A leak evaluated at 3,900 kg of ammonia (NH3) occurred in a 200-m3 ice water tank used to refrigerate the dairy workshop of a large dairy cooperative. The installation, subjected to authorisation, had introduced 12.3 tonnes of NH3. The quantity of gaseous NH3 emitted into the atmosphere was not evaluated. The leak resulted from the corrosion and perforation of one of the racks used to cool the tank water. Detected on 16th October, 2008 during an NH3 concentration inspection in the ice water tank, the leak was exacerbated by human intervention on 24th October intended to accelerate rack drainage. The water/NH3 mix (concentration: 17 g/l of NH3) in circulation throughout the dairy workshop caused odorous emanations in the work areas, but no discharge was reported into the natural aquatic medium.

According to the operator and 2 subcontractors specialised in corrosion phenomena and NH3 refrigeration installations, this event was primarily caused by:

• equipment malfunction: racks composed of steel with low carbon content;
• organisational deficiencies: faulty monitoring of the state of rack corrosion, an unsuitable procedure for injecting corrosion inhibitor (manual for the first few years of ice water tank use, initially installed in 1998, variable water level and/or surface foaming causing accelerated corrosion of surface racks introduced near the point of manual injection);
• human error: exacerbation of the leak tied to operations undertaken to accelerate rack drainage.

Compensatory monitoring measures were adopted to track NH3 concentrations in the air of production rooms as well as in the tank water. Tank contents were first neutralised with sulphuric acid on 7th and 15th November, 2008, and then oriented to an onsite storage facility to be treated at a rate of several m3/day in the plant’s internal purification station. The cost of property damage and operating losses was not known: design studies, replacement of defective equipment, programmed facility shutdown.

Notification of these events to the Classified Facilities Inspectorate was delayed. An order to implement emergency measures established prescriptions relative to: the handling of residual ammonia water, the protocol for overseeing its treatment at the site’s purification facility, and the verification of installations (inspection of rack corrosion as well as all refrigeration installations).

The racks were appraised by an expert and gradually replaced by stainless steel plates over the course of 2009.