The present Master’s Thesis is part of the Master’s degree in Nuclear Engineering of the Universitat Politècnica de Catalunya and the ENDESA Escuela de Energía, and it was developed during the internship in a Spanish Pressurized Water Reactor (PWR). The objective of the project is to update and test the nuclear plant model used for the Safety Analysis department which belongs to the Licensing Department mainly for Severe Accidents phenomenology studies to prepare for and respond to emergencies. The code used for this porpoises is the Modular Accident Analysis Program (MAAP), actually developed by the Electric Power Research Institute (EPRI). MAAP simulates the response of Light Water Reactors (LWR) during severe accidents; given a set of initiating events and operator actions,MAAP predicts the plant’s response as the accident progresses. The project includes a brief description of the severe accident phenomenology and an explanation of the MAAP code, including a summary of the reactor coolant primary system model, the containment systems and the Engineered Safeguards contemplated in the Reference Plant MAAP5 model. The latest version of the code that has been used to model the PWR Reference Plant is MAAP4.0.6 and several accidents have been simulated for the Probabilistic Risk Assessment. Therefore, starting from the MAAP4.0.6 model of the PWR Reference Plant it has been developed the new MAAP5.0.1 model, the so called Parameter File. Later on, the new model has been tested with the aim of gain confidence in the values of the Parameter File and hence, in the results of the accident sequences obtained with the new developed model. It has been test the new Reference Plant MAAP5.0.1 model by comparison against relievable results obtained with the RELAP5 code and the MAAP5 code. For the first case, it has been used the RELAP5 model of the Reference Plant and for the second one, the well-validated reference PWR Parameter File of the ZION Nuclear Power Plant, which is delivered with the MAAP code. Afterwards, it’s shown the results of the testing calculations, which accommodates a brief description of each simulated accident followed by the plottable results. Finally, the conclusions of the project are presented.