Biomedical Engineering Reference
In-Depth Information
Seveso
Plant
Disaster
This case is an example of a toxic cloud and is interesting and instructive as an example of how an
engineering failure can threaten public safety. As such, it can be used as a case study of engineers
not adequately attending to the first canon of the profession, i.e., to hold paramount the health, safety,
and welfare of the public. It was indeed a harbinger of the toxic scares that paved the way for
new environmental legislation around the world, calling for programs to address toxic substances and
hazardous wastes. It was also emblematic of potential exposures to toxic substances to those millions
of people living near industrial facilities and of the community “right-to-know” laws that have since
emerged. But, it appears here because the disaster represents so much more than the event itself in terms
of why it occurred, what were the real ethical issues, and consequently how risks from such incidents
can be better managed in the future.
On 10 July 1976, an explosion at a 2,4,5-trichlorophenol (2,4,5-T) reactor at a manufacturing plant
near Seveso, Italy resulted in the highest concentrations of the most toxic chlorinated dioxin (TCDD)
levels known in human residential populations.
25
Up to 30 kilograms of TCDD were deposited over the
surrounding area of approximately 18 square kilometer. To put this amount in perspective, scientists
usually are worried about dioxin, and especially TCDD, if a gram or less is released in a year. This
release of 30 000 grams occurred instantaneously.
The plant was operated by Industrie Chemiche Meda Societa, Anonima (ICMSA), an Italian subsidiary
owned by the Swiss company, Givaudan, which was owned by another Swiss company, Hoffman-
LaRoche.
26
The release resulted from a ruptured disk on a batch plant. The 2,4,5-T was formulated
from 1,2,4,5-tetrachlorobenzene and caustic soda (NaOH) in the presence of ethylene glycol. Normally,
only a few molecules of dioxins form from several trillion molecules of the batch, but in the events
leading up to the explosion, the reactor temperatures rose beyond the tolerances, leading to a “runaway
reaction,” bursting the disk. It is likely that a large volume of hydrogen gas (H
2
) was formed, which
propelled the six tons of fluid in the reactor into the Seveso region. Actually, the discharge could have
been worse, since upon hearing the noise being made at the beginning of the release, a plant foreman
opened a valve to release cooling waters onto the heating coils of the reactor. This action likely reduced
the propulsion.
The explosion is in part an example of doing the wrong thing for the right reason. The Italian
legislature had passed a law requiring the plant to shut down on weekends, whether or not a batch was
complete. On the weekend of the explosion, the plant was shut down after the formulation reaction, but
before the complete removal of the ethylene glycol by distillation. This was the first time that the plant
was shut down at this stage. Based upon chemistry alone, the operator had no reason to fear anything
would go awry. The mixture at the time of shutdown was at 158
C, but the theoretical temperature at
which an exothermic reaction would be expected to occur was thought to be 230
C. Subsequent studies
have found that exothermic reactions of these reagents can begin at 180
C, but these are very slow
processes when below 230
C. The temperature, in fact, rose mainly due to a temperature gradient from
the liquid phase to the steam phase. The reactor wall in contact with the liquid was much cooler than the
wall in contact with the steam, with heat moving from the upper wall to the surface of the liquid. The
stirrer was switched off so that the top few centimeters of liquid rose in temperature to about 190
C,
beginning the slow exothermic reaction. In 7 hours, the runaway reaction commenced. The reactions
may also have been catalyzed by chemicals in the residue that had caked on the upper wall that, with
the temperature increase, was released into the liquid in the reactor. Thus, the runaway reaction and
