A three-dimensional real-time nonlinear model is presented describing the transient situation of a pressurized water reactor nuclear power plant as dependent on both external control actions or disturbances and the inherent core dynamics. The plant has been assumed to consist of a three-dimensional core (subdivided into coarse-mesh boxes which, in turn, can be combined into superboxes), a natural circulation U-tube steam generator, and the main steam system (with safety, bypass and control valves, and a steam turbine). It can be disturbed from outside by a movement of control rod banks, an injection or dilution of soluble boric acid, and changes in the main coolant and feedwater mass flow, feedwater temperature, and, due to actions on the turbine control or turbine bypass valve, the secondary outlet steam mass flow. Restrictions were imposed by the requirement that the resulting code (named GARLIC) could be also operated either in parallel (i.e., in real time) or even in a predictive mode to the actual reactor process on a process computer (eventually in connection with a color display). These restrictions have been observed by replacing the diffusion term in the neutron kinetics equations by a combination of time-independent spatial coupling coefficients; calculating these coefficients from a comprehensive basic neutronic model; summing up the basic coarse-mesh elements into superboxes by homogenizing the corresponding local values and rebalancing the coupling coefficients over these superboxes; separating the neutron kinetics and thermodynamics and hydrodynamics part of the model, which could be treated in a pseudostationary way, from the nonlinear xenon-iodine dynamic part, then combining these decoupled parts in a recursive way. Taking advantage of the core symmetry properties, one obtains a nonlinear set of algebraic equations with a sparse power state matrix, which allows a further reduction in needed computer capacity when solving the resulting set of equations.