In a hydrogen laboratory hydrogen may accidentally leak from the high pressure systems and it may form a flammable mixture after mixing with air. CFD (Computational Fluid Dynamics) simulations of accidental hydrogen releases in a laboratory are performed in order to estimate the size and the resident time of the flammable cloud inside the laboratory. Moreover the effectiveness of the hydrogen sensor positions in detecting the presence of the hydrogen in the laboratory within few seconds has also been investigated. In a first scenario, the hydrogen coming from the storage is pumped by a compressor into the laboratory pipelines at constant pressure. To be conservative, it is assumed that the pressure can be maintained when the leak in the pipeline occurs. In a second scenario, the hydrogen flows directly from the storage into the laboratory pipeline without the compressor, and the pressure in this case is decreasing with time. In both cases, shut-off valves have been neglected in order to consider a worst case scenario. The simulation results from the two accident scenarios are compared, showing that the configuration with the compressor will potentially produce larger amounts of flammable mixture in the laboratory. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.