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Chernobyl Disaster (April 26, 1986)

TMI’s near meltdown of 1979 was bad; Chernobyl’s explosion in 1986 was worse. In fact, the incident at Chernobyl was the worst nuclear disaster, ever. Claiming a total of 56 lives, directly, between 1986 and 2004, the Chernobyl event still had little impact on anyone outside of the Eastern Bloc. Of the 56 who died, none of them were off-site. The event did cause irreparable damage to areas of Belarus, Ukraine, and Russia.

Events Leading to the Explosion

According to the World-Nuclear’s site regarding Chernobyl, the explosion occurred from a lack of training of the personnel, as well as a lack of general understanding of nuclear power. The reactor itself was flawed by design; the operators of the plant were ill-advised; the crew were untrained. All of these factors added together and came to a spearhead on April 26, in the early morning hours.

Reactor 4 was scheduled to be shut down for routine maintenance, having reached the end of its fuel cycle. The operators of the facility wanted to test a safety idea—if the main grid to the plant shut down, it would leave about a 1 minute gap between the outage and the generator’s ability to reach full-capacity to cool the reactor core. One minute was not acceptable.

Other errors included the following:

The Explosion

With the increased reduction of power due to the control rods being manually inserted beyond the point required for the experiment, the reactor would have entered into automatic-shut down—except that feature had been disabled in order to conduct the experiment. When the reactor dropped to 5% of its power, the rods were pulled out to increase the pressure in the core. However, what no one realized was the fact that high levels of xenon-135 began absorbing neutrons, inhibiting nuclear reaction. This was very bad.

The experiment continued, even though the power output was too low. More water rushed into the core, exceeding the safe limit at 1:19am on April 26. The core’s temperature did drop, however, because water absorbs neurons, the additional water reduced the reactor’s power output, so, the operators removed the control rods, but not completely.

This, of course, created a very unstable reactor unit, but the crew had no idea. The experiment began at 1:23am with the shut off of the steam turbines. The steam turbines controlled the flow of water, so the water stopped flowing, but the reactor was still producing heat. The control rods were never fully removed, so they were blocking the heat from reaching the coolant. This caused the reactor’s power output to jump, sending water into flash-steam. The water was still absorbing neutrons and the reactor core was still getting very hot. All very bad things, even if you don’t understand the technicalities.

At 1:23am (about 40 seconds after the experiment began), the reactor output was rapidly increasing. Operators pressed the AZ-5 button, which ordered a SCRAM (emergency shutdown of a nuclear reactor), though the reasoning for it was never certain. Sadly, the SCRAM had the opposite effect on the reactor. Because the control mechanism operated a slow speed (20 seconds to go 7 meters), and had a flawed tip, the SCRAM actually increased the production, overheating the core. The heat and pressure needed somewhere to go, so at 1:24am, the first of two steam explosions occurred, blowing the 2,000 ton lid off the reactor.

Aftermath

As a result of the explosion, two men died instantly, and another 52 were hospitalized and died later from radiation poisoning. The Soviet government tried to cover up the explosion and downplay the significance—they hadn’t even evacuated nearby Pripyat. When they finally agreed that the Chernobyl incident was massive, they informed the residents of Pripyat that they needed to evacuate for about three days. Pripyat remains unoccupied to this day.

For more information, please see the following sources:
Pripyat, Ukraine
Chernobyl (world-nuclear.org)
Chernobyl Disaster

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