Chernobyl was the world’s worst nuclear power accident. On the 25th – 26th April 1986 at the nuclear power plant in Chernobyl, Ukraine around 83 miles outside of Kiev the unthinkable occurred. The Chernobyl nuclear power plant had 4 reactors, while preforming what should have been a routine safety test, standard safety procedures where ignored while testing reactor number 4. At around 1:23am reactor 4 became lost control and with it began a chain reaction, of destruction. The reactor exploded which in turn blew off the reactors steel and concrete lid.
While real figures are hard to come by its thought that the Chernobyl disaster killed approximately 30 people immediately, radiation levels in the area spiked to astronomical levels which resulted in the evacuation of around 135,000.
A timeline of the events that led up to the Chernobyl Disaster.
On the 25th April 1986 reactor 4 was scheduled to be shutdown, the shutdown was planned in order to perform some routine maintenance work. The workers decided to take advantage of this short downtime to run an test on the reactor to see if in the event of a shutdown enough power would be generated to operate the core cooling pumps and emergency equipment until the diesel generators kicked in.
Reactor 4 began to be shut down, the reactor was running around half power when the electric load dispatcher refused to allow a continued shutdown. The reactors emergency core cooling system had been turned off as part of the test while the reactor continued to run at half power. At around 11pm the power was reduced further, agreed to by the grid controller.
The reactor was supposed to have been stabilised at 1,000MW before shutdown but fell to around 30MW due to operational errors. At 30MW the positive void coefficient became a problem. The operators realised that there was a problem, they tried to raise the power by manually freeing many of the control rods at 1am on the 26th April which resulted on the reactor stabilising at 200MW.
A short time later a drop in stream pressure combined with an increase in coolant flow occurred resulting in the operators to rapidly try to remove nearly all the remaining control rods. The reactor became dangerously unstable which required the operators to make constant adjustments to maintain power.
Next the operators reduced the reactors flow of feedwater in an attempt to maintain steam pressure, while the pumps that provided cooling to the reactor where slowing, becoming less and less effective which created additional steam in the key cooling channels. The reactors power surged to around 100 times its normal power. The power surge and increased temperature caused the fuel to rupture, fuel particles combined with the water creating a huge steam explosion destroying the reactors core. A second explosion followed the first to add to the devastation.