Numerical study on two-phase flow instability in multi-parallel channels of helical-coiled once-through steam generator of lead-cooled fast reactor

The helical-coiled once-through steam generator (H-OTSG) has the advantages of compact structure and strong heat transfer ability, and is appropriate for lead-cooled fast reactor. The two-phase flow instability may cause mechanical vibration and thermal fatigue of heat transfer tube bundles, posing a serious threat to the safe operation of the H-OTSG. In this work, the secondary side of the H-OTSG characterized as multi-parallel channels is modeled by RELAP5/MOD3.4 code, and the oscillation behavior during start-up and the influence of structural and operating parameters on system stability are studied based on time-domain method. The results indicate that the pressure, flow rate, and temperature of the secondary fluid exhibit density wave oscillations at the heating section in a (n-2,2) pattern. In addition, increasing inlet throttling, reducing outlet throttling, controlling subcooling within an appropriate range and avoiding operation under low loads are all beneficial for improving the system stability of the helical-coiled once-through steam generator.