Fuel assemblies are very expensive parts of the nuclear reactor. Initially they were used in Hungary for 3 years, now for 4 years and soon they will stay in the core for 5 years. Each year only 1/3rd, 1/4th later 1/5th of them is replaced, therefore the change of the fuel type is a lengthy process, with mixed cores used. The authorities require that the staff should be trained to each particular core before they operate it. For this reason the simulator should be upgraded to simulate the exact behavior of each core foreseen for the next 5 years. The RETINA code (Reactor Thermo-hydraulics Interactive) is a 3D offline code, developed in our department. KIKO3D - Neutron Kinetics 3D - has been developed in our Institute, too, in the Reactor Analysis Department. Both of them should be integrated into our full-scope replica simulator, coupled, and stressed to operate parallel in real-time, using four hi-power processors. The simulation-specific details are discussed in the paper.