Thermal design and transient analysis of nitrogen Brayton cycle coupled with sodium-cooled fast reactor

Abstract

Sodium-cooled fast reactors (SFRs) are one of the most promising reactor types for near-term deployment among Generation IV nuclear systems. Although the steam Rankine cycle has been generally adopted as a power conversion system (PCS) of an SFR, the potential chemical reaction between sodium and water has been known to be a major safety issue and economic disadvantage. To this end, a nitrogen Brayton cycle has been considered as an alternative PCS option for SFRs because of the chemical stability of nitrogen with liquid sodium. In this study, based on the thermal design of the nitrogen Brayton cycle PCS coupled with an SFR (hereinafter abbreviated as “N2-PCS”), an off-design performance curve or map of the main components in N2-PCS are derived. Quasi-steady-state analyses and transient behaviors of N2-PCS are discussed based on data obtained from the results of system code simulation. To obtain the main component design parameters and off-design performance curve/map, one-dimensional codes of printed circuit heat exchangers and the design codes of axial-type turbomachinery are developed. The design results and derived performance curve/map are applied to a commercial system code (Flomaster) for transient analysis. The outlet temperature of the compressor obtained from a quasi-steady-state simulation is discovered to be higher than the target conditions because of the discrepancy between the results calculated using the system code based on the perfect gas model and the compressor design based on the real gas model. By applying the effect of the gas model difference on the compressor to a cooler component located at the compressor outlet, the temperature differences between the simulation results and target conditions are reduced. A simple power-swing transient scenario is applied to the N2-PCS system code. Finally, it is noteworthy that the transient results provide perspectives on the performance of N2-PCS during power swing.