These models consider either the thermodynamic or mechanical non-equilibrium between the phases. The number of conservation equations in this case are either four or five. One of the most popular models which considers the mechanical non-equilibrium is the drift flux model. If thermal non-equilibrium between the phases is considered, constitutive laws for interfacial area and evaporation/condensation at the interface must be included. In this case, the number of conservation equations is five, and if thermodynamic equilibrium is assumed the number of equations can be four. Well-assessed models for drift velocity and distribution parameter depending on the flow regimes are required for this model in addition to the heat transfer and pressure drop relationships. The main advantage of the drift flux model is that it simplifies the numerical computation of the momentum equation in comparison to the multi-fluid models. Computer codes based on the four or five equation models are still used for safety and accident analyses in many countries. These models are also found to be useful in the analysis of the stability behaviour of BWRs belonging to both forced and natural circulation type.

(c ) Two-fluid models:

The two-fluid models take into account the thermodynamic and mechanical non-equilibrium between the liquid and vapour phases. In this case, the mass, energy and momentum balance equations are written separately for both the phases. The six conservation equations in the two-fluid model represent the propagation of phasic void, velocity, enthalpy and pressure disturbances. For closure, the conservation equations require constitutive equations for the interfacial and wall transfer terms. The most difficult task is the modelling of these constitutive relationships. In spite of several years of research, the interfacial constitutive laws are not yet well established. A classical approach is to define them based on flow regime maps used for identifying the flow patterns (bubbly, slug, churn, annular, etc.). The interfacial transfer relationships (mass, momentum and energy) can be calculated based on the flow pattern dependent relationships, which are selected by suitable flow pattern transition criteria. Most of the large system codes used for safety and accident analysis, for example RELAP5, CATHARE, ATHLET, etc. adopt the one-dimensional two-fluid model. The two-fluid model is not yet commonly used for the stability analysis of nuclear reactor systems.