest for application in small modular reactors (SMRs) is the recompression cycle, named for the flow split in which only a portion of the cycle fluid rejects heat while the other portion is recompressed.  This cycle efficiently converts SMR-generated heat to electricity under either wet or dry heat rejection conditions, thus allowing the combined power plant to operate efficiently—virtually anywhere in the world.

Process flow diagram for indirect-fired Brayton cycles for coal and natural gas.
Photo: SNL

Advanced Reactors (envisioned as <300 MWe SMRs) have three general classes:

  1. Sodium Fast Reactors: This reactor uses the metal sodium as the reactor coolant, operating at 550C. Development of this reactor is the principal objective of DOE-NE.
  2. High Temperature Helium Reactors: Using helium as the reactor coolant enables operating temperatures of >650C, improves safety, and reduces nuclear waste.
  3. Molten Salt Reactors: Molten salt as a reactor coolant allows for temperatures up to 800C.

Benefits, R&D, & Goals for Nuclear Energy:


  • Intelligent thermodynamics for optimal efficiency
  • Significantly reduced fuel usage
  • Capability for greatly reduced water usage
  • Very large range of scalability
  • Compact system reducing costs and footprint
  • Demonstrate performance at a commercial scale (currently moving a 550C lab-scale recompression closed Brayton cycle to the 10 MWe demonstration scale).
  • Develop structural materials, qualified seals, and bearings; solve maintenance issues; develop effective and cost-effective heat exchangers; study erosion and corrosion mechanisms.
  • Bearings and seals: Develop for turbomachinery operating in a high-temperature CO2 environment.
  • High-temperature turbines: Continue to advance technology readiness level by demonstrating proof of performance, reliability, and durability.
  • sCO2 compressors: Research parallel operation of two compressors, with one acting on a fluid very close to its critical point; develop designs and procedures to maintain stability in this challenging design.
  • Turbomachine rotor shafts: Develop designs and procedures to minimize asymmetric thrusting forces while minimizing windage losses.
  • Advanced materials: Test for erosion and corrosion in high-temperature, high-pressure CO2 environments.
  • Working fluid additives: Assess their effects on performance factors under various operating conditions.
  • CO2-to CO2 and CO2-to-other fluid heat exchangers: Research designs and manufacturing techniques to improve efficiency and reliability while lowering cost.


  • Develop operational procedures for parallel compression near the CO2 critical point.
  • Develop system operational procedures for optimal performance under various conditions.
  • Collaborate with industry to develop seals and bearings.
  • Collaborate with industry to optimize heat exchanger designs for performance and cost.
  • Develop the economic models for these systems.