A leak of refrigeration ammonia (NH3) occurred at night inside a food production facility using animals, vegetables and dairy input. The night watchman sounded the alarm after an NH3 detector had been tripped.

A technician proceeded by bleeding the NH3 buffer capacity in the cold production unit, but this operation was slowed by the presence of an ice plug blocking the drain orifice. The technician left the premises allowing the drainage operation to continue; he forgot to return to verify that the operation had been successful and to close the valve. The bottom ice plug and NH3 mixed with oil spilled around 3:50 am inside a 200-litre drum intended to collect the small volume of residual drips. For practical reasons tied to the drainage step, this drum placed away from the buildings was not part of the primary retention tank for the buffer capacity.

The ammonia effluent overflowed the drum, spilling into the stormwater collection network, then into the PREPSON brook. The environmental consequences appeared to be limited (a few frogs killed), both on the watercourse and at the municipal drinking water treatment plant. The level of flow infiltration was very minor through a sewer manhole not entirely sealed, but without any lighting inside.

The emergency response unit first assessed the leak at 25 l of NH3, then revised this figure to 250 kg when their intervention ended the next day at 9 pm. Plant activity was temporarily shut down, but no staff had to be made redundant. The watchman felt ill and was hospitalised out of precaution. The water management authority was notified. A municipal representative and the national gendarmerie were also dispatched to the site. Water samples were taken, even though no extraction point had been indicated along the course of this brook.

The procedure for treating NH3 buffer tank drainage remained incomplete, as evidenced by: an a posteriori validation of manual valve closure, the protocol to be followed by a technician during drainage incidents, the formation of an ice plug. Moreover, the measures adopted relative to retention during such drainage incidents did not account for a large-scale accidental overflow: a 200-l can on a 50-l mobile retention tank placed outside the primary retention zone containing the buffer tank.

The operator immediately executed an a posteriori validation of valve closure by the watchman on duty, who had not yet been fully trained in this task. Several 200-l drums were installed in the primary retention zone for the cold production facility; a regular transfer into a 2nd oil drum that had already been drained would prove necessary, yet this step followed the protocol adopted to ensure no spillage of major quantities of polluting substances into the environment via either the stormwater or wastewater network. In addition, a study was initiated for a technical servo-controller capable of restoring operations to refrigeration units upon closing all manual valves.