The formation and release of large amounts of hydrogen (H2) can accompany the course of a severe accident in a nuclear power plant. After the H2 is released into the containment, it is mixed and transported by natural and forced convection. If a sufficient amount of H2 accumulates and becomes flammable, different combustion modes may be possible: deflagrations, accelerated flames, and detonations. This report describes research carried out in the KOPER facility on spontaneous detonation initiation in H2-air mixtures by turbulent jet ignition. The KOPER facility is a large semi-confined volume designed to investigate conditions of explosion initiation and propagation in gaseous fuel-air mixtures. The effects of three variables were investigated: H2 concentration; jet orifice diameter; and the composition of combustion products in the turbulent jet. The possibility of initiation of two characteristic combustion regimes, turbulent combustion and local detonation, was demonstrated. Local detonation develops after a delay of 10-25 ms from ignition. Both high enough H2 concentration and large enough jet size are necessary for spontaneous detonation initiation. The minimum H2 concentration is within the range of 20 to 25 % vol., and the minimum jet orifice diameter lies in the range 100 to 200 mm for the givenmore » geometrical sizes and configuration of the facility. The existence of an optimum concentration of hot product gases was observed. A minimum ratio of turbulent jet size L and mixture detonation cell width l, L/l = 12-13 is required for detonation initiation. This minimum value corresponds to that measured for other type of turbulent jet initiation experiments (closed volume and continuous venting) and is supported by theoretical analysis.« less