At around 5:50 a.m., a 3 cm hole formed on a tube of a household waste incineration plant’s boiler. The 10 t of water above the hole immediately vaporised, causing the pressure in the furnace to rise. The mixture of flue gases and steam poured uncontrollably from the bottom ash hoppers and through the furnace’s interstices. The operator shut down the incinerator and lowered the furnace’s temperature by closing most of its air inlets. Once the temperature was under control, the waste was sprayed with a fire hose through the furnace’s sight glass to accelerate cooling. The boiler was placed back at negative pressure at around 9:00 a.m. Firefighters arrived at the plant but did not have to intervene.

The hole formed on a tube located immediately near a urea injector (urea is used to treat nitrogen oxides). The walls of the other tubes near this point had thinned. The leak may be related to corrosion of the tubes from urea splashes. The tubes had been monitored for wear. Their replacement by tubes that resist corrosion better was scheduled for October 2017. Pending this work, the operator replaced the punctured tube as well as eight tube sections considered vulnerable and conducted a hydrostatic test at the boiler’s operating pressure.

An approved air-quality monitoring association collected air samples around the site. The inspection authorities for classified facilities asked the operator to analyse the data from these samples and look for any impacts related to the accident. Plants, lichens, and soil were also analysed.