At 7:45 p.m., a main low-voltage switchboard transformer caught fire in the basement of a production building of a lower-tier Seveso pharmaceutical plant. The fire knocked out the building’s electrical power supply, causing the impellers and cooling systems of the reactors to turn off and thermal runaway to occur in one of the reactors. The reactor’s 4-bar rupture disc burst and the explosion vent opened to protect it and the 70 °C reaction mixture, which consisted of a dozen hazardous products (piperidine, acetic acid, calcium hypochlorite, MTBE, potassium carbonate, methanol, potassium, pyridine, hydroxylamine hypochlorite, isopropyl alcohol, nitrogen, benzyl chloroformate). One employee and six firefighters located nearby were sprayed (stains) with this mixture. A 60-m² puddle formed on the floor.

The plant operator initiated its internal emergency plan and had the plant evacuated. The firefighters did not find any signs of fire or risks of explosion. The gas supply was cut off at 9:30 p.m. The firefighters left at midnight. The inspection authorities for classified facilities came to the site. No comparable scenario had been identified by the hazard study conducted at the site. Operations in the damaged building and at the associated solvent tank farm have been halted and will resume only once the safety systems (FACP and automatic extinguishing system) are once again operational. In the meantime, an auxiliary operator will monitor the plant at all times. All the electrical facilities on the site will be analysed and a study will be conducted to consider installing
a backup power supply for critical equipment involving exothermic reactions (cooling, mixing, temperature and pressure sensors, etc.).