An empty 100-m reservoir of sulfuric acid (H2SO4) exploded at 9:15 am at a chemical site. Installed directly above its retention basin at a height of several metres, this 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 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.17th July, a leak of sulfuric acid was detected from the reservoir. 18th July, the leak is closed as temporary solution. 31st July, the reservoir is emptied and rinsed with water3rd August, the reservoir is emptied and rinsed with water, after which a subcontractor installed a scaffolding for the specific intervention (reservoir access, process insulation plating, etc.). 4th August, a plant employee, accompanied by 2 subcontractors, opens the dome when the explosion occurred (a grinder was used to unscrew the bolts).The accident appears to have been causes the release of hydrogen accumulated at the top of the reservoir, ignited by the spark of the 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.