This thesis presents the work carried out to develop a simulation system for nuclear power plants, denominated TRETA / COPMA-III integrated simulator, which allows the simulation of the thermalhydraulic processes that take place in this type of facilities, in normal operation and emergency operation, as well as the control room crew actions related with the management of the emergency situations. The simulation of the thermalhydraulic processes is carried out by means of the TRETA (PWR) or TIZONA (BWR) simulators, both developed by the Spanish Nuclear Council (CSN). In what concerns to the simulation of the human performance, and taking into account the fact that in this type of facilities the management of the emergencies is strongly proceduralized, the COPMA-III procedures simulator is used. This simulator has been developed by the Halden Reactor Project (HRP), and it was adapted by the HRP development team for its use in the integrated simulator. This new tool is characterized mainly by its modular structure and its interconnection ability with other codes. In an individual way, the different codes that compose the simulator present advanced capacities in its models. Firstly, the TRETA simulator presents great versatility in defining the grade of complexity in the simulation of the processes. On the other hand, regarding to the COPMA-III simulator, it enables the automatic simulation of human actions proceduralized or planned, that means, all of those manual performances of which it is possible to develop a deterministic scheme, including aspects of timing and work load. Concerning the simulator package modular structure, it makes possible even the substitution of the process or procedures simulators and the implementation of any other simulator that it is considered more appropriate for specific necessities. This simulation system not only could be applied to validate the procedures design, but rather could be use in the verification of the consistency of the analyses made in the safety analysis. This last aspect is specially relevant, because these studies don’t include, except for seldom cases, the dynamic evaluation of the operator actions impact in the analysis of the accidental sequences.