A small hydrofluoric acid (HF) leak occurred at a chemical plant producing uranium tetrafluoride from uranium ores while in the presence of HF. At 11:45 am, an alarm (whose 1st threshold had been reached) detected the presence of HF in the anhydrous HF storage building refrigerated to 5°C. A visual inspection, triggered by this alarm, failed to identify any leak. At 1:30 pm, the 2nd threshold was reached. The plant operator noted the presence of a minor leak at a weld bead on the wall of the black steel backup tank, along with a 1-litre pool of HF in this tank’s retention basin. A clogging layer was placed at the leak site, and the spilled HF was absorbed using a Carcel lamp, before cleaning and neutralising the retention basin. The internal emergency plan was not activated. This leak was caused by a crack on a weld bead of the black steel storage tank (not subjected to regulations on pressure vessels). Ultrasound controls, performed on the tank by the operator in October 2002, had not detected any anomaly. The inspection authorities for classified facilities requested the operator to investigate the cause of this crack in great detail and present conclusions on the origins, consequences and measures to be implemented.