The process of unsteady high-pressure hydrogen release observed on local vessel rupture is investigated experimentally. The flow structure during the formation of a supersonic free hydrogen jet is studied. The obtained data correlates well with the numerical simulation of sonic underexpanded hydrogen jet released into the atmosphere. In particular, it has been confirmed that the Mach disk fluctuates in the case of unsteady outflow of underexpanded hydrogen jet. It has been shown that in order to provide diffusion self-ignition, it is necessary to focus an air shock wave, formed after the vessel rupture, within the studied initial hydrogen tank pressure range. Lab and ground tests on self-ignition and explosion of hydrogen that outflows through a 15-mm opening when the flow interacts with a 37-mm-radius hemispherical cavity are described. A comparison of the diffusion combustion dynamics of releasing hydrogen in self-ignition and induced ignition scenarios is made. The observed visible flame front velocities of 150-220 m/s can result in dangerous blast waves in the case of large-scale hydrogen release. Measures to reduce the risk of explosion of unsteady high-pressure hydrogen jets releasing into an encumbered space are proposed. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.