In a chemical plant, an explosion occurred on an immersed rotary pump supplying nitrogen peroxide (N2O4 in a mix with low-concentration nitric acid) to the concentrated nitric acid (HNO3) unit. The accident happened at the time of restarting the unit after 1 week of downtime, during which maintenance operations had been performed at the base of the unit’s columns (in a part separate from the affected zone). As for the human toll, 2 technicians had to be hospitalised for perforated eardrums, as the explosion could be heard several kilometres around. Fire-fighters were notified by neighbours concerned about the explosion, yet the site’s internal emergency plan was not activated. The pump’s suction and discharge pipes, which were heavily damaged by the explosion, released in addition to toxic smoke (NO2) 15 to 30 litres of a reaction mix that reached the unit’s cooling water circuit. Detection of the pH drop allowed for channelling this water into the overflow basin and thereby avoided polluting the natural environment. Air pollution measurements (in NOx), recorded by a chemical emergency squad, proved negative. A safety perimeter was nonetheless set up. The damaged unit was closed for 8 to 10 days to allow for a complete verification of the installation. The explosion, which occurred following the 4th attempt at restarting the pump (after changing the fuse on the starter), was apparently due to the mix moving to inside the pump stator chamber subsequent to an electric flash between phases and/or ground. The contact created by the mix with both copper from the coil and iron from the stator chamber caused formation of an explosive mix of copper nitrate/nitrite and organic matter. During pump restart, another short-circuit apparently triggered the explosion. The use of immersed rotary pumps on the reaction mix was discontinued in favour of a magnetic drive pump, since this technology had already been implemented on the unit.