A chlorine release (Cl2) was detected at around 5:50 p.m. in a chlorine/caustic soda workshop in a chemical plant. The internal emergency plan was initiated, and the personnel secured the installations. The incident was brought under control at 6:10 p.m. The operator estimated that 180 kg of chlorine was released and a press release was issued. Although not located downwind from the plant, staff at a nearby train station contacted the operator to report that one of its agents was feeling ill. A resident of a neighbouring village also contacted the plant to report chlorine odours. The site’s firefighters conducted reconnaissance at both sites but did not detect odours.

The accident analysis showed that a recirculation pump in a chlorine storage tank had been left in operation inadvertently. The extended operation of the pump caused a rise in temperature of the tank. The liquefied Cl2 heated up and the resulting internal overpressure caused progressive degassing to the scrubber via the pressure regulator. A malfunction of the RedOx sensor, which measures the concentration of caustic soda in the scrubber, did not allow the shift operators to realise that it was unable to neutralise all the chlorine. A sample of the soda in the scrubber would have allowed the situation to be understood, but it was no longer in service.

The risk analysis prior to the recirculation operation on the tank was not sufficiently thorough. A detailed procedure for avoiding this event was not formalised. The operator reinforced the maintenance of RedOX sensors on the scrubbers to improve their reliability and decided to systematically analyse the conditions for taking safety-related equipment out of service. A response sheet shall be associated with each of the situations were equipment is taken out of service.