In the recent past, the analysis of plant transients and the analysis of reactor core behaviour were performed separately. Usually, the core was represented by a point kinetics model to analyse plant transients and, for the core physics calculations, boundary conditions were imposed at the inlet and the outlet of the core. In reality, these boundary conditions depend on the power generation in the core. To ensure a realistic description of the physical phenomena in an accident analysis, the application of coupled codes is required. In recent years code developers began coupling three-dimensional (3-D) neutron kinetics codes with advanced thermal-hydraulics system codes. Such complex computer codes allow modelling of the entire reactor system, including a 3-D neutronics core. When reactivity initiated accidents with an asymmetric neutron flux distribution in the core are analysed, only such coupled codes are capable of estimating the real feedback effects. These codes can perform safety analyses in order to replace the conservative estimations with best-estimate calculations.