On March 11, 2011, the earthquake of magnitude 9.0 and subsequent tsunami led to a multi-unit severe accident at the Fukushima Daiichi Nuclear Power Station (Daiichi). As a result of high temperature oxidation of the core materials a large amount of hydrogen had been produced and released from the N2 inerted containments, and in case of Unit 4 - due to transfer through a common ventilation system from Unit 3 to Unit 4, leading to explosions that damaged the reactor buildings of three reactor units (Units 1, 3 and 4). Even though efficient hydrogen counter measures have been considered by the inertisation of the containment atmosphere with N2, the explosions outside the containments in the reactor buildings could not be prevented, as these buildings were not provided with mitigation measures. This underlined the challenges caused by hydrogen and other combustible gas accumulation to the building’s integrity in general and the need to further improve the hydrogen/combustible gas mitigation measures within the containment (if nothing was implemented so far) as well as in surrounding reactor building. Under the OECD/NEA/BSAF and BSAF2 programs, analyses of the Fukushima accident were performed using various severe accident tools with the goal of improving understanding of the accident progression and providing relevant information on the condition of the reactor buildings and containment. As follow up of the BSAF2 project, the ARC-F project aims to address the remaining issues, as hydrogen explosion analysis and its consequences. For this purpose, dedicated Task-group, composed of representatives from IRSN, CEA (France), IBRAE (Russia), NRA and JAEA (JAPAN), has been charged with providing information on the analysis of hydrogen transport and combustion. This paper summarizes the results and lessons learned from the analyses performed by the Taskgroup members for gas explosions that occurred in Unit 1 and Unit 3.