Why meltdown before venting

According to the accident investigation report of the Fukushima Daiichi nuclear power plant, the meltdown always started before the vent.
It can be seen from the fact that it is the same in the CG that appears in the NHK reproduction drama.

However, this is a serious mistake that anyone with a little knowledge will immediately notice.

To briefly summarize the accident, the tsunami came after the emergency stop due to the great earthquake, and the emergency battery and generator were flooded.
Combined with the damage caused by the earthquake, the external power supply disappeared, and the pump of the equipment that returns the steam that was turning the power generation turbine to water stopped.
The control of the emergency condenser becomes impossible, and the amount of water in the pressure vessel cannot be controlled.

In this case, the first thing to think about is restoring power and repairing the condenser.
Since it is only necessary to procure a generator from the outside to restore the power supply, it should have been possible to procure it with the support of the TEPCO headquarters.
The problem is the repair of the condenser.

According to the materials, the condenser was not repaired and the water level dropped.
In CG, the meltdown starts here, but since the pressure vessel should be sealed (normally operated at high pressure), the meltdown does not occur from the theory of "critical water phenomenon".
However, when the water level drops and the fuel rods are exposed, hydrogen is generated and it becomes a very dangerous state.

It is probable that the cause of the sudden explosion of the building was that the water level in the spent nuclear fuel storage pool dropped and the spent fuel rods were exposed, causing hydrogen to fill the building (due to the radiolysis of water vapor).
(It was confirmed that the radiation dose was rising when the workers entered the building.It also matches the reason why the stopped building No. 4 exploded.)

If the water level in the pressure vessel drops and the fuel rods are exposed, water vapor is decomposed by radiation to generate hydrogen, and the pressure vessel is filled with hydrogen.
Due to the high temperature of the fuel rods, the pressure vessel may explode if left untouched.
(In Chernobyl, the explosion of the pressure vessel scattered fuel rod debris over a wide area.)
This is the true "worst result".(Government officials at the time now describe the accident at Fukushima Daiichi as the "worst result.")

In order to avoid the worst results, it was decided to vent (open the pressure vessel) after confirming the drop in water level.(NHK's CG has already melted down at this time, but it is not actually done.)
When vented, internal pressure is applied (imagine opening the lid when boiling in a pressure cooker), so it evaporates at once.
It is natural that the fuel rods melt down at this time.

The question is, "Why did they have to have melted down before venting?"
The reason is that it would be a problem if the fact that the meltdown occurred after venting was known.
Therefore, it is possible that the instructions for venting were not decided within TEPCO.

If TEPCO was seeking to repair the condenser and it was possible to repair it, it would mean that there was no compensation for damages to the local residents, let alone the decommissioning of the Fukushima Daiichi nuclear power plant.
If the person who pushed them away and instructed the vent was a person other than TEPCO, it could be a very big responsibility problem.

In other words, TEPCO is currently the perpetrator, but in the case of a meltdown after venting, there is still the possibility of becoming a victim. That's why they wanted to separate the meltdown from the vent.

TEPCO admits that it is the perpetrator of this accident, but it has to admit that it is just because the result of the accident investigation is so.
Moreover, since it is a state-owned enterprise that has received a loan from the government, it cannot say anything even if there is any doubt.
But it's really unpleasant for consumers.