Computational Fluid Dynamics is an effective tool to develop safety standards related to the sudden release of hydrogen from a high pressure reservoir. In this work, a three-dimensional in-house code is developed to numerically simulate the release of high pressure hydrogen (70 MPa) from a reservoir when the release area into air is expanding with time. Furthermore, high pressure hydrogen flows cannot be accurately simulated by the ideal gas equation; therefore the Abel-Noble real gas equation of state is applied. A transport equation is solved to find the concentration of hydrogen and air in the hydrogen-air mixture generated soon after release. The novelty of this work is to simulate and to study the flow when the release area enlarges rapidly. To obtain this capability, the solid boundaries of the release area are moved and the mesh follows based on a spring method. All the nodes in the mesh are moved at each time step accordingly to have a good quality mesh. Three initial diameters of 1.0 mm, 1.5 mm and 2.0 mm are tested for the release area, and opening wall speeds of 80 m/s and up to 300 m/s are discussed. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.