Stability of supercritical water-cooled reactor during steady-state and sliding pressure start-up conditions

Full text of publication follows: The drastic change of fluid density in the reactor core of a supercritical water-cooled reactor (SCWR) gives rise to the concern of density wave stability. Using a single channel thermal-hydraulic model, the stability boundary maps of the U.S. reference SCWR design have been constructed for both the steady state and the sliding pressure start-up conditions. For the steady state, the supercritical water in the reactor core has been simulated using a three-region model - (1) a 'heavy fluid' with constant density, (2) a mixture of 'heavy fluid' and 'light fluid' similar to homogeneous-equilibrium two-phase mixture, and finally (3) a 'light fluid' which behaves like an ideal gas or superheated steam. The governing non-dimensional groups have been determined from a non-dimensional analysis of the conservation equations for this three-region supercritical water model. Two important non-dimensional groups, namely, Pseudo-Subcooling number (Npsub) and Expansion number (Nexp), which are similar to the Subcooling number (Nsub) and Phase Change number (Npch) commonly used in the analysis of the subcritical two-phase flow, have been identified for the supercritical region. The stability map in the supercritical region has been plotted in the Pseudo-Subcooling number versus Expansion number plane. It has been found that the U. S. reference SCWR design operates in the stable region with a big margin. Sensitivity studies have been performed with respect to the inlet orifice coefficient, system pressure and the inlet coolant velocity. It has been found that the stability boundary in the Npsub - Nexp plane is not sensitive to the system pressure and the inlet coolant velocity, but it is sensitive to the inlet orifice coefficient. This is consistent with the findings of the earlier research done for the subcritical two-phase flow. During the sliding pressure start-up operation of the SCWR, two-phase steam-water mixture at subcritical pressure will appear in the reactor core. A non-homogeneous (e. g., drift-flux) nonequilibrium two-phase flow model was applied. An exponential vapor generation rate for subcooled boiling was assumed along with a simplified but recent correlation for vapor drift velocity. The characteristic equation was numerically integrated, and the stability boundary map was plotted on the traditional Subcooling number versus Phase Change number plane. The effects of different two-phase flow models on the stability boundary have also been investigated. Thus, stability boundary maps have been constructed using four different models, namely, the Homogenous-Equilibrium model, the Homogenous-Nonequilibrium model, the Non-homogenous-Equilibrium model and the Non-homogenous-Non-Equilibrium model. It has been found that the Homogenous-Nonequilibrium model predicts the most conservative stability boundary at high Subcooling numbers, while the Homogenous-Equilibrium model yields the most conservative boundary at low Subcooling numbers. These maps are being used to develop the sliding pressure start-up strategies of the SCWR. Work is underway to expand the single channel model to a multi-channel model with neutronic feedback to study the local flow instabilities in a SCWR. (authors)