[en] The report reviews external vapour cloud explosion phenomena, especially transition from deflagration to detonation, in the context of nuclear power plant safety, for explosions postulated according to the B.M.T. guideline ''Chemical Explosions''. Damage analyses of recorded accidents show that fast deflagration, not detonation, explains their consequences. Subsequently, laboratory- and large-scale trials are surveyed and safety-related conclusions drawn. Large unconfined cloud field trials (Maplin Sands, China Lake), for propane/butane, evidence negligible overpressures (a few millibar). Balloon tests, for stoichiometric hydrogen-air mixture, up to 2000 m³, yielded about 60 millibar max. Flame acceleration due to obstacles, nets, turbulence-inducing structures was investigated. Small- to large-scale trials, with hydrogen-air mixtures, show that transition to detonation is restricted to jet ignition. Main safety-related conclusions are: - detonation need not be postulated. Even a partially-confined stoichiometric hydrogen-air cloud generates less than 0.3 bar; - jet ignition should be avoided by appropriate design; - turbulence-inducing obstacles: gratings, pipes, corners merely produce local overpressures and cannot cause transition to detonation