Small modular reactors (SMRs): Comparison of saturated versus superheated steam Rankine cycle under equal thermodynamic constraints

With part-load operation is increasingly becoming important issue for small modular reactors (SMRs), two secondary Rankine cycles for a 500 MWth pressurized-water-reactor small modular reactor were compared under identical thermodynamic constraints: a saturated-steam cycle with a U-tube steam generator and a superheated-steam cycle with a once-through steam generator were designed under identical primary-side conditions and the same steam-generator pinch-point temperature difference. Moreover, feedwater heaters satisfied fixed terminal temperature difference and drain-cooler approach limits. Part-load performance (25–100%) was evaluated using the Stodola-based off-design turbine model with wet-steam penalties and steam-generator pressure/temperature programs consistent with each generator type (i.e., constant-average-temperature for the U-tube steam generator; constant-secondary-pressure for the once-through steam generator). High-pressure turbine pressure ratio and the split between reheater bypass and third feedwater-heater extraction were optimized for maximizing net efficiency. Equipment sizing was assessed via total heat-transfer coefficient–area product. Both cycles achieved similar peak net efficiency. The superheated-steam cycle required lower steam mass flow and smaller total heat-transfer coefficient–area product (i.e., UA), indicating a compact power-conversion system. The saturated-steam cycle was slightly superior at full load, whereas the superheated-steam cycle maintained higher efficiency and smaller reheater and extraction-flow variations at part load. These results provide a constraint-matched benchmark for small modular reactor power conversion systems prioritizing compactness and part-load robustness.