In a chemical plant, 15 kg of chlorine (Cl2) were released into the air during the restart of a membrane electrolysis unit. The site’s internal emergency plan was activated, the unit was stopped and the staff made their way to the meeting rooms. Finally, the alert was lifted 45 minutes later, after technicians had checked the Cl2 concentrations in the air; no other consequences were reported. An increase in pressure at the inlet of the 1st column of the unit’s gaseous effluent sodium scrubbing system caused siphoning of the evacuation of condensates at the bottom of the column and the release of chlorinated gas into the air (flow rate of 50 g/s # 15 kg of pure Cl2). Beforehand, an air inlet in the Cl2 manifold had led the regulation system to channel all of the Cl2 produced towards the absorption system, and the flow to be treated therefore rose above 12 t/h. Exhaustion of the sodium at the bottom of the 1st scrubbing column caused a foaming phenomenon producing clogging of the column and a pressure buildup at the fan discharges; the operator was aware of this phenomenon. The temperature of the sodium recirculation then rose from 67 to 92 °C and the pressure at the column’s inlet rose from 85 to 214 mmCE, i.e. the height of the condensate recovery guard chamber through which the Cl2 had escaped. Several measures were taken following this accident: new guidelines requiring shutdown of the electrolysis unit in the event of pressure exceeding 200 mmCE during restart phases pending the installation of an automatic high pressure safety mechanism on the column inlet, study for collecting releases from condensate evacuation guards, experiments with an anti-foaming product on the sodium recirculation, in particular, during restart operations, and revision of operating procedures to emphasise the importance of checks before restarting installations.