In a synthetic rubber plant, a valve broke on a 12.5 m³ tank half filled with butadiene; 5 m³ of the substance were released into the atmosphere. Production was halted and the internal emergency plan activated (under the scenario of an internal/external plan). All traffic around the plant was prohibited and rail traffic (Paris-Bordeaux high-speed trains) was blocked for 3 hours. A strong smell was noticeable beyond the site boundary. Considerable human resources were deployed to control the leak. Explosibility measurements proved to be negative. The suspected valve (diameter: 50 mm, nominal pressure: 16 bar) used to bypass a remote controlled valve had remained closed during the accident. Polymerised butadiene was found in the cap and above the gate of the valve that failed due to brittle fracture of the cast iron following a pressure surge related to the volume increase caused by polymerisation. A manual valve upstream of the defective valve, immersed in the butadiene layer surrounding the tank, could not be operated to stop the leak. During a preventive maintenance service performed 3 weeks prior, it was noticed that the defective valve had locked. Repairs planned for the subsequent week had yet to be completed. An action plan was adopted, involving: an inventory of all valves in comparable situations; a critical analysis of the given installations; a facility status report; monitoring rounds; and the installation of steel valves.