Accurate computational simulations of three-dimensional steam-water flow behaviours in steam generators are indispensable in two respects: maintaining the integrity of the plant under normal conditions; and preventing the occurrence of plant abnormalities.

Steam Generator (SG) numerical simulation codes are composed of mass, momentum, and energy conservation equations and constitutive equations to match the number of two-phase flow parameters with the number of equations. Homogeneous-slip, drift-flux, and two-fluid models are available to formulate the mass, momentum, and energy conservation equations; In SG simulation codes, these equations are averaged over a control volume, which includes flow channel and heat transfer tubes: porous formulation.

The constitutive equations include correlations for the drift-flux model, interfacial area correlation, heat transfer coefficient, and drag coefficient; these equations should be developed depending on tube bundle arrays, such as square and triangular tube bundles. When a steam-water mixture flows in a riser section in the SG, the flow direction is parallel to the heat transfer tubes: internal flows. However, when the mixture flows in a U-tube section, the flow direction is no longer parallel to the heat transfer tubes: external flows. In developing constitutive equations, the dependence of two-phase flow behaviours on internal and external flows should be considered.

Developed SG simulation codes should be validated by experimental data obtained by facilities that simulate actual steam-water behaviours at operating pressure (7.6 MPa). SF6-ethanol system is a promising simulant system, because the density ratio of SF6-ethanol at the pressure of 0.68 MPa and temperature of 30 oC is equal to that of steam-water at the SG operation pressure. Cutting-edge measurement techniques, such as optical probe sensors, are used to measure local void fraction and gas-liquid interface velocity at several measuring locations in a simulated SG test facility.

Several SG codes have been developed as Electric Power Research Institute (EPRI) ATHOS, Electricite de France CAFCA based on the homogeneous-slip model, Mitsubishi Heavy Industries (MHI) Fluent-based code, Xian Jiao Tong University STAF based on the drift-flux model, EPRI PORTHOS, MHI Fluent-based code, Korea Atomic Energy Research Institute CUPID-SG, North China Electric Power University, THAC-SG based on the two-fluid model.

This article is contributed by Prof Takashi Hibiki, Chair Professor of Thermal-Fluid Engineering, City University of Hong Kong, with the coordination of the Nuclear Division.