In the nickel ore (matte) acid attack unit of a metallurgy factory, a 3-kg gaseous chlorine leak occurred around 2:30 pm on a ¾-inch tap of a PVC pipeline bound with polyester resin and fed with both wet and dry Cl2. The accident occurred during installation works for a metal floor subsequent to a tap perforation (creating a 5-mm² gap) caused by a subcontracted employee. One of the 4 responders overcome by the discharge was taken to hospital as a precaution; he was released at 8 pm that evening. Activation of a detector (alarm threshold: 20 ppm), located 10 m from the leak, caused closure of both the dry and wet Cl2 intake valves inside the unit. This detection also caused closure of the wet Cl2 outlet valves on the electrolysis unit as well as this unit’s dry Cl2 valves; the internal emergency plan was triggered and the installations placed in safe operating mode. Once alerted, external first responders conducted site surveys. Neighbouring municipalities and industries were informed of the incident; a press release was broadcast at the end of the afternoon. No impact outside the facility was indicated. A prevention plan had been adopted for these works. Subsequent to the accident, the operator issued a three-year plan to eliminate disused taps on the wet Cl2 lines and perform an evaluation of risks due to pipe punctures, a modification of the facility’s procedure for establishing prevention plans in order to better incorporate the risks stemming from the presence of chlorine pipes, a modification of the list of emergency plan contact phone numbers, and a reminder of the plan’s rules addressed to lorry drivers.