Thermal-Hydraulic Simulation of Supercritical-Water-Cooled Reactors

Abstract

At supercritical pressures the distinction between the liquid and gas phases disappears, and any fluid stays in a single continuous phase: no evaporation or condensation is observed. At supercritical state the thermo-physical properties of the fluid, such as density and viscosity, change smoothly from those of a liquid-like fluid to those of a gas-like fluid as the fluid is heated. Because of the single-phase nature, a one-phase model would be ideal for thermalhydraulic simulation above the critical pressure. However, in the nuclear power plant applications the one-phase model is not sufficient, because in transient and accident scenarios, the pressure may drop below the critical pressure, turning the coolant abruptly from a one-phase fluid into a two-phase mixture. Therefore the thermal-hydraulic model has to be able to reliably simulate not only supercritical pressure flows but also flows in the two-phase conditions, and thus the six-equation model has to be used. When the six-equation model is applied to supercritical-pressure calculation, the questions how the model behaves near and above the critical pressure, and how the phase transition through the supercritical-pressure region is handled, are inevitably encountered. Above the critical pressure the latent heat of evaporation disappears and the whole concept of phase change is no longer meaningful. The set of constitutive equations needed in the six-equation solution including friction and heat transfer correlations, has been developed separately for both phases. The capability of constitutive equations, and the way how they are used above the supercritical pressure point, have to be carefully examined. In this article, the thermal hydraulic simulation model, which has been implemented in the system code APROS, is presented and discussed. Test cases, which prove the validity of the model, are depicted. Finally, the HPLWR concept is used as a pilot simulation case and selected simulation results are presented.