As a part of Sodium-cooled Fast Reactor (SFR) development, the supercritical CO2 (S-CO2) Brayton cycle is considered as an alternative power conversion system to eliminate sodium-water reaction (SWR) from the current conventional steam Rankine cycle is utilized with SFR. The leakage flow of S-CO2 from turbo-machinery via seal becomes one of important issues since not only it influences the cycle efficiency due to parasitic loss but also it is important for evaluating the system safety under various operating conditions. Thus, a transient simulation for estimating the critical flow in a turbo-machinery seal is essential to predict the leakage flow rate and calculate the required total mass of working fluid in a S-CO2 power system.

This paper briefly reviews the advantages of supercritical CO2 Brayton cycle relative to a typical steam Rankine cycle used in the Sodium-cooled Fast Reactor. In addition, this paper describes the test data using a CO2 critical flow experimental facility with three orifice configurations to model the flow resistance of a rotating shaft labyrinth seal. This data is used for validation of an existing transient analytical tool developed for transient hydraulic system analysis. Contained within this code is the analysis approach of Henry/Fauske from 1971 for two phase critical flow of one-component mixtures. This paper presents prediction of transient pressure, temperature, and flow profiles for critical and sub-critical flows of CO2 relative to blowdown test data to show that reasonable results are obtained. Similar analyses relative to test results of three orifice configurations are conducted and it shows that multiple orifices increase the time to equalize pressure in the blowdown system and therefore equates to higher flow resistance and lower leakage.

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