Preliminary Studies of Na-CO2 Interaction Byproduct Cleaning Agent for SFR coupled to S-CO2 Power Cycle

Abstract

The Sodium-cooled Fast Reactor (SFR) with a traditional steam Rankine cycle was operated successfully in the past. However, the potential hazard from a sodium-water reaction (SWR) has posed a design challenge for a SFR. For this reason, the supercritical CO2 (S-CO2) Brayton cycle was suggested as an alternative power conversion system for a SFR to eliminate SWR. The studies on the S-CO2 Brayton cycle revealed that several advantages (i.e. relatively high thermal efficiency, compactness of turbomachinery and heat exchangers, and elimination of SWR) can be found for this design option. Despite these competitive features, the reaction between sodium and CO2 will take place when the pressure boundary fails. So far, there have been active studies on thermodynamic and kinetic features of Na-CO2 interaction. It is important to investigate Na-CO2 interaction with phenomenological approach. However, in order to improve safety and economics of a system, the study on the consequence of Na-CO2 interaction through a systematical approach is essential. Furthermore, if the CO2 flows into the sodium side due to the pressure boundary failure, the reaction products will be formed. Most of reaction products are in solid state, and sodium carbonate (Na2CO3) is the main reaction product from Na-CO2 interaction. If the flow channel is plugged or the system is contaminated by the solid reaction products, the economic losses will be caused by replacing the plugged channel or removing contaminants. Therefore, this study was performed to find a material which can clean up the reaction byproduct in solid state, mainly Na2CO3, created from Na-CO2 interaction. From a screening process, it was identified that three potential substances (i.e. sodium bromate (NaBrO3), sodium chlorate (NaClO3), and sodium tetrafluoroborate (NaBF4)) can act as a cleaning agent. For the screening process, several criteria were imposed: 1) melting occurs below 400ºC, 2) neither decomposing nor boiling below 600ºC, 3) no H or H2O in the compound, and 4) MSDS (Material Safety Data Sheet) should show that the material is not highly toxic or hazardous. As a result of TGA (Thermogravimetric Analysis) and DTA (Differential Thermal Analysis) methods, all of three sodium-based compounds decomposed before 600ºC and did not react with Na2CO3. Based on this study, the other potential substances will be identified from the extended candidate list, and the other methods to clean the solid reaction products will be investigated in the future too.