Analysis Of Flow Instabilities In Supercritical Watercooled Nuclear Reactors

Abstract

Near the supercritical thermodynamic point, coolant density is very sensitive to temperature which gives potential to several instabilities in the supercritical water-cooled nuclear reactors. The flow stability features of the U.S. reference Supercritical Water- Cooled Reactor (SCWR) have been investigated. Single channel stability features were studied by the decay ratio calculations for Density Wave Oscillations (DWO). The system response matrix was developed through perturbation and linearization of the conservation equations in the time domain. Then, the DWO decay ratio was calculated from the dominant eigenvalue of the system response matrix. It was found that the U. S. reference SCWR will satisfy the stability criterion at steady state if an inlet orifice coefficient was properly chosen. Simplified stability maps that define the onset of DWO instability have also been constructed based on a frequency domain method for both the single channel and the channel-to-channel DWO. At supercritical pressure, a three-region model consisting of heavy fluid region, heavy-light fluid mixture region and light fluid region has been used. New non-dimensional governing parameters, namely, the Expansion Number and the Pseudo-Subcooling Number have been identified. It has been found that the U.S. reference SCWR will be stable at full power operating condition with large margin. Although the SCWR operates in the supercritical pressure region at steady state, operation at subcritical pressure will occur during a sliding pressure startup process. At subcritical pressure, the stability maps have been developed based on the traditional Subcooling Number and Phase Change Number (also called as Zuber Number). The sensitivity of stability boundaries due to different two phase flow models has been studied. It has been found that the Homogenous-Nonequilibrium model (HNEM) yields more conservative results at high subcooling numbers while the Homogenous Equilibrium (HEM) model is more conservative at low subcooling numbers. Based on these stability maps, a stable sliding pressure startup procedure has been suggested for the reference SCWR design.