An empty 100-m³ reservoir of sulphuric acid (H2SO4) exploded at 9:15 am at a chemical site. Installed directly above its retention basin at a height of several metres, this “F2” reservoir was projected and fell nearby, toppling the scaffolding set up for reservoir maintenance and causing 3 workers (2 subcontracted personnel) to fall. Two of the three were seriously hurt: the plant employee sustained cranial trauma after falling some ten metres; and a subcontractor was pinned between the tank and the scaffolding, injured to the face by the grinder he had been using. The internal emergency plan was activated, bringing both internal and external responders to the site. The injured were taken to hospital. The operator notified the Prefecture, local city halls and the general public. The classified facilities inspectorate initiated an investigation.

The reservoir had been temporarily plugged on 18th July using a sealant box after discovering a leak on 17th July. Scheduled for repair at the beginning of August following its drainage, the tank was rinsed with water throughout the weekend of 2nd August, then a subcontractor installed the scaffolding for the specific intervention: reservoir access, process insulation plating, etc. A plant employee, accompanied by 2 subcontractors, then assembled the tank in order to open the dome when the explosion occurred.

The accident appears to have followed a hydrogen (H2) accumulation at the top of the tank due to ignition of the inflammable mixture formed with air when cropping corroded bolts from the dome manhole with a grinder. The tank was torn over half the circumference of the shell/bottom junction, and its anchorages were stripped. The tank had only been rinsed once, which strengthened the hypothesis of insufficient tank rinsing causing the acid attack of metal with an H2 release.

This reservoir had been repaired in 1989 following an incident that caused delamination without failure of its fastenings, due to a CO2 release during sulphuric acid neutralisation (ARIA 23705); an angle bar had been added and the cleaning procedure had been modified.

Available feedback pointed to several H2 explosions after a steel reservoir attack with diluted acid at other plants (ARIA 169, 22278, 31082).

Given the succession of accidents arising over the previous few months in the chemical and petroleum industries, combined with the pipeline transport of hazardous substances, a meeting was organised in September 2009 to discuss industrial safety challenges and environmental protection between the Secretary of State for Ecology and industry leaders in these sectors. The industry delegation submitted proposals for improving the safety of their facilities, led by enhanced ageing controls on installations and their maintenance, while committing to greater focus on sensitive ecological zones in order to improve species protection and preservation zones.

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