During the preparation of reagents (240 litres of hydrogen peroxide at 35%, 100 litres of demineralised water and 1 kg of copper sulphate) to treat liquid effluent in an annex building of a nuclear power plant, a major vapour release occurred around 3 pm. The fire alarm sounded. In accordance with procedure, the nuclear zone was evacuated, both internal first responders and the local fire department were called (arriving respectively at 3:05 and 3:15), and management was notified. Vapour permeated the room and partially condensed on the walls. The 9 employees on duty were examined and showed no signs of injury. After acknowledging the absence of fire at 3:25, in-house responders injected the reagents into the tank of effluent to be treated in order to stop the vapour release. Two of these responders who had entered the room felt itchy hands after touching the door handle (covered with condensate) despite wearing individual protective gear. This irritating sensation disappeared following washing with water. At 9:30 pm, measurements taken by the fire-fighters’ mobile chemical emergency squad indicated normal air quality in the rooms adjacent to the impacted zone. Management authorised access to this zone as of 10:40 that evening and initiated rinsing operations at 12:20 am on 6th Sept. No damage was reported.

The plant operator’s analysis ascribed the cause of this vapour release to an exothermic disproportionation reaction of the oxygenated water, triggered by adding copper sulphate in solid form into the reagent preparation tank. This mix was used to treat non-contaminated fluids that had transited in the “secondary” part of a vapour generator. Hydrazine, which had been incorporated to protect the metal from corrosion while capturing oxygen, in turn had to be neutralised before discharging the liquid. When the accident occurred, the procedure detailing the operating protocol had not integrated this risk of disproportionation from two products coming into contact. Although copper sulphate in soluble form had been added in the past without any complications, this accident arose upon introducing copper sulphate in solid form for the first time. The operator proceeded to: update the procedure, raise awareness among chemical unit personnel of the hazards related to oxygenated water, and disseminated these lessons to the Group’s other divisions.

The operator, along with the Prefecture, Sustainable Development Ministry, and ASN and IRSN nuclear agencies, all issued press releases. This event was rated a level “0” on the INES scale.