Effect of the two phase flow models in the nuclear reactor single channel stability analysis

The single channel or parallel channel density wave oscillation (DWO) type flow instability is a well-known and important issue in the operation of Boiling Water Reactors (BWR), which needs to be avoided during the reactor design. The proper simulation of the two phase flow is very important in the BWR single channel thermal-hydraulic stability analysis. The supercritical fluids are widely proposed to be used in the Gen-IV reactor designs such as Supercritical Water Cooled Reactor (SCWR) and Gas Cooled Fast Reactor (GFR). For those supercritical fluids cooled Gen-IV reactor designs, although the coolant is in single phase supercritical pressure condition during steady state operation, the two phase flow in subcritical pressure will occur during some off-normal conditions. Therefore, the appropriate two phase flow models also need to be developed for the stability analysis of the Gen-IV reactor designs. To investigate the two phase flow modeling effects on the nuclear reactor single channel thermal-hydraulic stability analysis, four two phase flow models, namely, the Homogenous-Equilibrium model (HEM), the Homogenous-Nonequilibrium model (HNEM), the Nonhomogenous-Equilibrium model (NHEM) and the Nonhomogenous-NonEquilibrium model (NHNEM), are developed and applied to a typical SCWR hot channel. The neutral stability boundaries were derived using a linear model in the frequency domain. The stability boundaries are compared and plotted in the traditional Subcooling number versus Phase change number plane. It was found that the homogenous models predict more conservative stability boundaries than the nonhomogenous models, and the difference of the stability boundaries predicted by four two phase flow models is reduced in the higher pressure conditions. (author)