Pollution
Humain
Environnement
Economique

6 to 800 l of ammonia (< 500 kg NH3) were released to the atmosphere from an ice cream factory at the end of the night by an exhaust manifold following overpressure in the high pressure circuits of a refrigeration unit and the malfunction of a pressure relief valve on an oil cooler. Calibrated to 18 bar, it opened at 15.7 bar, as the safety high pressure switch was set at 16.5 bar. According to witnesses, NH3 in liquid form at the start of the emission prevented the pressure relief valve from "closing" after discharging. All of the NH3 in the affected section was released until the circuits were permanently closed. The NH3 detected in the engine room caused the installations, whose safety mechanisms were operating correctly, to shut down. An alarm alerted a surveillance officer who followed the safety guidelines and the factory's internal emergency plan was activated less than 15 minutes after the alarm. The external emergency services that arrived at the scene 10 min after being alerted used 2 "water shields" to limit any propagation of NH3 towards a residential area close by. 59 ppm of NH3 were recorded at the source, 39 ppm at 200 m and 11 ppm at 300 m. The gendarmerie (French military police) asked the population, including 150 people from a neighbouring estate, to stay inside within 200 m radius. Site staff were evacuated, along with neighbouring businesses within a 50 m perimeter downwind. The municipality rainwater treatment system operator for the industrial area was also alerted. For technical reasons, the firefighters only closed this network's outlet valve 30 to 45 minutes after the start of their intervention. An initial discharge of heavily alkalised water (water shields) into the VILAINE river was unavoidable and a few fish were killed. Faced with a significant increase in pH (>10) noted by firefighters immediately downstream of the point of discharge, the water police carried out several releases from a dam upstream and, supported by the inspection authority, asked the operator to install a pH metering system along the river, relaying with that of the firefighters in order to monitor the progress of the wave. As a precaution, a drinking water abstraction point was suspended downstream. Upon arrival of the pollution wave, a 2nd release was made from a forebay on a tributary of the VILAINE river. The measures taken were lifted after observing the effect obtained by dilution and the return to normal conditions. The 40 employees were placed on technical unemployment for one day. The loss of feedwater from an evaporative condenser had caused an abnormal pressure buildup of the NH3 in the condensation circuits. All of the pressure relief valves were replaced, the HP safety mechanism threshold was lowered, a water loss alarm and pressure relief valve operation detection system were installed, and the containment pond closing procedure was modified.