Hydrogen explosion risk needs to be carefully assessed and evaluated in nuclear facilities because of the potential catastrophic consequences: breakdown of safety equipments, failure of containment, dissemination of radioactive materials in the environment. When studying an indoor release, one possible simplification is to assume a perfect gas mixing inside the room. This assumption is effectively often used to evaluate toxic risks in the environment outside a building (Mastellone, Ponte, & Arena, 2003). However, perfect gas mixing assumption is only a rough approximation, as indoor concentrations can largely differ from mean values, due to buoyancy, recirculation zones or obstacles for example. In order to better evaluate the risk of explosion in case of an accidental release of hydrogen, IRSN conducted a numerical study using FLACS CFD software. Several parameters have been studied to identify dangerous situations and draw a representative picture of the risk: room size, position and direction of hydrogen leak, ventilation characteristics. Hydrogen release flow rates used for numerical simulations have been chosen as the highest leak rate which, by applying the assumption of perfect mixing, produces an average concentration in the room equal to hydrogen lower flammability limit (LFL). Simulation results indicate that in some particular configurations, especially for impinging hydrogen jets, hydrogen concentrations can locally be above LFL and then create explosive atmospheres with significant volumes. (C) 2010 Elsevier Ltd. All rights reserved.