At a facility specialised in manufacturing basic plastics, a biphasic leak occurred on the safety valve of a bromation reactor containing 2 tonnes of dichloroethane with 5% bromine chloride (BrCl). Spreading of the mix remained confined to the production plant, as well as within the released cloud of hydrogen chloride. The facility’s internal emergency plan was activated. Internal responders used a “water blanket” to limit both evaporation and the risk of inflammation, in addition to cleaning the premises ; all liquid wastes were transferred to an impermeable tank. Rail traffic on a nearby line was halted as a precautionary measure. The emergency plan was lifted 2 hours after the accident, with no injuries reported. Excessive reagent (BrCl) flow was the cause of pressure rise inside the reactor and safety valve operations: the intake control valve had been temporarily replaced by a manual valve as part of a

maintenance operation. During the days prior to the incident, repeated clogging had led to changing the recommended position of the manual valve. To avoid such an accident from recurring, various remedial actions were implemented: design of the reagent loading line to ensure the flow rate always lies below the effective vent discharge rate ; improved tracking of temporary modification requests ; installation of a filter on the reagent loading line to prevent the risk of clogging; a study of the safety valve network in order to minimise the risk of vent clogging via a biphasic drive.