Study of innovative Brayton cycle design and transient analysis for Sodium-cooled fast reactor application

Abstract

The closed Brayton cycle systems have been considered as a promising candidate to replace the steam Rankine cycle for a Sodium-cooled Fast Reactor (SFR) application while the competitive performance can be achieved while vigorous sodium reaction can be inherently substituted. This thesis investigates the perfor-mance and size comparison of the supercritical carbon dioxide ( $S-CO_2$ ), helium and nitrogen cycles for the 150MWe SFR system. By the virtue of high efficiency and compactness, the $S-CO_2$ cycle is selected and the recompression layout is confirmed to be the most efficient among numerous candidates which includes several recuperation, inter-cooling and re-heating processes with optional flow split cases. A comparison of single shaft and triple shaft turbomachinery configurations is further conducted under the part load operating conditions. The thermal efficiencies of single shaft and triple shaft $S-CO_2$ cycles are 43.0% and 43.6% respectively, which is slightly higher than the steam Rankine cycle performance. Based on the previous works on the $S-CO_2$ test facilities from various research institutions, Supercritical $CO_2$ Integral Experiment Loop (SCIEL) is designed to achieve higher efficiency with higher pressure ratio with $S-CO_2$ power cycle compared to other pre-existing facilities. The underlying design principles of the integral experiment facility is discussed and the preliminary compressor experiment data are compared to the system analysis code results. To assess the part load performance with respect to inventory, bypass valve and turbine throttle valve control schemes, a quasi-steady analysis is conducted. Based on the preliminary heat exchanger and turbomachinery design, the off-design performance can be estimated. A system analysis code, GAMMA+ code, is utilized to verify the quasi-steady state result with the inventory control scheme. While the inventory is controlled in the $S-CO_2$ cycle, the main compressor condition must be carefully checked as the thermody-namic properties change rapidly.