Preliminary events

The nuclear power plant situated 20km from Chernobyl in northern Ukraine employed reactors of the type RBMK-1000, a graphite-moderated pressurised water reactor cooled with light water. Routine testing of the electrical network in reactor unit 4 started a tragic chain of events that caused the reactor to melt down and explode.

Nuclear power stations not only generate power, they also consume it. When a reactor supplies its own power grid, it needs to be designed in a way that there is always sufficient electricity to shut down the reactor in case of an emergency. A test had been scheduled to determine if the remaining electricity generated by the turbine after being run down could bridge the time needed to start the emergency diesel generators (approximately 40-60 seconds).

A similar test conducted previously in unit 3 had failed, because the voltage had dropped too rapidly. The new test, scheduled to take place during a routine reactor shutdown, was to check if an improved voltage regulator would remedy the problem. The consequences proved to be fatal.

At 1.00am (CET) on Wednesday 25 April, the first stage in reducing the reactor’s power output from the nominal level of 3,200 megawatts (thermal) down to 1,000 megawatts was carried out. This power level reduction was normal in routine reactor shutdowns. At 1:05pm, the electrical load dispatcher in Kiev instructed the plant to stabilise the power output at 1,600 MW. The second reduction stage was to be carried out 10 hours later. At 12:00am, the plant personnel changed shifts. The tragic series of events leading to the reactor meltdown followed almost immediately.

(CET - Central European Time)