The Brayton Loop facility at Sandia National Laboratories (SNL) has been instrumental in identifying technology gaps and developing solutions critical for the commercialization of supercritical carbon dioxide (sCO2) power cycles. The benefits afforded by these new conversion technologies are required to fully leverage the potential from the next generation of nuclear reactors: increased efficiency, smaller footprint, and the ability to use dry cooling. While the recompression closed-loop Brayton Cycle (RCBC) test loop test program has been successful, its full potential has been stymied by the poor reliability of the turbine-alternator-compressor (TAC) bearings. While many of the early events were due to poor controllability issues, resulting from the first-generation power electronics, it demonstrated their lack of robustness necessary for these high-power density machines.

Despite contactless operation at speed, foil air bearings are in contact with the shaft at all speeds that are below that required for lift-off; for the foil bearings in the TAC it's around 8,000 RPM. Therefore, surface to surface contact occurs during start and shutdown, leading to wear. These unpreventable events contribute to loss of low friction surface treatment and are life limiting. While fully developed foil bearings are commercially applied to air cycle machines and small gas turbines (microturbines), rotors of low mass, this failure mechanism is amplified for large commercial sCO2 systems with heavy rotors and larger loads, rendering them unsuitable for future commercial small modular reactors (SMRs). It was therefore decided to identify and test more robust bearing solutions that would not only improve run time accumulation and facilitate performance and cycle configuration testing, but also demonstrate solutions which are scalable and better suited for large commercial systems.

As part of the TAC improvement program, and after the control system upgrade, the bearings were identified as the main technology to increase TAC reliability and efficiency. TACs currently employ aerodynamic foil type bearings. This report considers the retrofit design, acceptance testing, and the initial testing of a TAC retrofitted with externally pressurized porous (EPP) gas bearings. Both radial and axial constraint are required to support the rotor and react the axial thrust load. This was accomplished by replacing the compressor journal and thrust bearings with a single EPP bearing assembly, and a simple replacement of the journal on the turbine side.