Forschungszentrum Karlsruhe conducted and sponsored a study on containment loads from hydrogen combustion pressures that could occur in case of an unmitigated severe accident in a future 1500-MW(electric) pressurized water reactor. The analysis of large-scale distribution tests leads to the conclusion that the full spectrum of combustion modes from slow deflagration to global detonation must be considered in the absence of any hydrogen control system. New experimental and theoretical results are presented for fast flames, deflagration-to-detonation transitions, and marginal and stable detonations in hydrogen-air mixtures on reactor relevant scale. Maximum possible combustion loads for severe accidents are predicted for a typical dome geometry. The results provide a database on global combustion loads for design studies on future severe accident resistant containments.