Design, Fabrication, and Testing of a High Temperature Foam Core Heat Exchanger for Small Modular Reactors

A helium-cooled small modular reactor (SMR) can take advantage of the Brayton power conversion cycle with greater efficiency compared to the steam Rankine cycle. Alternatively, a more advanced supercritical carbon dioxide (sCO2) Brayton system would have the added benefit that much less pumping power is required to convert a given thermal input to electricity compared to helium. Both helium and sCO2 are applicable to a wide variety of power generation applications including nuclear, geothermal, solar-thermal, fossil fuel, and shipboard propulsion. Refractory metallic foams operating at extremely high temperatures can increase heat transfer efficiency in gas-to-gas and liquid-to-gas heat exchangers by providing an extended surface area for better convection, i.e., conduction into the foam ligaments providing a “fin effect,” and by disruption of the thermal boundary layer near the hot wall and ligaments by turbulence promotion. These new heat exchangers have compact size and use advanced manufacturing techniques to avoid costly machining. The primary objective of this project is to establish the initial feasibility of a foam core heat exchanger for use in a 200-MWt small modular reactor operating with a helium Brayton cycle. In previous work, Ultramet and Sandia National Laboratories designed and constructed the components of a high temperature refractory regenerator (closed- loop recuperator). When using helium gas as the working fluid, modeling results indicated that the regenerator could operate with a maximum hot leg inlet temperature of 900°C and transfer 180 kW to the cold leg using 100 g/s helium at 4 MPa. Regenerator components were constructed using molybdenum open-cell foam and a solid molybdenum alloy outer shell per specifications defined by Sandia; however, the components were not fully assembled. In this project, Ultramet will complete assembly of the foam core regenerator and provide an initial demonstration of performance using helium as the working fluid. Oak Ridge National Laboratory will further refine the existing models and continue design optimization and scaleup toward small modular reactor applications. The proposed foam core heat exchanger is potentially applicable to a wide variety of power generation applications including nuclear (small modular, next-generation, conventional water-cooled, and fusion reactors), fossil fuel, waste heat, shipboard propulsion, and renewable heat sources such as solar-thermal or fuel cells. Higher cycle efficiency will lead to lower fuel cost, lower water usage, and in the case of fossil fuel heat sources, lower greenhouse gas emissions. For a nuclear power plant, a smaller plant footprint and smaller equipment sizes (compact heat exchangers and turbomachinery) are anticipated, resulting in lower capital cost.