In a chemical plant shut down for the weekend, a 125-m³ tank experienced a pressure increase at around 3 pm subsequent to the sudden polymerisation of its acrylamide content. The tank roof, located amidst 5 other identical containers (2 of which were full), deformed and the manhole pad on the upper part of the tank was torn off. Polymer splatter in the form of small geysers of gel through the tap fell as a mist up to 35 m from the tanks and over a 300-m² area inside the plant. A water vapour cloud could also be seen from a motorway passing adjacent to the facility. No injuries were reported and consequences of this incident remained confined to the site. This untimely polymerisation had been caused by a leak on one of the tank’s heating circuit valves. The supervisory crew had noticed the rise in tank temperature but did not adequately process this information. Some 30 fire-fighters and about 10 vehicles were dispatched to the site, where a safety perimeter was set up. A control room was installed and men wearing diving suits performed sampling. Site clean-up was completed by 6 pm. The inspection authorities for classified facilities proposed several immediate measures: recording the locations of tanks containing identical chemical substances (one tank was identified with 88 tonnes of content), measuring the polymerisation inhibitor concentration in the damaged tank, verifying the inhibitor injection system based on analytical results, enhancing the monitoring of a damaged tank during the weekend, and specifying a rapid means of intervention in the event temperature in the second tank changes abruptly. The plant operator wound up connecting the tank to a water distribution outlet to enable diluting, as required, the chemical product according to a hazard-free procedure and then cooling it.