In a chemical plant, during normal operation of a reaction of concentration of hypophosphorous acid, an explosion followed by ignition take place. Due to a failure of stirer in the mixing tank, a crystalline incrustation of hypophosphorous on the rim of the mixer tank at liquid level acid formed and decomposed spontaneously. A part from phosphoric acid an amount of phosphine was formed as decomposition product sufficient to destroy the vacuum and to drive back the unfastened manhole cover. The mixture ignited when mixing with air. A part of the substance flowed via the vacuum pump into the piping system that contained hypophosphorous acid and an ignition took place.7 persons are injured by release. The entrances and the roads in installation are closed. The damages to buildings and technical devices is evaluated at 125 TDM and 1000 DM the channel cover.