AbstractAs a central part of an interactive colour-display core-surveillance system for giving an operator a better insight into the present and future operational status of a PWR, a 3-D real-time nonlinear digital core and plant simulator GARLIC has been developed.The simulated PWR is assumed to consist of a core, a natural circulation U-tube steam generator and a main steam system. By computing the distribution of important local core parameters (power, xenon and iodine densities, fuel rod and coolant temperatures etc.) and the behaviour of global plant parameters (system pressure, water level, inlet and outlet temperatures) as a function of both inherent core dynamics and external actions or disturbances during long range (quasistationary) transients, GARLIC can be used in a predictive mode. The results of transient computations are stored in a database and displayed on colour terminals using an interactive system.The main assumptions and restrictions used to develop the theoretical model as well as the main characteristics, properties and requirements of the code are reviewed and results of transient runs on a process computer are presented.