On the 26th of April in 1986, at 1:23AM (Kiev time zone) two big explosions occurred at the Chernobyl power plant (V.I. Lenin) 40-60 seconds after a test at the plant had started.
A system test, which was ordered by the soviet government, was planned in the night of the 25th of April by the plant staff.
This test was meant to clarify whether the nuclear reactor would have been able to keep the cooling water circulating in case of an emergency shut down.
In order to do so, the turbines and pumps should have generated the needed power to overcome the time until the start of the emergency generator.
BUT there was a design failure known for this type of reactor (RBMK) which left the installation without the supply of cooling water for about 75 seconds, in case of an emergency shut down.
This gave „enough “time to evolve the heat which did lead to the catastrophic failure.
By virtue to this kind of test the automatic shutdown system needed to be deactivated because every working control system would have recognised this test as a problem and an alarm would have been set off.
Actually, the test should have been carried out before taking the reactor into operation. Due to staff problems and the „urge “to finish the construction earlier than expected, this test was skipped!
Additionally, this test should have taken place one shift earlier. Therefore the delay resulted in unprepared / unsuitable / not trained / ,,, staff carrying out the test.
At 1:23 AM the test started.
The main turbine got shut down which started another issue: The turbine had way less torque than expected which caused a high steam evolution due to not enough cooling water being in the reactor.
Above lead to a high reactor performance that the control system tried to avoid with adding control rods.
Due to the higher amount of energy emerging, more water was transformed into steam in the core which provoked a chain reaction.
„Luckily“ the automatic control system could „successfully“ contract the chain reaction with using one control rod after another until it used all 12. Those control rods were not under manual control.
Around 1:23:30 the emergency shut down was activated: The AZ5 button.
The exact reason is not known but most assume that this was a reaction to the even higher power levels which evolved on the base of the chain reaction.
This high-power level caused retracting the manual control rods additionally to the automatic ones above, which needed estimated 18 seconds.
These days it's known that also the control rods were designed with a mistake. The rods inserted by pushing the AZ5 button had a graphite tip, not the neutron uptake cooler used in standard rods, with neutron absorbing boron.
The core overheated and fractured the fuel rods, that blocked the control rods after 1/3 of the insertion.
The energy production went up to 530MW in just a couple of seconds.
The steam pressure raised extremely high as a result, wherefore radioactive material leaked into the cooling water and the channels of the fuel elements were wrecked.
Suddenly the energy production skyrocket to 30GW - this is ten times higher than usual!
The last measured energy level visible on the control panel showed 33GW.
This sudden raise resulted in an uncontrolled increase of the steam pressure with fatal consequences.
An explosion did tear off the 1500 tons heavy reactor cover and pushed it through the roof of the reactor building. This caused the destruction of more fuel rods and several cooling lines of the core, which again emerged a further raise of the thermal power level/production.
Three seconds later a second explosion, triggered by the heated up fuel rods being exposed to oxygen(air), occurred.
Thereby parts of the reactor spread over a huge area which was discovered only at sunrise the next morning.
Source of all pictures on this side: google.com