The Chernobyl accident

Unit 4 of the Chernobyl nuclear power plant was to be shutdown for routine maintenance on 25 April 1986. On that occasion, it was decided to test whether the residual capability of the turbine was enough to provide electrical power for the reactor core cooling system and emergency equipment during the shutdown of the reactor until the emergency power generator was fired up.

Unfortunately, this test was carried out without proper co-ordination between the operators and the plant safety personnel, resulting in inadequate safety precautions in the test. Moreover, the operators took a number of actions which bypassed some of the safety systems, causing a potential dangerous situation. This course of actions together with the design defects of the reactor led to loss of control of the reactor and subsequently the accident.

The reactor when operated at low power exhibited a sudden and uncontrollable power surge. Without emergency cooling and a sufficient number of control rods inserted, the power surge resulted in violent explosions and almost total destruction of the reactor. The consequences of this catastrophic event were further worsened by the fact that the graphite moderator and other materials caught fire. This contributed to a widespread and prolonged release of radioactive materials to the environment.