A numerical investigation of the sCO2 recompression cycle off-design behaviour, coupled to a sodium cooled fast reactor, for seasonal variation in the heat sink temperature

Abstract

Supercritical CO2 cycles are particularly attractive for Generation IV Sodium-Cooled Fast Reactors (SFRs) as they can be simple and compact, but still offer steam-cycle equivalent efficiency while also removing potential for Na/H2O reactions. However, CO2 thermophysical properties are very sensitive close to the critical point which raises, in particular, questions about the compressor and so cycle off-design behaviour when subject to inevitable temperature increases that result from seasonal variations in the heat sink temperature. This publication reports the numerical investigation of such an issue that has been performed using the Plant Dynamics Code (ANL, USA), the cycle being optimised for the next French SFR, ASTRID (1500MWth), as a test-case. On design, the net plant efficiency is 42.2% for a high pressure (25MPa) turbine with an inlet temperature of 515°C and considering a cycle low temperature of 35°C.The off-design cycle behaviour is studied based on preliminary designs for the main components and assuming the use of a fixed heat sink flow rate. First results obtained using a common fixed shaft speed for all turbomachines, without any other active control, show no stability issues and roughly constant density (and volumetric flow rate) at the main compressor inlet for the range of heat sink temperature considered (21–40°C). This occurs because the new stationary states are found without requiring a significant shift of mass to the higher pressure level, meaning the compressor inlet pressure rises in concert with temperature. A significant fall in the loop thermal power and efficiency is observed however, which analysis reveals to be caused by a fall in pressure ratio that is an inevitable result of the non-ideal nature of sCO2. Indeed the difference in the compressors off-design performance (the recompression cycle arrangement features 2 parallel compressors) is such that more mass-flow is attracted in the bypass line, which has a negative impact on cycle efficiency. A second series of results are taken for which the main compressor speed alone is controlled (between 50 and 56rev/s) and successfully maintains a constant thermal power across the sodium–CO2 heat exchanger. The resulting higher pressure ratio (compared to the fixed speed results) and greater flow rate through the main compressor also lead to higher cycle efficiencies that are close to the optimum achievable for a given heat sink temperature. The series of tests reveals that to achieve a constant thermal power and high efficiency with the sCO2 cycle at elevated heat sink temperatures, a degree-of-freedom in the compressor performance is necessary.