In severe accidents, hydrogen may be generated in a nuclear power reactor and may explode damaging safety and containment systems. In a multi-compartment safety containment, phenomena due to partition walls and vents between compartments may cause a sort of 'amplification' of the maximum deflagration overpressure in single compartments; then greater pressure differences may destroy internal structures with equipment supported. Department of Mechanical and Nuclear Constructions of the University of Pisa carried out experiments with multi-compartment small scale containers about this problem. Peculiar phenomena have been identified, like 'jet ignition' oxyhydrogen torch' and 'recoil'. A new version of the experimental apparatus "LargeView" was built with a 0.3 MPa design limit pressure. A set of transducers takes the overpressure transients, while a videocamera system records the flame evolution through the glass walls of the container. Tests have been held with equipment models put inside the first and second chambers of the facility. Sensible differences in recorded maximum overpressures in tests held under the same nominal experimental conditions might be related to slight errors in the hydrogen concentration measurement, because the pressure seems to be very sensitive to the initial hydrogen concentration. These results suggest to reduce the present safety limit for the hydrogen concentration in a nuclear reactor containment.