| Date | Time | Event |
|---|---|---|
| April 25, 1986 | 1:06 am | Normal operation: reactor was at full power. Steam power was directed to both turbines of the power generators. The purpose of the test was to determine if one of the turbines could supply power to feedwater pumps until standby diesel generators came on line in the case of a local power failure. |
| 1:00 pm | In preparation for the test prior to routine shutdown, reactor power was powered down to 1600 MWt for a test sequence. Now only one turbine was needed to take in the decreased amount of steam from the reduced power and one of the turbines was switched off. | |
| The test was designed to be conducted at 22%-32% full power. However the demand in electricity was still high at the time, and authorities asked to keep the reactor at at 50% power for additional 9 hours. | ||
| April 26, 1986 | 12:28 am | Permission was received to resume the reactor power reduction. One of the operators failed to reprogram computer to maintain the power at 30%, which caused the power to fall down to 1% and reactor was filled with solid water (no boiling) - an extremely unstable situation because of the positive void coefficient. This amount of power was too low to conduct the test. |
| 12:32 am | To try to bring the power back to the desired level operator pulled number of control rods out of the core (less than 26 control rods remaining in the core), but the power only increased to 7%, still well below the test design value. The reason the power could not be brought back up was the "xenon poisoning" effect. Xenon is a decay product of I-135 and is a strong neutron absorber which "poisons" the fission reaction. It reaches an equilibrium at normal operating power levels by being "burned away" by neutron absorption and further decay. When the power level was decreased from the 1600 MW level, there were lots of I-135 to decay into xenon, but a small neutron flux with which to burn it away, so it built up rapidly. | |
| 1:15 am | In order to keep reactor from automatically shutting down under these conditions, emergency core cooling system (ECCS) and several of the automatic scram circuits were disconnected. More control rods were pulled out to overcome "xenon poisoning". 6 control rods were still remaining on the core. At this point reactor could not be shut down rapidly if needed. | |
| 1:20 am | All eight cooling water pumps were running at the low power, compared to a normal six even at full power, so there was nearly solid water with almost no void fraction, which increased the vulnerability to any power excursion which produced boiling. | |
| 1:22 am | The turbine was tripped to initiate the test, which caused the switching off of four of the eight recirculation pumps. This would have scrammed the reactor if the automatic scram circuit had not been disconnected. | |
| 1:23.35 am | Reduced coolant flow caused voids to form rapidly in the pressure tubes, increasing reactivity because of the positive void coefficient. The higher reactivity produced more steam. As the system fed itself, heat production soared, casing uncontrollable steam generation. | |
| 1:23.40 am | Operator, recognizing an emergency, hit the button that was to lower all control rods into the core and shut down the reactor. | |
| 1:23.44 am | Ironically, this last attempt was the final straw. The reactor system had a design problem that no one had foreseen. The control rods had six-inch graphite tips at their lead ends. These tips enter the core before the boron carbide section. As they enter the core, they displace water, which is essentially displacing absorber with moderator. Being inserted all at once they caused power to increase for a few seconds. Under normal operating conditions, this small temporary increase has little effect. But Chernobyl Reactor Number Four was not operating under normal conditions. As control rod tips entered the core, power surged to 100 times normal in four seconds. | |
| 1:24 am | Under intense heat, the core began to break down. Fuel assemblies fragmented, control rod channels warped, steam built up furiously and, finally, steam tubes burst. Tons of steam and water shot into the reactor, causing a tremendous steam explosion. Steam pressure blew the 1000-ton steel- and cement-filled biologic shield off the top of the reactor, destroying the roof of the reactor building along the way and exposing the hot core to the atmosphere. | |
| 1:28 am | 14 firemen arrived on the scene of the accident. | |
| 2:00 am | The largest fires on the roof of the reactor hall were brought under control by a group of 100 fire fighters called in from the Pripyat. | |
| 5:00 am | Most of the fires were put out, but graphite fire had started. Graphite fire caused the dispersion of radionuclides high into the atmosphere. |
Table 1 Timeline of the Chernobyl accident compiled by Natallia Pinchuk.
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