Chronic of the accident

The accident

The beginning of the Chernobyl accident goes back to the 25th of April when the unit 4 of the reactor was shut down for purpose of a routine maintenance. The engineers wanted to conduct an experiment during which they would shut off the reactors emergency core cooling system (ECCS) to see weather, in the case of a power reduction/emergency, the cooling system would still be able to cool the core.
14:00 – The power of reactor operates at about half power and the ECCS is switched off.

23:00-00:28 – Further power reduction leads to operational error and the power falls to about 30 MWt, although it should be stabilized at about 1000 MWt.
•    The operators try to raise the power by switching off the automatic regulators and freeing the control rods manually.

26th of April 1986
01:00 – In order to retain the reactor control at least 30 control rods were necessary in the reactor core. During the experiment only 6 to 8 control rods were actually used, which made the reactor very unstable.

01:03 – The reactor is stabilized at about 200 MWt.

01:23:04 – The engineers decide to proceed with the experiment.
•    Flow of the feedwater is reduced in order to maintain the steam pressure. Less cooling water is provided to the reactor, since the energy that powered the pumps was reduced. The loss of cooling water increases steam production in the cooling channels and the engineers are not able to prevent the runaway increase in power that was 100 times the nominal power output.

01:23:40 – The operator presses the emergency button. Control rods start to enter the core. Consequently the reactivity at the bottom of the core is increasing.

01:23:43 – Power exceeds 530 MWt and continues to rise.
•    Sudden increase in heat production ruptures part of the fuel and small hot fuel pellets, which react with water. A steam explosion follows and destroys the reactor core. The rupture of several fuel channels increases the pressure in the reactor to such extent that the 1000 t reactor support plate becomes detached. The control rods, which were on the halfway down, are now jammed.

01:24 – A note of the Chief Reactor Control Engineer: “01:24: Severe shocks; the rods stopped moving before they reached the lower limit stop switches; power switch of clutch mechanisms is off.”
•    The initial steam explosion is few seconds later followed by a second explosion. Fuel, core components, structural items, highly radioactive debris are sent into the air and the destroyed core is exposed to the atmosphere. The plum of the smoke and radioactive debris rise up to about 1 km into the air.
•    In the remains of the Unit 4 building fire brakes out.

01:28 – First group of 14 firefighters arrives to the scene.

02:10-02:30 – The largest fires on the roof of the machine and the reactor hall are under control.

05:00 – The Unit 3 of the Chernobyl Nuclear Power Plant is shut down.

The whole chronic…