No one was sure whether the Chernobyl V.I. Lenin Nuclear Plant's emergency equipment and core cooling pumps would function through a shutdown until the diesel power system came online. Since a routine maintenance shutdown was already scheduled on Reactor #4 for April 25, 1986 , staff decided this would be a good time to run some basic tests. And, so, on April 24, they began powering down the reactor.
Later that evening they received a call from the Kiev grid operator requesting additional electricity. Chernobyl staff complied with his request, but forgot to restart the automatic control system which would take over the cooling system in the event of an emergency. At 11:10 p.m. Kiev no longer needed the additional electricity, and the staff resumed powering down. Unfortunately, they set the regulator incorrectly, and the power dropped from 1,000 megawatts to 30 - too low to run the test. At this point the test should have been abandoned; the reactor was not designed to run at such low levels, and the core quickly became contaminated with fission by-products, poisoning the reaction and producing instability.
However, to avoid confrontation with higher-ups, staff decided to increase the reactor's power. They did this by manually removing the control rods, graphite tubes which dampen the nuclear reaction and keep the core from overheating. The Chernobyl reactor was designed to work with a minimum of 30 control rods in place at all times; the technicians removed all but six. Within an hour the power output had risen to 200 megawatts… and the temperature in the core was becoming dangerously high. By 1:32 a.m. it became clear that things were getting out of hand. Somebody pushed the red emergency shutdown button, and motors began pushing all 205 of the control rods into the reactor in an effort to dampen the reaction. At this stage human error joined with flawed design. As the graphite rods slid into place they displaced the water coolant. In seconds the power surged from 7% of normal to 10,000% -- far more than the structure could handle.
When you take away all the bells and whistles, an atomic reactor is essentially a massive steam turbine, with nuclear fission providing the heat instead of coal. The explosion which took place at Chernobyl was not a nuclear explosion: rather, it was the inevitable result of large amounts of steam in a confined area, combined with gases from the now-melting reactor core. Most modern reactors are built within a "containment building," providing multiple layers of protection. The Chernobyl plant had no containment walls: as a result, the explosion sent chunks of radioactive material sailing throughout the complex. Superheated chunks of graphite landed on roofs around the plant, setting approximately 30 smaller fires around the smouldering crater which had once been Reactor #4.
Within a few hours firefighters were able to quell the smaller blazes; reactor #4 would continue burning, and sending radioactive material into the air, for several days. Elevated levels of radiation were detected throughout Europe. The Soviet bureaucracy was less than forthcoming with details, leading to rumors of thousands of dead and dying. Ultimately the official death toll provided was 31 dead, with over 300 cases of acute radiation sickness. Some scholars estimate as many as 20,000 people were exposed to levels of radiation sufficient to cause immediate threats to health, while millions received lower doses whose long-term effects remain unknown. The smoldering reactor was finally encased in a "sarcophagus" of concrete and steel: despite that, a 30-kilometer radius of rich farmland surrounding the accident site was evacuated and remains uninhabited to this day. The economic aftermath of Chernobyl was as devastating to the Soviet Union as anything else. Already reeling from the Afghanistan conflict, they found themselves forced to prop up farmers throughout the Warsaw Pact as the radiation cloud spread and the world banned imports from Eastern Europe. Chernobyl was yet another nail in the Soviet coffin and another step on the road to Communist bankruptcy.
Bad as things were, they could have been far worse. The damage was confined to one reactor; at one point it looked like the fire might spread to the nearby units. In the years afterward, the area around Chernobyl has become a bit of a wildlife preserve. Freed from human habitation, the lush green fields and recovering forests have become home to many animals. Some scientists claim the Chernobyl region is better off than many of the other ecologically devastated areas of the post-Soviet Union. There is little in the way of chemical damage and industrial pollution: all that remains there is a bit of background radiation.