In a plant manufacturing iodised products, the safety disk of a hydriodic acid (HI) synthesis reactor failed just after the reactor had been shut down and secured. Between 2 and 3 kg of hydrogen iodide released into the atmosphere escaped into the open air via the doors of the building that remained open. Driven by the North-East wind, the opaque cloud made its way outside of the establishment and headed towards the other companies in the industrial area, where the staff were asked to stay inside, and crossed a minor road, fortunately without causing any accidents. It dissipated within10 minutes. During this time, the building’s doors were closed to keep the gaseous discharge inside and progressively treat it using the gas scrubbing unit. No human or environmental consequences were reported. A leak on an isolation valve in the reactor’s steam-heating circuit caused the accident: the undesired heating of the reactor caused the temperature and pressure to rise and the rupture disk to burst. Furthermore, the incorrect attachment of the disk’s discharge pipe caused HI to be discharged inside the building whose doors should have been closed. Finally, the organisation system ensuring the allocation of tasks in the event of an incident was inadequate (diagnosis too slow, no involvement by some of the staff in managing the incident, etc.). The corrective actions implemented to reduce the likelihood of a recurrence of such an accident include the following: installation of double isolation valves on the steam line, attachment of the discharge pipe on the rupture disk, revision of the operating instructions to make sure that the building’s doors remain closed, and revision of the incident organisational system (organisation chart, definition of each employee’s specific responsibilities, and an annual simulation of incidents), etc.