At a demineralisation unit, 1 m³ of 35% hydrochloric acid spilled into a retention basin and onto the concrete slab of nearby tanks. The gas desorbed in forming fog. The transfer pump shut down 1 min later, the siren was triggered and fire-fighters notified in accordance with a systematic call procedure. A nozzle dispersed the cloud and the absence of all external danger was verified. The accident was brought under control in 6 min (ventilation, neutralisation), before the arrival of first responders. A plastic lip union, broken during assembly and then re-welded 1 month prior to the incident by a subcontractor without advising the plant, separated. The unit was accepted without testing. Subsequent to an inappropriate modification of automated mechanisms on the day of the accident, some valves remained closed when the pump started. The investigation listed 14 points requiring further study before restart (oversized pump, insufficient support elements, etc.). HCl storage and distribution were to be simplified and procedures recalled.