CHERNOBYL ACCIDENT

Chernobyl Accident

Chernobyl Nuclear Accident

Introduction

On April 26, 1986, around 1:23 in the morning, unit 4 of the Chernobyl nuclear power plant near Pripyat, Ukraine, exploded, sending millions of radioactive particles into the atmosphere. More than 350,000 people had to be evacuated or resettled from the most contaminated areas of Belarus, Ukraine, and Russia. The nuclear cloud traveled over much of Europe, falling as radioactive rain. Decades after the accident 4.5 million people continue to live in irradiated areas. This entry provides an overview of the multiple causes of devastation in addition to the consequences that continue to be felt by generations of Chernobyl survivors. Communication failures were an important component, and the fate of the disaster's survivors continues to appear in contemporary news reports.

Timeline of the Disaster

December 31, 1983, Viktor Bryukhanov, director of the Chernobyl Nuclear Station, signs an acceptance document that the annual works had been completed successfully.

March 1984, Chernobyl Reactor Four is brought into commercial operation.

April 25, 1986, Chernobyl Reactor Four is prepared for its safety test, and the reactor's output is slowly reduced to 50 percent of capacity in preparation for the test. As another nearby power station unexpectedly goes offline, the Kiev Grid-Controller requests that further reductions in power be postponed to keep supplying power to the city of Kiev.

April 25, 1986, 11:04 P.M. The transient fault condition has passed, and the Kiev Grid-Controller gives permission for the test to resume. By this time, the reactor's day shift had departed some time ago, the evening shift is preparing to leave, and the night shift is ready to take over operation of the reactor (Abbott, 2006).

April 26, 1986, 1:23:04 A.M. The experiment begins. Steam to the turbines is shut off, with the reactor crew unaware of the unstable reactor condition. As the turbine flows, the water flow to the reactor decreases, allowing voids in the cooling water to form as a result of the massive temperature rise within the reactor.

April 26, 1986, 1.23:40 A.M. A SCRAM of the reactor is initiated with the pressing of button AZ-5 (Rapid Emergency Defense 5). The control rods of the reactor are automatically inserted; however, this takes place at slow speed. (The rods take 20 seconds to travel the full 7-meter RBMK core height. The tip of the control rods is made of graphite, which displaces the moderating coolant and increases the reaction rate.)

April 26, 1986, 1:23:45 A.M. The reactor output rises rapidly to 530 MW; control rods break as a result of the massive spike of energy and enormous heat being generated within the reactor core.

April 26, 1986, 1:23:47 A.M. The reactor is now producing 30 GW of thermal power, which is more than the reactor's operational output by a factor of 10. Steam pressure rises to the point at which fuel channels within the reactor are destroyed; large-diameter cooling pipes rupture and fuel rods begin to melt.

April 26, 1986, 1:24 A.M. A massive steam explosion takes place, blowing the lid of the reactor (known as the biological containment shield), which weighs 2,000 ...