POST 1: CHERNOBYL
Early morning, Saturday, April 26, 1986. Chernobyl Nuclear Power Plant (ChNPP), Ukraine, Soviet Union.
Reactor No. 4, an RBMK-1000, is scheduled for shutdown for maintenance. The closing program includes a turbine test. A minimum output of 700 MWt (Mega Watt thermal) was prescribed for the test.
The night shift came on duty at midnight. The reactor's thermal power output has already been reduced from 3200 MWt to 1600 MWt.
The deputy chief engineer, Anatoly Dyatlov, decides to carry out the test on 200 MWt. Leonid Toptunov, the senior reactor engineer on duty, switches the computerized control system to another control mode. This will help him control the reactor at such a low power output. (The RBMK-1000 is notorious for been difficult to control at low power output.)
Toptunov neglects to program the minimum level at which the computer must stabilize the power output. As a result, the output drops to 30 MWt, which is only about 1% of the reactor's maximum output. The reactor is highly unstable and according to nuclear standards and protocols the test had to be canceled and the reactor stabilized and shut down.
However, this is not done. The test must continue and Toptunov and another operator succeed in pushing the output up to 200 MWt. Due to the build-up of Xenon-135 gas in the reactor core and additional factors, they cannot get it higher.
At 01:23:04 the test starts and is over after only 36 seconds at 01:23:40. Toptunov's supervisor, Aleksandr Akimov, orders the reactor to be shut down.
Toptunov presses the red AZ-5 button to initiate the shutdown. Dyatlov, Akimov and Toptunov are blissfully unaware that during the 36 seconds of the test, things have started to go horribly wrong in the reactor core.
The AZ-5 has the expected effect of reducing the reactivity in the core for a mere 1 second. However, then design flaws and other factors kick in and push the already highly unstable reactor over the edge.
At 01:23:44 reactor No. 4 explodes with a force equivalent to the detonation of 60 tons of TNT. The explosion destroys the reactor and hurls the 2000 ton shield that sits on top of the reactor into the air like a coin. It destroys a section of the reactor chamber and breaks open the roof before falling back and landing diagonally on top of the exposed reactor. Large-scale nuclear radiation and contamination begin immediately.
The final events within the reactor core that caused the first explosion lasted approximately only 2 seconds. It happened so quickly that it exceeded the capabilities of the measuring instrumentation within the reactor and could not be measured.
It has been reconstructed post facto using advanced mathematical models, but it is still not 100% accurate.
Two seconds later, at 01:23:46 a second and more powerful explosion rocks the reactor.
At that time, the biggest nuclear disaster in the world took place. Radioactive material is spread over most of Western Russia and Europe. Russia's economy was crippled due to the disaster with half a million workers carrying out the clean-up work at a cost of an estimated 18 million roubles.
Along with Japan's Fukushima disaster, it is the only other disaster that has been classified as a Level 7 nuclear disaster. (The Hiroshima and Nagasaki nuclear explosions do not qualify as disasters because both are acts of war.)
The RBMK reactor program was finally shut down in 1988. The construction of Chernobyl's No. 5 & 6 reactors was also stopped due to, among other things, political pressure after the disaster.
On Friday, 26 April 2024, the disaster happened 38 years ago, however the world is still living with the consequences.
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