In a fertiliser factory, NPK fertiliser dust ignited while working with a cutting torch inside the pelletising workshop. Staff attempted to contain the accident using available extinguishers and sounded the alarm. The action of internal first responders protected by self-breathing apparatuses proved to be ineffectual. Fertiliser decomposition would in fact resume whenever sprinkling ceased. External fire-fighters provided backup. A thermal camera made it possible to localise the source: decomposition was occurring in an enclosed case (containing structural components, the concrete slab and edge fixtures of a conveyor belt undergoing dismantling), where a small quantity of fertiliser had accumulated. Use of a highly pressurised and very fine water spray led to stopping the decomposition, while cutting out metal sheets forming the case made it possible to snuff out the last remaining hot spots detected by the camera. At the same time, the factory’s sewer network was isolated so as to avoid polluting the LOIRE River. A power pump was installed to direct extinction water into the nitric acid storage retention basin. Despite all the precautions taken before initiating works intended to prevent this specific risk (i.e. cleaning equipment, obtaining work authorisations and hot work permits, building personnel awareness, allocating fire-fighting resources, etc.), the presence of fertiliser in this nearly hermetically-sealed container under the slab had not been anticipated. To mitigate the risks related to continuing these works, several corrective actions were adopted, including: enhanced monitoring, alteration to the openings in structures capable of enclosing fertiliser, drafting of a revised procedure relative to works execution. The factory operator also decided to install a thermal camera to improve first responders’ efficiency during an outbreak and streamline monitoring as worksites were winding down.