Hypothetical accident scenarios in a chemical plant for hydrogen production coupled to a nuclear power plant are investigated numerically in order to estimate the potential consequences on the nuclear plant. Computational Fluid Dynamics is used to reproduce and analyse the formation and propagation of a flammable cloud due to a hypothetical accidental hydrogen release and of a toxic cloud due to the release of hydrogen chloride in different wind conditions. The simulations show that the damage on the nuclear power plant due to the explosive cloud can be expected to be minor for the specific assumed accident scenario. More critical could be the effect of the hydrogen chloride release. The presence of a hill as mitigation measure between the hydrogen production plant and the nuclear facility increases the minimum time for the toxic cloud to reach the nuclear plant but it has a non-uniform effect on the amount of toxic mass and the minimum distance between the two plants depending on the wind speed and on the toxic threshold. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.