A chlorine leak was detected at 1:30 a.m., in an upper-tier SEVESO chemical plant, on the hydraulic seal of the intake fan on the sector’s chlorinated vents. The leak occurred after a liquid chlorine evaporator had been put back into service. The technicians switched off the evaporator’s supply at 1:38 a.m. and secured the installation. The chlorine detectors at the site’s property line measured 10 ppm of chlorine for 5 minutes, then 2 ppm for 4 minutes. The internal emergency plan was initiated. The chlorine workshop was secured, a safety perimeter was established around the evaporator, and the personnel were confined. The facility operator installed water curtains to suppress the gaseous chlorine. The water was then recovered and neutralised. The situation was brought under control at around 3 a.m., and the confined personnel were released at 7:40 a.m. The operator issued a press release.

Four employees had fallen ill due to the releases. A local resident living just 150 m to the south of the site had noticed chlorine odours. It was estimated that less than 100 kg of gaseous chlorine had been released.


On the day before the incident, an intervention took place on a section of the evaporator requiring that a manual degassing valve be opened toward the chlorinated vents’ intake fan. Following this intervention, the valve remained open causing liquid chlorine to pass through the trap downstream and then into the fan’s intake circuit when the evaporator restarted. The incorrect valve position went undetected during the evaporator restart leak test because the valve was not covered by the test.

Moreover, in the trap, the low temperature safety controls, set at -30 °C (the boiling temperature of chlorine is -34 °C at atmospheric pressure), activated only at 1:45 a.m. as the minimum temperature at the beginning of the incident had been -28 °C.

Immediate corrective actions:

  • safety interlock threshold raised to -20 °C with a pre-alarm at -15 °C in the trap. This modification has been made systematic for all traps in the sector;
  • review of the evaporator restarting procedure.

Medium-term corrective actions:

  • review of the risk analysis focused on the arrival of liquid chlorine in the chlorine
  • gas effluent system – implementation of a more reliable liquid detection strategy in the traps
  • reinforcement of lockout/tagout, alignment control procedures.