Chernobyl Accident Events

Michelle Ramadan
March 5, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016

Power Plants

Fig. 1: Chernobyl over 20 years later. (Source: Wikimedia Commons)

Nuclear power plants require an immense amount of precision. Any shift in the delicate balance of fuel rod, coolant, or neutron absorbers can result in a total disaster. All it takes is one simple mistake to cause utter disaster. In April of 1986, a power plant in Chernobyl experienced the world's biggest power plant disaster to this day. It took a mere five seconds for Chernobyl to go from a small error to a huge catastrophe.

Technical Details

During the height of the cold war, the Soviet bloc began building nuclear reactors in Chernobyl to power Ukraine. However the construction of these reactors did not incorporate many modern day safety features required by the International Atomic Energy Agency. This, combined with human negligence, caused the disaster that is Chernobyl. On April 25th, the day before the accident, a routine safety test took place. Reactor Unit 4 was powered down to determine whether cooling of the core could continue in case of a power loss. This test had been done successfully the previous year before installing new voltage regulators. A miscommunication between those in charge of safety protocols and those in charge of operating the test caused a fatal oversight. The operators failed to reset a controller that prevents the power from decreasing below 30%. This caused the power to drop to 1% and water and xenon to fill the core. In attempt to force the power back up, one of the operators removed some controller rods from the core. Studies on Chernobyl have shown that the operator had to remove all but eight control rods, while the safety guidelines require at least fifteen control rods at all times. This violation of the safety rule caused the reactor to become very unstable. Ultimately, this chain of events during the test caused a catastrophic power surge.

At the molecular level, the extra energy caused the water cooling the fuel rods to turn into steam. This lack of water exposed the fuel rods and caused them to over heat. There was also no coolant to prevent the rapid neutron multiplication rate. Under these circumstances, it takes less than 1 second for a neutron emitted to cause another fission. The fuel rods got so hot that they melted into the rest of the coolant and the steam broke all the pressure tubes and blew off the top of reactor number 4. [1] Just moments after this steam explosion, a build up of hydrogen caused a second explosion. Lumps of fuel were thrown causing a ring of fires and further chaos. Two operators were killed during this explosion. The radioactive steam lifted up into the atmosphere and began to spread based on the weather pattern. The particles came back to the ground in the form of radioactive rain in places as far as Wales.


With only five seconds to respond combined with a power surge, it was unlikely that anyone could have prevented the disaster based on the way Chernobyl was constructed. This disaster brought global attention to the safety of nuclear reactors. Five seconds of disaster has caused the site to be encased in a concrete dome due to the half-life of the radioactive material. Fig. 1 depicts the desolate power plant, still emitting radiation 24 years later. The area will not be inhabitable for almost 20,000 years due to the 4.5 billion year half life of low enriched uranium. [2] With 450 similar power plants around the world, it is important that we learn from the mistakes made in the construction and operations of Chernobyl.

© Michelle Ramadan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] "The Chernobyl Accident: Updating INSAG-1," International Atomic energy Agency, Safety Series No. 75-INSAG-7, November 1992.

[2] A. Taylor, "The Chernobyl Disaster: 25 Years Ago," The Atlantic, 23 Mar 11.