Early in the morning on April 26, 1986 a steam explosion blew the pressure dome off a 1.5-gigawatt nuclear reactor in the city of Pripyat, Ukraine. The release of fission products in the explosion and subsequent fire resulted in the worst civilian nuclear disaster in history, which you can read more about here:
The Chernobyl disaster
The events cascade that led to the Chernobyl disaster isn't really shocking because it happened, but because it was possible to happen. Every step of the causal chain, from the design of the reactor to the management of the plant to the Soviet Union's preparation for unplanned contingencies contributed to making things as bad as they were in the spring and summer of 1986 around Pripyat. Knowing this, I think it's silly to use the disaster as an argument against the use of nuclear energy, as some advocacy groups have done. Let's take a look at that chain to see whether the argument makes sense.
First, the design of the RBMK-1500 reactor that exploded was fundamentally flawed. At low power the reactor chugged along in fits and stops like an old rough-idling diesel engine. Spikes and dips in thermal output on the order of megawatts while idling were considered normal, and technicians had to proceed with extreme caution during low-power operations. At any power setting the reactor was designed with a positive void coefficient, meaning that if excess heat energy caused the coolant surrounding the fuel elements to begin to boil the reaction rate of the pile would increase, further increasing the rate of boiling. This is a runaway cycle requiring active control to prevent failure, and is unheard of in western reactors since it's entirely possible to design reactors with a negative void coefficient. On top of all this there was no adequate containment structure around the reactor. Any release of radioactive material from the reactor was designed to go straight into Pripyat's air supply, rather than be contained in an easily-quarantined concrete chamber.
Second, the immediate cause of the accident was a willingness to rush demonstration of an experimental procedure without adequate modelling and testing. The Chernobyl managers were concerned that in the event of a sudden reactor shutdown combined with a blackout on the electrical grid, it would take too long for a backup diesel generator to spool up and begin running the reactor's circulation pumps to keep the decaying uranium fuel from overheating. This is exactly the type of accident that occurred 25 years later at Fukushima, when a megathrust earthquake and tsunami simultaneously cut power to most of Japan's gird and destroyed the auxiliary power infrastructure at the plant. The plan was to scram the reaction in Chernobyl's reactor 4 but keep the steam generator running, using the tail-off of thermal power as the reaction halted to generate enough power to keep the turbines spinning and the circulation pumps running until the diesel generator come online. Had the procedure been followed as documented, things probably would've turned out fine. In the event, the reactor was inadvertently almost shut down early, and in their attempts to bring reactor 4's power back up to the prescribed level, technicians changed the initial conditions of the test, a massive power spike occurred, pressure built in the core faster than the unintended reaction could be stopped, and when the pressure exceeded the failure strength of the vessel it exploded in a flash phase change of the water coolant.
Had the reactor been sealed off from the outside world in a containment vessel, this still would've been bad, but not a disaster for the community. Due to the Chernobyl plant's design with no containment structure, the explosion ripped open a large section of the plant, allowing a plume of intensely radioactive smoke and ash generated by a fire in the graphite reactor moderator to escape into the surrounding environment. With no plan besides sending lots of people into a terrible place quickly, hundreds of emergency responders (later known as the liquidators) would be poisoned by the radiation, and many more workers and members of the surrounding communities will see cancer in their lives as a direct result of a few boneheaded design and management choices.
Remarkably, the disaster wasn't the end of the Chernobyl plant's operational life. Though a 30-kilometer radius around the plant has been evacuated of human inhabitation, and thus inadvertently become one of Europe's largest and most vibrant wildlife sanctuaries, once a sarcophagus was built to contain the remains of reactor 4 rotating shifts of workers came to Chernobyl to operate the remaining three reactors until its final decommissioning in 2000. As eager as Ukraine was to get the nightmarish history of Chernobyl and Pripyat behind it, people do like keeping the lights on.
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