On 26 April 1986 reactor number four at the Chernobyl Nuclear Power
Plant, near Pripyat in the Ukrainian SSR, suffered a massive,
catastrophic power excursion resulting in a steam explosion which tore
the top from the reactor exposing the core. The plant had no
containment structure as does, for example, the three-mile-island
reactor. Further explosions and the resulting fire sent a plume of
highly radioactive fallout into the atmosphere and over an extensive
geographical area. Four hundred times more fallout was released than
had been by the atomic bombing of Hiroshima. The death toll has been
estimated to be up to 4000 but only about 56 deaths are directly
attributable to the accident.
The accident occurred because a new safety device was being tested during shutdown at the end of the plant's first fuel cycle. The device was supposed to supply electricity to cooling pumps for 60 seconds during abnormal shutdown. This was necessary because it takes 60 seconds for backup diesels to get up enough power to do the job. Without power for just a few seconds the lack of coolant would cause a major catastrophe. The catastrophe would be the worst possible (for technical reasons) if most of the fuel had already been spent.
Some political pressure resulted in the reactor going online before the device was adequately tested and it was not installed. The director of the Chernobyl station signed a document declaring the reactor ready - if he had not done so thousands of people would not have received bonuses or other awards. Records were falsified to hide all this.
The plant operated for two years without the safety device. At that point the plant had to be shut down to refuel. The station managers saw this as an opportunity to install the device and test it. Procedures for the test were planned. The day shift was quite competent and familiar with these procedures and the shut down was scheduled during that shift.
As fate would have it, a regional station suddenly went offline and Chernobyl was requested to stay online to make up the difference until demand diminished sufficiently. It wasn't until 11:00PM that permission was given to continue the shut down. By that time the day shift was home, the evening shift was packing up and the night shift was about to start work. The shutdown process would have to continue while one shift was leaving and the other arriving. The night shift did not understand the procedures and was not prepared for the shutdown. The following dialogue between two personnel was recorded:
|worker: What shall I do? In the programme there are
instructions of what to do, and then a lot of things are crossed
boss of worker: Follow the crossed out instructions.
The sequence of events during the shutdown was as follows: