This project is categorized as an atmospheric dispersion study about how a toxic gas external dispersion might reach a nuclear power plant control room with dangerous concentration. In fact, it is contained within a larger project, which studies how likely a train accident, related with hazardous materials transport, could end up in a dangerous toxic vapour concentration in the control room, affecting the plant safety. The scope of the actual study is to obtain control room toxic vapour concentrations by performing a probabilistic approach of atmospheric dispersion calculations. The goal is to apply a prospective approach, aiming to use the results in the probabilistic safety assessment of the plant. The tasks developed consist of regulations review, atmospheric dispersion codes comparison and selection, scenario design and release points distribution, meteorological analysis and calculations automation, and finally, control room vapour diffusion calculations. The Nuclear Energy Agency (NEA) has provided the Nuclear Engineering Research Group (NERG) from the Technical University of Catalonia (UPC) with the dispersion and diffusion codes, HABIT and ARCON96, for the development of the present study, under consideration of academic purposes. The whole project is developed within the frame of a collaborative agreement between NERG group (UPC) and Nuclear Plant Licensee. Since this agreement is confidential, no real names and codification for the plant and its systems, components and materials are used. Therefore, the generic expression “The plant” is written instead.