On April 26th, 1986, Reactor 4 of the Chernobyl nuclear reactor exploded. It was caused by a combination of a test gone wrong and the design of the reactor, and its effects amplified as a result of Soviet policy.
About 7% reactor's heat is produced by the decay of fission products, even when there is no fission occurring. Though this is small, it is still enough that an inactive core still needed to be actively cooled by water pumps. The reactor had diesel generators to operate the water pumps, but the time it took for the generators to to fully rev up was between 60 and 75 seconds, long enough to be considered a hazard.
It was thought that the residual energy of the steam turbine could be utilized to bridge the gap, and three tests were run - one in 1982, another in 1984, and the third in 1985 - by cutting off the flow of steam to the turbine, but these ended in failure despite numerous modifications. The test procedure required that the reactor's thermal output would gradually be reduced from 3,200 megawatts to 700 megawatts before the steam was shut off. On April 25th, a fourth test was to be done, but was postponed at 4:00 PM when a regional power plant in Ukraine shut down. Despite this, preparations for the test were still carried out, which included the shutting down of an emergency cooling system.
At 11:04 PM that evening, the go-ahead was given to resume shutting down of the reactor, but instead of the gradual shutdown that was planned, the control rods were inserted rapidly, leading to a buildup of Xenon-135, a "poison" that inhibits the nuclear reaction, causing a greater than expected power drop. Control rods were removed to compensate.
The test began at 1:23:04 AM. Shift supervisor Alexander Akimov opposed the test, but his objections were overridden by Anatoly Dylatov, Vice Chief Engineer (who would serve five years in prison as a result of his role in the disaster). As the turbines wound down, so did power to the pumps. At 1:23:40 AM, the reactor was "Scrammed" and the control rods were inserted into the core. The control rods were tipped with graphite, which increased fissioning in the core, and displaced the neutron-absorbing water, compounding a problem caused by steam voids (bubbles) in the water. Both fuel and control rods fractured under the intense heat, stopping them at 1/3rd their way into the core. The water inside the core boiled with explosive force, and the tips of the rods, made of graphite, ignited in the intense heat. A second, more powerful explosion occurred three seconds later, caused either by a runaway chain reaction in the nuclear fuel or the production of hydrogen. The explosions threw radioactive material into the air, while the fires sent more up in the smoke.
The firefighters were not informed of the radiation danger, though even when it became known, some decided to "kamikaze" themselves. The population of nearby Pripyat however were not warned and the city was evacuated until 11AM that morning, when illness struck some of the residents. Despite the policy of Glastnost, the Soviet government maintained a tight restriction on the media, and the disaster was not admitted to the outside world until two days later, radioactive fallout from Chernobyl set off radiation alarms in Sweden. In fact, Reactor 1 of the Chernobyl complex suffered a partial meltdown in 1982 that itself was not publicly known until 1985.
|The Elephant's Foot, made of previously molten fuel and concrete|
31 people died directly from the radiation, mostly firefighters sent to extinguish the fire, and at least 4,000 cases of cancer are believed to be attributed to it. The cost of cleanup was a major factor in the fall of the Soviet government.
Though there has been an exclusion zone around the reactor complex ever since the disaster, the other three reactors continued to operate for years after the disaster. Reactor 2 operated until 1991 when it was decommissioned after a fire, while reactor 3 was decommissioned in 1996 and the final reactor shut down in 2000.
Though the exclusion zone remains uninhabited by people, life has returned.