In a paper mill, chlorine dioxide (ClO2) used to bleach the paper pulp leaked and 4 explosions were set off during plant downtime. The accident occurred as a dioxide generator reactor was being drained into a polyester tank with a capacity of 18 to 20 m³, while an operator had already begun rinsing the pipes. Methanol forced into the reactor then came into contact with chlorate residue. The ClO2 formed by this association decomposed exothermically in triggering a considerable and near-instantaneous increase in the gaseous volume, which was responsible for the explosions. The pressure surge was discharged via the open reactor manhole. The chemical reaction propagated into the polyester tank, and one of the explosions caused its destruction. The plant’s POI emergency plan was not activated, yet gate staff did notify firefighters subsequent to the erroneous risk assessment. A safety zone was cordoned off around the workshop, which could be confined using a water curtain; all air conditioning units were shut off as a precautionary measure. ClO2 emissions were contained: 20 ppm over a 1-min period and a peak recorded at 35 ppm shortly after the water curtain was set up. No casualties were reported and property damage estimations amounted to 1 million francs (150,000 euros), consisting of: collapsed tank roof, blown-out cladding, damaged generator valves and heat insulator, and broken pipes. The procedures for shutting down the workshop, and in particular the rinsing step, underwent modification