At a plant manufacturing and packaging swimming pool products, a chlorine (Cl2) release occurred during a chlorine waste neutralisation step through immersion in an open-air water container. The chlorinated emission had practically dissipated upon arrival of the fire-fighting crew 45 min later. No environmental impacts were recorded. At the time of the incident, 900 g of chlorinated waste, composed of workshop sweepings and dust recovered in vacuum removal (comprising in large part trichloroisocyanuric acid, TCCA), were placed into a 1,000-litre water container so as to solubilise the TCCA. Water saturation caused this Cl2 release; since excess chlorinated powder was unable to dissolve, it became loaded with humidity leading to an exothermic decomposition. The gas release ceased once the decomposition residues finally dissolved. To avoid a repeat accident, the TCCA waste deactivation procedure was revised to include a daily accounting of the wastes left to be deactivated, in order to prevent water saturation in the container (max. 100 kg of TCCA / 1,000 litres of water). The TCCA safety fact sheet was from then on to be posted alongside the treatment station and the existing fire hose modified to allow intervening by flooding the waste awaiting destruction. In time, the TCCA ingredient was to be replaced by a less reactive product.