At 9.55 a.m., at a pesticide production plant, vapour containing ammonia was emitted in a nitric acid storage workshop. The audible and visual alarm was triggered to evacuate staff present in the unit. At 9.57 a.m., the unit was shut down manually from the control room. The unit restarted at 11.45 a.m. The facility had restarted 3 days before the incident.

At around 12.15 p.m., 3 technicians carrying out maintenance in the workshops reported that they had been exposed to ammonia, because they had not been able to put on their masks to evacuate the area. They were treated by emergency services for a check-up and returned to work the next day. The operator estimated that the quantity of ammonia discharged from the chimney was between 10 and 100kg.

The vapours from a reactor outlet contained ammonia following sudden shutdown of the nitric acid supply. This shutdown should have caused automatic shutdown of the ammonia supply by the low nitric acid level and nitric acid/ammonia ratio deviation safeties. They had been shunted to not trigger the workshop, due to the reduction of the reactor’s rate, as a result of nitric acid supply disruptions. Shunt viewing was not directly available on the reactor’s control screens. The shunt had not been reported in the folder intended for this use in the shunt management procedure. In addition, the shunt had not been checked when shifts changed. The ammonia supply had therefore been shut off manually by the technician in the control room, who was managing 3 workshops at the same time. They had attempted several operations to restore the acid supply before shutting off the ammonia supply, which caused the reactor to fill with only ammonia for a few minutes.

The sudden shutdown of the nitric acid supply was due to untimely closure of the storage tank’s outlet valve. Only one tank on the site was available to supply the reactor, as the other was being used to store non-compliant acid. The simultaneous use of this tank to supply the reactor and load a truck resulted in a higher transfer rate from the tank. The product’s flow speed caused the outlet valve to close. The technician in the control room did not have any information on either the truck’s loading rate or manual operations at the fill stand.

Following the event, the operator put in place the following actions:

• repairing the nitric acid storage tank’s outlet valve;
• checking that shunts are taken into account at the start of every shift;
• auditing shunt procedure and adjusting access rights to perform a shunt;
• improving viewing of shunts in the control room;
• studying evacuation conditions in the workshop.

A BEA-RI (Bureau d’Enquêtes et d’Analyses sur les Risques Industriels, i.e., industrial risk investigation and analysis bureau) investigation was conducted.