The drastic change of fluid density in the reactor core of a supercritical water-cooled reactor (SCWR) gives rise to a concern about density-wave stability. Using a single-channel thermal-hydraulic model, stability boundary maps for the U.S. reference SCWR design have been constructed for both steady state and sliding pressure startup conditions.

The supercritical water flow in the reactor core has been simulated using a three-region model: a heavy fluid with constant density, a mixture of heavy fluid and light fluid similar to a homogeneous-equilibrium two-phase mixture, and a light fluid, which behaves like an ideal gas or superheated steam. Two important nondimensional numbers, namely, a pseudosubcooling number Npsub and an expansion number Nexp, have been identified for the supercritical region. The stability map in the supercritical region is then plotted in the plane made of these two numbers. The U.S. reference SCWR design operates in a stable region with a large margin. Sensitivity studies produced results consistent with the findings of the earlier research done for the subcritical two-phase flow.

During the sliding pressure startup of the SCWR, a two-phase steam-water mixture at subcritical pressure will appear in the reactor core. A nonhomogeneous (e.g., drift-flux) nonequilibrium two-phase flow model was applied. The characteristic equation was numerically integrated, and stability boundary maps were plotted on the traditional subcooling number versus phase change number (or Zuber number) plane. These maps have been used to develop a sliding pressure SCWR startup strategy avoiding thermal-hydraulic flow instabilities.