At around 3.30 pm in an establishment specialising in the treatment of sedimentation sludge produced by the adjacent safe drinking water plant which supplied 500,000 inhabitants, a delivery driver unloaded 23 t of sodium (30.5%) by mistake into a 35 m³ silo containing 3 t of quicklime. An exothermic reaction occurred with risk of rupture of the tank due to overpressure. The silo was cooled using 2 nozzles. A safety perimeter of 100 m was set up, 14 employees were evacuated and the traffic on CD 404 was interrupted in both directions. A laser pyrometer was used to assess the temperature of the silo: 120°C at the bottom, 80°C in the middle and 30°C at the top. The O2 concentration was 16%. Firefighters responded equipped with respiratory protection. To anticipate possible ruin of the tank, it was decided to increase the cooling by adding a 3rd nozzle, opening the vent, emptying the 900 m³ retention tank in order to store in it the cooling water and the mixture contained in the tank. The emergency services noted a drop in temperature (43°C at the bottom, 38°C in the middle and 14°C at the top) and observed the weakening of the bottom valve despite good stability of the tank. At around 9 pm, leakage (10 l/min) of the mixture appeared on the silo. Given the rate of cooling and the volume of the retention tank, the emergency services divided the flow rate by 2, and then stopped the cooling at 9.56 pm to avoid any overflowing. The temperature of the silo rose again: 80°C at the bottom and 60°C in the middle. The pH in the tank was 12. According to the company’s managers, a pH of 14 would cause the tank’s mixing pump to malfunction. The silo access metal elements were damaged due to the action of the sodium: 2 components in the middle section of the silo were dismantled to accelerate the flow into the retention tank. 10 m³ were drained in this way. The removal of 2 probes and a base plate ensured the flow of 13 m³ of sodium. At 3.15 am, the temperature was 50 °C at the bottom and 25 °C at the top. The installations were rinsed to reduce their corrosion by the sodium. The reaction no longer caused the temperature to rise. The risk of explosion and tipping over of the silo were eliminated. Traffic was resumed. The company took responsibility for unloading the mixture that was in the silo. The intervention by the emergency services ended at around 6 am on 29/08. The incident had no consequences on the safe drinking water production. A legal enquiry was carried out.