A fire broke out in a swimming pool product formulation, packaging and storage plant. The establishment’s internal and external emergency plans were activated. The staff of the plant and of 2 neighbouring establishments were evacuated and brought together in a municipal hall; the main traffic links were suspended (A43 motorway, RN 6 trunk road, RD 74 minor road, Modane-Chambéry railway line) and containment measures were established for 6 neighbouring municipalities (La Chambre, St Martin sur la Chambre, St Etienne de Cuines, Notre Dame du Cruet, and Ste Marie de Cuines). Chlorine (between 600 and 800 kg according to the emergency services and less than 160 kg according to the operator) from the decomposition of sodium dichloroisocyanurate (NaDCC) used on the site, was released into the atmosphere. Nevertheless, the sensors of the local air quality monitoring network did not detect any significant atmospheric pollution. Finally, the fire was brought under control after 5 hours of fire fighting and the external emergency plan was lifted 6 hours after its activation. Six people with minor exposure (2 technicians, 2 fire-fighters and 2 inhabitants) were hospitalised. There was significant damage to equipment : 3 out of the 7 production lines were destroyed. The plant was permanently closed for economic reasons. The fire broke out on packaging line no. 7 that was restarted 4 hours earlier after being shut down for maintenance. This line consisted of an upper hopper in which NaDCC was loaded and then fed down the packaging machine by gravity. A filtration system was used to trap chlorinated dust in the upper hopper then recycle it in the packaging machine. The causes of the fire were not clearly determined. The fire was started and fed by the self-sustained decomposition of NaDCC contained in the packaging machines. Several hypotheses were put forward on how this decomposition had started: introduction of a reactive substance into the production hopper or presence of organic grease in the flow regulation valve following the maintenance operation, contamination of the filtration system (presence of moisture and organic dust after a 4-week stoppage) causing the decomposing particles to fall onto the packaging machine.