This is a transcript of the video https://www.youtube.com/watch?v=zC83a6YGeZc . In my opinion, it is the simplest and most accurate description of how the whole thing happened. My transcript starts at 1:18.
Because of political pressure, the power plant was put into operation sooner than it should have been. As a result, not all safety experiments were carried out on the fourth reactor, the one that exploded. When the reactor was scheduled to be shut down for a routine inspection, they decided to perform the fateful experiment.
Ironically, it was an experiment to find out what would happen if a failure occurred. They wanted to know whether, in the event of a power outage both inside and outside the plant, the inertia of the turbine would generate enough energy to shut the reactor down safely.
So they wanted to spin up the turbine, cut off its supply of fresh steam, and see whether it would generate enough energy for all systems to be shut down safely. The experiment was not meant to run at the reactor’s full output; only twenty-five percent was planned. However, that was not the output they were getting. The reactor’s power kept dropping to a dangerous level, and more and more xenon 135 was building up in the reactor.
The process in which a large amount of xenon builds up and stops the reaction is called xenon poisoning of the reactor. They were also worried that the reactor could fall into a so-called iodine pit, which happens when a large amount of iodine 135 appears. Iodine also absorbs neutrons and therefore stops the reaction.
If that happened, the reactor would have to be shut down for half a day to a full day until all the iodine decayed, and under the pressure to produce electricity, that was unacceptable. They decided to prevent it by pulling out the control rods, which should have increased the reactor’s power. Unfortunately, the power only rose to about seven percent of the planned twenty-five percent.
So they decided to pull out additional control rods as well, the ones that are there precisely for safety reasons. That is something they should never have done, but unfortunately, the Chernobyl nuclear power plant did not have properly trained staff. This was largely because the way the reactor worked was a military secret, and not everyone was allowed to know how it operated.
This means that even a trained technician was not properly trained on how this particular reactor worked. There was also water in the reactor, which was constantly being pumped in, and in this case water can also act as a reaction inhibitor, because it absorbs neutrons itself.
When the experiment began, the control rods were raised higher than they should ever have been, and some control rods had also been removed manually. So the reactor was functional, but poorly controllable and unstable, and under these conditions the reaction really took off.
When they tried to lower the control rods into the reactor, the rods were too high, which meant it took a long time, and the longer it took, the higher the temperature in the reactor rose. In the end, the rods even got stuck, because the mechanism for lowering them stopped working due to the heat.
Unfortunately, these control rods were not even properly filled, so the part that entered the reactor essentially displaced the water that had been helping to regulate the reactor at least a little. By displacing the water, fewer neutrons were absorbed, and the reaction only sped up.
At the moment of its failure, the reactor was running at a thousand percent of its planned output. Pressure in the reactor was rising because the water was rapidly turning into steam, which ultimately caused the explosion.
Due to mistakes by the management, the reactor explosion then led to further problems. The first problem was that they had only three dosimeters. The first one had a limit far too low to measure such high radiation.
It could measure only about 3.6 roentgens per hour. The second stopped working right after it was switched on, and the third was blocked by the explosion. When they later brought in another dosimeter and it measured the maximum value, the management declared that it was probably faulty and that the levels could not be that high.
Then the asphalt roof of the plant collapsed and, because of the heat, began to burn on its own. Another problem was that the firefighters who helped put out the fire did not know they were fighting a reactor fire. They were pouring water on a reactor at two thousand degrees, and at such high temperatures water, H2O, splits into oxygen and hydrogen.
When you have a high concentration of oxygen and hydrogen, at some point it ignites and an explosion occurs. So this firefighting actually caused further explosions that made the whole situation worse and worse.
At the Chernobyl power plant there was an incredible number of mistakes, whether due to improperly trained staff, the management’s inability to communicate, or above all the fact that, in most cases, the information needed to deal with the situation was never passed on.