A blast due to ignition of a coolant fluid (eutectic mix of diphenyl oxide and biphenyl) took place in a nylon factory. An abrupt pressure drop was recorded in the primary circuit supplying calories to most facility installations. Between 3:34 and 3:42 pm, pressure in the circuit decreased from 7.4 to 7 bar. A log of the coolant fluid level variation concerning both expansion tanks connected to the circuit made it possible to evaluate the coolant loss at 600 litres. This loss was limited by the automatic closing of 2 pneumatic shut-off valves on the primary circuit section, where the pressure drop had occurred. The coolant fluid leak was localised at the level of a spinning workshop, only recently placed into service, following the break of a flange weld on a double casing pipe. Flowing out by gravity, the coolant (300°C) had made contact with electrical cables (possible formation of an electric arc). The highly confined blaze released a black plume of smoke. The emergency information procedure and internal emergency plan were both activated. Alerted by fire detectors, 2 employees carried out a successful initial intervention with 10 extinguishers (7 powder and 3 CO2). Fire-fighters brought the blaze under control with foam and 2 small hoses. The RD7 departmental highway was closed by precaution and 2 adjacent stores were evacuated. The fire was extinguished at 5 pm. Structural damage was limited to the building roof and electrical installations (cabinets, motors, cable paths and regulation devices). No injuries were reported, just 3 employees from adjacent stores experienced irritation in their throats and were taken to the teaching hospital for observation. The particular unit was shut down for 2 months, but no other site activity was affected. An internal investigation was conducted, as were insurance and judicial appraisals. The factory operator responded by installing organic vapour detectors and sprinklers.