Time Line The entire scenario surrounding the events which took place on April 26, 1986 at the Chernobyl Nuclear Power Plant were the result of a safety experiment gone awry. All power reactors produce significant amounts of heat even when not in use ( due to the residual radiation with the materials surrounding the fuel). As a result, when a reactor is in its shutdown state, there is still a need to employ cooling systems to remove this heat to keep the fuel from melting. Normally, the electric power necessary to keep these systems running is drawn from the offsite power grid. However, if there is a loss of offsite power with a shutdown, it is customary to employ large diesel generators to supply the needed electricity to keep the required emergency core cooling systems (ECCS) in operation.

In the United States, the time required to start one of these diesel generators is on the order of 6 seconds. At Chernobyl, this time was on the order of 3 minutes. In an effort to supply the necessary power to run the safety systems in the 3 minute window between reactor shutdown and diesel generator start, Chernobyl engineers thought that useful electric power could be extracted from the turbine while it was in the process of coasting to a halt. It was during a safety experiment designed to test this theory that events were allowed to unfold which would ultimately lead to the Chernobyl accident. It is the purpose of this section to outline, in as simple language as is possible, the events which unfolded on April 26, 1986.

Chernobyl Time Line


1.) 13:05 Plant Power decreasing, signifying the start of the reactor shutdown.

2.) 14:00 Emergency Core Cooling System (ECCS) is disconnected.
3.) 23:10 Power reduction resumed.


4.) 1:00 Increased power to 200 Mwt by withdrawing rods.

5.) 1:07 Two additional recirculation pumps started - all 8 pumps running.
6.) 1:19 Increased feedwater flow to steam drums.
7.) 1:22:30 Feedwater flow to steam drums decreased to very low value - 30 secs later reactor inlet temperature begins to rise.

8.) 1:23:04 Turbine feed valves closed.

9.) 1:23:40 Emergency Scram initiated by button AZ-5. Button pushed because:

10.) 1:23:43-45 Power increasing rapidly due to positive void coefficient.

11.) 1:23:48 Explosion occurs followed by a second explosion a few seconds later.
Important Lesson:

The Chernobyl accident reminds us of the continuing importance of safe design in both concept and implementation; of operational controls, of competence and motivation of plant management and operating staff to operate in strict compliance with controls; and of backup features of defense against potential accidents.

Although a large nuclear power-plant accident somewhere in the United States is unlikely because of design and operational features, we cannot relax the care and vigilance that have made it so.

Back to Main Page