According to the Chernobyl movie, we see that the Cheif try to operate the reactor a low power but the reactor becomes completely stalled as for the Xenon poisoned it all over the core. It was decided to operate the reactor at 700 MW. So they tried to lower down the reactor power from 1400 to 700 MW. But it was a shock that after the rector cross 750MW it began to drop spontaneously and turned to zero within a few minutes!!
The cheif than decided to up the power from O to 700 MW. But at this moment, if he ordered to shut down the reactor for 24 hours, the xenon went away from the core and this agonizing incident might not be happened.
But the reactor was drowning in poison. The make matter worse, the reactor wasn't hot enough to produce steam. So as they try to increase from 0 MW, the power was not increasing. So they pulled down all the controls rods. Remember, "Control rods are like breaks in the car". After completely pulled out the control rods, they managed to increase power upto 200MW!! The core was full with Xenon and water.
At 200MW the test of operating the reactor began. The uranium fuel installed. There was nothing to check and balance the fuel inside the core. Therefore, the balance immediately swings the opposite direction. In less than a second, reactivity increases!!
That moment, Inside the core the remaining water quickly converter to steam. The heat of the steam increased rapidly as well as the reactivity. The remaining Xenon decays away. The power of the reactor was rising. There was nothing left to stop it!!
And the final climax finished, when they push the shut down button. The situation was such an worse than even the shut down button cant save them from the vital explosion.
The chernobyl destroyed!!
The post goes above my head. i don't understand anything. keep writing. Stay Safe. Thank you.