[ARROW] Timeline of Events
April 25
Day 1
1:00 am The reactor was running at full power with normal operation. Steam power was directed to both turbines of the power generators. Slowly the operators began to reduce power for the test. The purpose of the test was to observe the dynamics of the RMBK reacto r with limited power flow.

1:05 pm Twelve hours after power reduction was initiated the reactor reached 50% power. Now only one turbine was needed to take in the decreased amount of steam caused by the power decrease and turbine #2 was switched off.

2:00 pm Under the normal procedures of the test the reactor would have been reduced to 30% power, but the Soviet electricity authorities refused to allow this because of an apparent need for electricity elsewhere, so the reactor remained at 50% power for another 9 hours.

April 26
Day 2
12:28 am The Chernobyl staff received permission to resume the reactor power reduction. One of the operators made a mistake. Instead of keeping power at 30%, he forgot to reset a controller which caused the power to plummet to 1% because of water which was now fil ling the core, and xenon (a neutron absorber) which was building up in the reactor. This amount of power was too low for the test. The water added to the reactor is heated by the nuclear reaction and turned into steam to turn the turbines of the generator.

1:00-1:20 am The operator forced the reactor up to 7% power by removing all but 6 of the control rods. This was a violation of procedure and the reactor was never built to operate at such low power. The RBMK reactor is unstable when its core is filled with water. The operator tried to take over the flow of the water which was returning from the turbine manually which is very difficult because small temperature changes can cause large power fluctuations. The operator was not succesful in getting the flow of water corr ected and the reactor was getting increasingly unstable. The operator disabled emergency shutdown procedures because a shutdown would abort the test.

1:22 am By 01:22, when the operators thought they had the most stable conditions, they decided to start the test. The operator blocked automatic shutdown on low water level and the loss of both turbines because of a fear that a shutdown would abort the test and t hey would have to repeat tests.

1:23 am
(The test begins)
The remaining turbine was shut down
1:23:40 am Power in the reactor began to gradually rise because of the reduction in water flow caused by the turbine shutdown which lead to an increase in boiling. The operator initiated manual shut down which lead to a quick power increase due to the control rod design.

Diagram-3 Effect of shutdown rods.
[Effect of shutdown rods]

1:23:44 am Disaster Point- The reactor reached 120 times its full power. All the radioactive fuel disintegrated, and pressure from all of the excess steam which was supposed to go to the turbines broke every one of the pressure tubes and blew off the entire top shie ld of the reactor.

Source #[9]

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