The fatal event occurred in Fukushima Daiichi NPP, focused the e orts of many R&D organiza- tions towards the comprehensive study of the accident sequence, in order to better understand the phenomenology which took place throughout the whole accident. The development of powerful analytical tools and advanced plant models, underwent a surge since the event occurred in Three Mile Island NPP. From Fukushima Daiichi accident on, plant modeling has been notably enhanced, and furthermore it has pointed out the necessity of having tools capable to provide emergency preparedness and emergency response against this kind of events. This necessity has given rise to the creation of the European FASTNET project, within the Horizon 2020. Nowadays, advanced plant modeling combined with powerful analytical tools, have allowed to obtain more accurate results of severe accident at nuclear power reactors than at any time in the past. FASTNET gathers all these previous e orts, in order to develop tools capable to obtain fast response concerning source term evolution which eventually reaches the environment. The foundation of FASTNET is a large database of accident scenarios. This project is enclosed in the creation of the mentioned database for a Station Blackout scenario in a GE BWR 3 Mark 1 reactor design, by means of the analytical code MELCOR 2.1. Given the fact that source term evolution cannot be conceived without thermal-hydraulic evolution, present work focuses on both elds of study, and attempts to provide a comprehensive analysis of the accident for a reactor technology quite similar as the one of the Unit 1 in Fukushima Daiichi.