This work addresses the hydrogen safety issue of an International Thermonuclear Experimental Reactor (ITER) in case of a loss of vacuum accident (LOVA) scenario. In this scenario, accidentally generated hydrogen mixed with injected air could be ignited at reduced, sub-atmospheric pressures. The general question is whether the induced combustion pressure could exceed the ITER vacuum vessel design pressure. The paper presents the results of large scale dynamic experiments on hydrogen ignition and combustion at reduced pressures in the presence of a turbulent air jet injected into the hydrogen atmosphere. Experiments have been performed in a cylindrical vessel with a volume of 8.8 m(3) filled with hydrogen at an initial pressure of 200 mbar. The orifice size of 6 mm i.d. was chosen to model in the same time scale the real leak through a 100 mm x 100 mm area into the ITER vacuum vessel with a volume of about 3000 m(3). The structure and dynamics of the air jet into the hydrogen atmosphere at different initial pressures were investigated using microscopic liquid droplets as markers. During the combustion tests, it was found that more distant ignition positions and stronger ignition energies lead to maximum combustion pressures that are lower than the design pressure of the ITER vacuum vessel. (C) 2011 Elsevier B.V. All rights reserved.