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Do you know human-error can cause the deaths of thousands people? Now, I’ll tell you the worst nuclear accident, Chernobyl. This accident is the explosion of nuclear reactor near Pripyat, a small city in Ukraine, in April 26, 1986. This explosion caused direct deaths for 46 workers and more than 600,000 peoples have been contaminated, and about 4000 of them died because of thyroid cancer in 1986-2004 periods.

I’ll tell you a little about how nuclear reactor works. In the core of reactor, there are radioactive materials that produce heat continuously. To keep reactor core in the safe temperature so there’s not overheated, coolant water must be flowed continuously. In reactor 4 Chernobyl (that exploding happened), there consisted of 1600 fuel channel and each of them need 28 tonnes of coolant water per hour.

That have been planned, in daytime April 25, 1986, reactor 4 will be shutted down for routine maintenance and an experiment will be held to test emergency core cooling safety during shut down process. Although reactor is in shut down condition, radioactive materials inside continue to produce heat and hence control rods must be entered here to stop it, and then removed by coolant water.

It’s the chronology about fatal human error that cause explosion of reactor 4 and blow so many radioactive materials.

Daytime, April 25, 1986
A team of engineers had arrived to test the new voltage regulating system. During shut down, the output of reactor was decreased about 50%. Unfortunately, a regional power station went offline. It can cause lack of producing electricity, hence Kiev grid controller commanded to delay the test.

April 25, 11:04 pm
This delay caused serious result. According to plan, the test should end in the daytime and in the night it just for maintenance after shut down. But because of the delay, the test was held in the night. Thus the procedure for test in the night wasn’t prepared. At that moment, there are two people who were responsible for the night shift: Alexander Akimov (the chief) and Leonid Toptunov (the reactor’s operational manager).

Because of the test, the reactor’s output was decreased, from 3200 MW to 70-100 MW.
Toptunov did a serious mistake, entered too many control rods, thus the reactor was near shut down condition (the output became 30 MW).

In that condition, the test was impossible to be held. And there was a condition that they didn’t realize: in the lower output condition like that, xenon-poisoning happened, where much of xenon-135 absorb neutrons and hence inhibit nuclear reaction.

The operators that unaware it, thought that the cause of low heat producing was too many control rods. Then they pulled out the control rods and turned off automatic control rods system, but because of xenon poisoning that inhibit nuclear reaction, the output was just 200 MW. Then the test was continued.

April 26, 1:05 am
Coolant water activated, as the routine procedure.

1:19 am
Coolant water exceeded safe limit, caused decreased heat producing and steam void. This is caused operators removed all the control rods manually.

Control rods removal caused unstable reactor condition. Steam production failure happened. In that condition, shut down should be activated but the operators disabled the shut down system.

1:23:04 am

Coolant water flow decreased and steam generators produced heat again. Unfortunately, the control rod remnants inhibited radioactive materials to reach the coolant water. Consequently, overheated happened, exceed that xenon can be absorbed. Within automatic and manual control rods disable, nothing can avoid this overheated condition.

1:23:40 am
Operators pressed AZ-5 (Rapid Emergency Defense-5) and activated SCRAM system (to shut down reactor and enter all the control rods). Unfortunately, this mechanism worked slowly, about 18-20 seconds. Ironically, the erroneous graphite tip control rods design caused control rods to remove the coolant water before nuclear reaction decreased. This condition actually increased nuclear reaction rate, therefore nuclear core overheated. Some of control rods fractured and trapped the next control rods that should be entered. In three seconds the output became 530 MW.

1:23:47 am
Seven seconds afterward, the output became 30 GW, ten times from the normal output. Consequently, it ruptured fuel channels and the pipes that was used to flow coolant water.

1:24:20 am
The first steam generators explosion happened and destroyed reactor’s roof, blew radioactive materials in the air. This explosion caused more fuel channels, control rods, and pipes destroyed.
1:24:23
Second explosion happened. More radioactive material released, and the reactor’s roof that is composed by graphite aggravated it.

All operators that work died within one week. They worked without protective suit. In short time after the accident, firefighter team arrived to extinguish this reactor. Ironically, they didn’t know if this wasn’t a regular fire. “We don’t know if it’s a nuclear reactor. No one tell us”, said Vladimir Pravik, the commander of that firefighter, that dies in May 9, 1986 because of acute radiation.

In 5 am the fire can be extinguished, but the fire inside the reactor continued until May 10, 1986. That fire was handled by many helicopters that dropped more that 5000 tonnes of sand, lead, clay, and boron, and also sprayed liquid nitrogen.

The eyewitnesses, before they died, were asked how that they felt about the radioactive contamination. They said that they tasted “metal sensation”, and they felt their face was sticked by thousands of pins and needles.

The next results, all of Pripyat residents were evacuated. Victims were inevitable. Twenty seven workers in Moldova died because of thyroid cancer in that year. Great evacuation happened in Russia, Ukraine, Belorussia, and Moldova, the areas which felt the worst effect of Chernobyl disaster. Further, radioactive materials were found in the Sweden workers’ clothes.

These are the results in human. How about the environment? Pripyat River wasn’t suitable to be consumed. Radioactive materials were accumulated in the fishes’ body. Forests that are in 4 square kilometers around Chernobyl reactors were badly damaged; the pine trees were browned and died. Some animals died, and the others stopped reproduce.

So, beware about nuclear. I think it’s better to use water or earth power plant rather than nuclear power plant. Although water power plant produce less energy than nuclear, but it’s safer.
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Original source: wikipedia