Tribute to Chernobyl Disaster - Sleeping Sun

Why did the reactor explode?

A safety test was due to be carried out on the reactor in the winter. This meant shutting it down. Consumers used too much electricity, so the test was delayed until the spring of 1986. The purpose of the test was to find out wheather the water pumps could supply enough coolant if power levels drop. The power plant ran on its own electricity, which it produced. So if in any case, there is a need for an emergency shutdown (SCRAM), would there be enough time to fire up the diesel generator to keep the water pumps supplying cool water through the core? A nuclear reactor needs coolant for some time even after a complete shutdown. 1 minute is all the time that is needed for the diesel generator to kick in.

- April 25, 1986: at 1 am in the morning, operators started to decrease the 3,2 GW thermal output.
- By 1 pm power was reduced to 1,6 GW.
- By 2 pm, operators were informed: despite the coming weekend, consumers used more electricity, than what was expected. Further power deduction was stopped.
- By 11:15 pm, the operators were informed: consumer needs dropped, the 4th reactor can be disconnected from the grid.

So the test began with junior operators. The senior operators usually had the benefits of working in the daytime. The more junior staff was also experienced and qualified, but junior.

Anatoly Diatlov, chief engineer of the Chernobyl Nuclear Power Plant was to lead the operations. To his command, various automatic systems, inculding the SCRAM, were disabled.

- April 26, 1986, 0:30 am: to be sure, that the core won't suffer any damage, the coolant flow rate was increased. Control rods (accelerator and brakes of the nuclear reactor - if lowered, power decreases, if pulled out of the core, power increases) were also used to power down the reactor to 0,7 GW. Due to the positive void coefficient (if power increases, it increases a lot, and same with decrease) of the RBMK type reactor, power dropped to 0,03 GW. Way too low and dangerous to begin any test. A low-power reactor is very unstable. They can run at normal power or stop at a complete halt, but low-power operation is the same as flying any aircraft slow. They can stay on the ground, or fly at a fast speed, but they will crash at low speed.
By power levels dropping rapidly, xenon poisoning also took it's effects. Xenon sticks onto the uranium cassettes, preventing neutrons from splitting. In this case, a reactor would need to be completely shut down, until 2 days later, the xenon poisoning would dissolve, leaving "clean" cassettes. The only way xenon poisoning could be removed manually, is heating up the reactor. But this means, power will rocket, once the xenon is gone. Operators all over the world have been informed, never to try, since it is way too dangerous.
- 1:07 am: the two operators realized: the reactor could be very unstable. They questioned Diatlov, but despite several warnings from fellow operators, he ordered power to be increased by taking control rods out. They got pulled out all toghter. A nuclear expert after the accident said: "even the prime minister can't order a reactor to be operated in that way".
- 1:22 am. The Nuclear Regulatory Commission ordered the test to be carried out at 0,7 GW. A safe power level. Now, with all the control rods out of the core, the reactor was only making 0,2 GW, beacause of the xenon sticking onto the uranium, preventing it from splitting.
- 1:23 am. The turbine is shut off. This means less and less electricity is being made from steam, so less and less water is pumped through the core (the water pumps recived less and less power).
- 1:23:20 am. Coolant water is far hotter, than normally.
- 1:23:30 am. By now, power level is 0,3 GW. So the xenon, which was stopping uranium from splitting is now gone, leaving the uranium cassettes to split rapidly without brakes.
- 1:23:42 am. Power level is now 1,4 GW. The operator, Akimov realizing rapid power increase, decides to press the emercency shutdown button (SCRAM). Though the power was still under normal use, the operator pressed the button because of the speed it increased in!
- 1:23:47 am. Control rods reach the upper part of the core. At that short moment, when they are inserted, they increase power, due to faulty design. The increase of power they would normally do, is not worth mentioning. But the reactor is now a steam pressure cooker. Heat levels are far beyond normal. The graphite rods catch fire, increasing the power output considerably. And they don't even reach the core...
- 1:24 am. The last reading on the control panel was 33 GW. 10 times more, than the maximum power output. The reactor's 2000 tonne lid is lifted by the enormous pressure. Radioactive steam quickly escapes, ripping everything apart in it's path. It is more than 4000 degrees, setting fire to the whole buliding.

The rest...you probably heard...