Flashing-Induced Instability Analysis and the Start-Up of Natural Circulation Boiling Water Reactors

To help achieve the necessary natural circulation flow, a fairly long chimney is installed in a natural circulation boiling water reactor (BWR) like the ESBWR. In such systems, hot water near the chimney exit could flash, thus leading to thermal-hydraulic instability during low-pressure start-up. A BWR stability analysis code in the frequency domain, named the Flashing-Induced STability Analysis for BWR (FISTAB), was developed in this work to address the issue of flashing-induced instability. The FISTAB code was benchmarked against the experimental results from the SIRIUS-N facility at the Central Research Institute of Electric Power Industry in Japan. Both stationary and perturbation results agreed well with the experimental observations.

The proposed ESBWR start-up procedure under natural convection conditions was examined by the FISTAB code. It was confirmed that the examined operating points along the ESBWR start-up trajectory from TRACG simulation would be stable. Furthermore, to avoid the instability resulting from the transition from single-phase natural circulation to two-phase circulation, a simple criterion was proposed for the natural convection BWR start-up when the steam dome pressure is still low.