(a) Homogeneous equilibrium model:

This is the simplest model for analysing the thermal hydraulic phenomena in a two-phase system. This considers one conservation equation each for the mass, energy and momentum of the mixture. Hence, it assumes complete thermal and mechanical equilibrium between the phases. This model is also referred to as Equal Velocity and Equal Temperature (EVET) model. Here, the mixture is considered to behave as a single fluid. An equation of state takes into account the variation of fluid properties. To take into account the differences in velocity between the phases, empirical slip models are used. The success of the model depends on the accuracy of the empirical relationships used in the code for the wall friction and wall heat transfer. Use of computer codes based on the homogeneous equilibrium model for accident and safety analysis of nuclear reactor systems is perhaps becoming a thing of the past. However, in the earlier design stages, codes based on this model are still extensively used. Further, most analyses to generate the stability behaviour (basically codes used to generate stability maps) of natural circulation systems are based on the homogeneous equilibrium model.