Icmesa chemical company, Seveso, Italy. 10th July 1976

Accident summary

At approximately 12:37 on Saturday 10th July 1976 a bursting disc on a chemical reactor ruptured. Maintenance staff heard a whistling sound and a cloud of vapour was seen to issue from a vent on the roof. A dense white cloud, of considerable altitude drifted offsite. The release lasted for some twenty minutes. About an hour after the release the operators were able to admit cooling water to the reactor.

Among the substances of the white cloud released was a small deposit of TCCD, a highly toxic material. The nearby town of Seveso, located 15 miles from Milan, had some 17,000 inhabitants.

Over the next few days following the release there was much confusion due to the lack of communication between the company and the authorities in dealing with this type of situation.

No human deaths were attributed to TCCD but many individuals fell ill. A number of pregnant women who had been exposed to the release had abortions. In the contaminated area many animals died.

Failings in technical measures

• The production cycle was interrupted, without any agitation or cooling, allowing a prolonged holding of the reaction mass. Also, the conduct of the final batch involved a series of failures to adhere to the operating procedures. The original method of distillation patent specified that the charge was acidified before distillation. However, in the plant procedures the order of these steps was reversed.
• Operating Procedures: safe operating procedures
• The bursting disc was set at 3.5 bar, and was to guard against excessive pressure in the compressed air that was used to transfer the materials to the reactor. Had a bursting disc with a lower set pressure been installed, venting would have occurred at a lower and less hazardous temperature.
• Relief Systems / Vent Systems: venting of excessive pressures, sizing of vents for exothermic reactions
• The reactor control systems were inadequate both in terms of the measuring equipment for a number of fundamental parameters and also in the absence of any automatic control system.
• Control Systems: sensors
• Alarms / Trips / Interlocks: loss of cooling, agitator failure
• The company was aware of the hazardous characteristics of the principal exotherm. However, studies showed that weaker exotherms existed that could lead to a runaway reaction.
• Reaction / Product Testing: calorimetry methods, thermal stability
• There was no device to collect or destroy the toxic materials as they vented. The manufacturer of the bursting disc recommended the use of a second receiver to recover toxic materials. No such vessel was fitted.
• Design Codes - Plant: nature of hazardous releases
• Secondary Containment: catchpots
• Information on the chemicals released and their associated hazards was not available from the company. Communication was poor and failed both between the company and the local authorities and within the regulatory authorities.
• Emergency Response / Spill Control: safety management system, site emergency plan

References

Lees, F.P., ‘Loss Prevention in the Process Industries – Hazard Identification, Assessment and Control’, Volume 3, Appendix 3, Butterworth Heinemann, ISBN 0 7506 1547 8, 1996.