Preliminary Studies of S-CO2 Critical Flow for Leak Modeling in Sodium-CO2 Heat Exchanger

Abstract

The supercritical CO2 (S-CO2) Brayton cycle has been receiving attention as an alternative power conversion system to the steam Rankine cycle for the SFR system. Even though a S-CO2 Brayton cycle can eliminate the sodium-water reaction, there is a potential reactive process between sodium and CO2 if the pressure boundary fails in the sodium-CO2 heat exchanger. The pressure boundary is an interface enduring a high pressure difference between sodium at 0.1 MPa and CO2 at 20MPa. Thus, when it fails, high- pressure CO2 will be injected into the sodium side to react with sodium. The amount of chemical reaction between sodium and CO2 will vary depending on several factors; the crack or rupture size, the interfacial area between sodium and CO2, the amount of released CO2, and so on. These factors are as influential as the reaction temperature of Na-CO2 interaction. To specify these factors, it is important to predict the CO2 leak mechanism during the CO2 leakage. However, only limited number of studies has been performed for understanding the CO2 leak mechanism. The system dynamic response with respect to Na-CO2 reaction was numerically simulated by assuming a double-ended guillotine break in a shell-and–tube type heat exchanger previously [1]. The modeling of the CO2-gas jet into water (before CO2-gas jet into sodium) has been investigated from both experiment and numerical analyses to obtain kinetic parameters of Na- CO2 reaction and understand the behavior of CO2 leak flow as a jet [2]. However, several limitations can be found from the previous studies. The assumptions such as maintaining steady conditions in the CO2 side or fixing the mass flux at the nozzle inlet at constant over the course of time are neither practical nor reasonable as the CO2 side conditions. Since the CO2 side conditions will change during the depressurization due to the leak, more realistic assumptions should be applied to the CO2 leak model. Before simulating the CO2 flow behavior close to the actual scenario, an isentropic critical flow model was numerically developed with several assumptions in this study. From this model, the variation of conditions of sodium and CO2 sides and the consequences of Na-CO2 interaction can be predicted in the future. The numerically obtained results can be used for evaluation of the consequences of Na-CO2 interaction.