Fire broke out inside a chemical plant when completing drainage of a Lactam-12 storage cell within a retention basin prior to a 4-week technical shutdown of the installation. External fire-fighters and employees controlled the outbreak using 2 variable-speed nozzles in less than 2 hours. The fire was moderate in magnitude and consequences were minor: one employee was slightly burned by an ignited particle; no environmental deterioration was reported, and property damage was limited to equipment present adjacent to the basin (pipe heat insulation, electrical cables, etc.). Two tonnes of lactam were drained from the storage facility via a pipe whose end flange had been deliberately loosened to generate seepage of the liquid chemical product (heated to 190°C) into the water containment basin in order to solidify the substance. At the end of tank drainage, lactam processing speed was increased at the level of this flange (4 m/s) due to the fact that the medium was transitioning from a simple liquid phase into a vapour / liquid / solid mix, thereby causing an accumulation of static electricity on the drain pipe. A disruptive discharge between the pipes on one side and on the other the spaced flange (given the absence of electrical continuity as a result of the spacing and the insulating nature of lactam) was responsible for igniting lactam vapour around the flange (flash point: 175°C). This exceptional operation had not been included in a preliminary risk analysis, nor had it been addressed in special operating guidelines. The previous operation of this type had been conducted over 5 years prior using a sampling tap that was plugged on the day of the accident. The Classified Facilities Inspectorate logged these observations.