During a transfer operation involving a cistern located in the acid production plant and nitrated by-products from a chemical plant, a rupture disc broke around 6:45 am on a line connecting a cryogenic ammonia (NH3) storage facility containing a 340-tonne theoretical capacity (-33°C at 1,013 mb, 310 payable tonnes) with a gas holder. Within a span of 105 min, between the presumed onset of the leak and its discovery by site personnel, 2.4 tonnes of NH3 were released into the atmosphere. A passer-by notified the fire department 25 minutes later.

A toxic cloud wafted over 4 km at an average speed of 0.3 m/s. 20 min and 3 attempts were required by the team of technicians, alerted around 7:15 am, when arriving on duty to close the chain valve that isolates the ruptured disc. The first attempt failed when the chain slipped and the pulley got stuck. Bothered by the NH3 gas entering his protective gear worn for the occasion but not properly adjusted, the technician decided to abandon the second attempt. This older-generation storage facility was not equipped with any internal pressure measurement recording system. The neighbouring population was asked to remain indoors and no injuries were reported, aside from odour nuisances and ocular discomfort from time to time until 9:30 am.

Several installation malfunctions downstream of the release preceded the accident. The operator drew a number of lessons, namely: a degraded operating regime not fully acknowledged by the technician when the cistern was being prepared for transfer; inadequate maintenance and control of both the measurement instruments (poorly calibrated flow meter) and protection systems (poorly installed rupture disc, a safety valve difficult to manipulate, etc.); design flaws (manual pressure setting, lack of visibility between the transfer station and the tank being filled); absence of leak detection device (notification relayed by neighbours); use constraints specific to the protective gear (choice of equipment, user instructions, practice); sharing of recommendations and information between line staff; intervention protocol (installation safeguards, isolation of storage compartments, etc.); staff training (under normal or degraded conditions, relevance to intervention techniques).

This accident, which generated a strong psychological impact, received heavy media coverage. Both the installation and operating guidelines were revised: installation of 2 pressure sensors, one connected to an alarm the other to a recorder; a relief valve on the tank; an NH3 detector alarm; an automatic transfer shutoff device; and the scheduling of frequent inspection rounds at the time of transfer operations.