Accidents
Chernobyl Nuclear Reactor Melt Down
A series of six human errors committed during testing at the Chernobyl nuclear power plant led to the 1986 accident which claimed 31 lives
The chain of errors began on April 25th during tests designed to find out how long stable power could be maintained in the event of a power failure that required a switchover to a diesel generator. Normally, the generator is powered by steam from the reactor.
"It sounds paradoxical now," a commentator said, "but they (the staff at Chernobyl) were concerned about a power failure where the reactor could become non-operational. There was a time lag between the switching off of the electrical system and the start up of the diesel system."
The official investigation described the six critical human errors as follows:
* The number of reserve control rods in the reactor core was allowed to drop below permissible levels, making it more difficult to control the reactor.
* The automatic controls for the reactor's power level were shut off. This allowed the power to drop below safe levels, also rendering the nuclear reaction more difficult to manage.
* Both the main water-circulation pumps and the backup pumps were turned on at the same time, forcing the coolant to flow too quickly; this created air bubbles which hampered the cooling process.
* In attempting to prevent a reactor shutdown, the staff cut off automatic blocking devices that would have shut off the reactor when steam failed to reach the generator.
* Staff also switched off systems that controlled water level and steam pressure.
* Staff turned off the emergency safety cooling system for the reactor.
Such a sequence of human actions was "deemed so unlikely by the engineers of this project" that they designed no technical system to defend against it”.
This is a classic example of Prof James Reason’s SWISS CHEESE model of accident causation.
COST: UNKNOWN
DEAD: 31
ENVIRONMENTAL DISASTER
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