Nitrous vapours were released in a plastics manufacturing plant at around 10:45 a.m. The accident occurred during an operation to empty nitric and sulphuric acid pipes into a recovery tank, known as a drum. A workshop manager, present at the site, reported the release. An operator stopped the pipe draining operation and sprayed down the tank. Having noticed the presence of acid in the sump where the drum was located, they drained the sump, thereby stopping the release at around 11:45 a.m.

In a school located 2 km from the site, people reported stinging eyes, and the town hall was alerted, which then informed them to remain confined. The fire brigade conducted atmospheric measurements in the school and lifted the alert at around 12:15 p.m.

During the incident, several hundred kilograms of nitric acid, oleum sulphur, and sulphuric acid were released into the atmosphere. The operator simulated the shape of the plume and estimated the zones of reversible effects and olfactory effects. Majoring assumptions led to a zone of reversible effects extending 163 m and an olfactory threshold of 1,906 m.

Following this release, the workshop manager tested the draining of the lines to the drum and found a leak at the drum’s drain valve in the sump. There had been water in the sump and heat from nitric acid’s reaction with the water resulted in the release of nitrous vapours. The pump inside the sump did not extend into the bottom of the sump.

The remote control system of the leaking drum’s purge valve had not closed properly. Due to its design, the valve’s position could not be checked, and the operator had no record of the position.

Following the event, the operator managed to repair the bottom valve and rendered the valve’s operation more reliable by allowing its position to be checked. A procedure was established for emptying the pipes, checking the closure of the drum’s valve and the position of the sump bottom components. A procedure for emptying the drum was also drawn up, thereby ensuring that nitrous vapours could not be generated. The action sheet concerning acid leaks was modified to include a risk assessment designed to put the internal contingency plan in motion. The operator is studying the possibility of incorporating containment balls in the sump or an emulsifier to contain the release of nitrous vapours. The operator is also considering its ability to analyse a nitrous vapour release situation in order to provide factual information on the plume’s passage near the site.