In general, thermal hydraulic modelling of nuclear reactor systems is based on the one­dimensional approach. Thermal hydraulic modelling of the steady state, transient and stability behaviour of two-phase natural circulation systems is no exception to this general approach. System codes have reached a highly developed modelling status and a wide acceptance. They can reproduce accurately enough most of the existing safety related steady state and transient experiments, so far as the dominant physical mechanisms are known, as adequate models are included in the codes, and so far as the dominant phenomena are also understood by the code users. Thus, these codes are an excellent tool to analyse in a parametric way the dynamics and interrelation of a larger number of components in complex systems with different physics involved. However, the use of multi-dimensional modelling of two-phase natural convection in large vessels (including the calandria vessel of pressure tube type heavy water reactors), sumps or plenums of nuclear reactors is essential as the flow in these cases are multi­dimensional in nature.

Some of the most commonly adopted thermal hydraulic models applicable for the 1-D and multi-dimensional analyses are briefly described below: