Recently I watched a disaster film Fukushima 50 base on true events
Some people asked why they must call a fire engine to water the nuclear reactor. 
So, if there is a shortage of water, since the nuclear power plant is so close to the sea
why not just use sea water to pour over the nuclear reactor? 
And why is it important to restore electricity?
So today this video will discuss these topics.
First of all, we must first understand how humans obtain energy from nuclear energy.
We all learned it during our secondary school days. radioactive decay this subject. 
Let’s use the Americium-241 atom as an example for discussions
Basically, when radioactive atoms undergo radioactive decay, they generate energy.
The process by which radioactive atoms undergo radioactive decay is
The Americium-241 atom is converted into two atoms with smaller and different elements
and generate energy
But this process is very long and its half-life is 432 years.
Then the radioactive atoms used in nuclear power plants are Uranium-235, 
and its half-life (hundreds of thousands to 4.5 billion years).
This is too long for humans.
There is no way for humans to obtain useful energy solely by means of radioactive decay process.
So scientists have tried their best to speed up this process
and how to obtain the energy of radioactive materials more effectively.
This method is to make a neutron collide with Uranium-235 atom
When both of them collide, Uranium-235 atom will undergo a nuclear fission reaction.
1 Neutron and 1 Uranium-235 atom will be converted into two atoms of smaller and different elements
and 3 neutrons.
Then these three neutrons will collide with the other three Uranium 235 atoms,
and then the next nuclear fission reaction will occur.
These continuous collisions and nuclear fission reactions become a cascading event of chain reaction.
The huge energy produced by these chain reactions can be used by humans. 
This huge energy is converted into heat energy.
Then let's look at the general structure diagram of the nuclear power plant. 
Its main components are:
Reactor Pressure Vessel – All the fuel of the radioactive material Uranium-235 is placed here
All Uranium-235 substances are placed in a long metal rod container.
Then these metal rods are arranged side by side.
There is some space between the metal rods, to make space for the control rod to be inserted in between them (the two long black lines between them).
It is to control the speed of the chain reaction and adjust accordingly.
Main turbine
AC generator,It is connected to the turbine.
When the main turbine spins, it produces electricity.
Its principle is the Electromagnetic induction that we learned in secondary school, I won't discuss it here.
Condenser
Its main function is to cool the steam.
When the steam cools, it turns into water (liquid state)
This red circle represents the primary circuit,
and the light blue color is fresh water.
This fresh water is in direct contact with radioactive materials,
and this fresh water must never leak outside.
This green circle represents the secondary circuit,
and the dark blue color is sea water.
All the water pipes of the primary circuit do not connect to the water pipes of the secondary circuit.
It is because the fresh water is directly contacted with radioactive materials and they are very dangerous.
These fresh water can never leak into the sea water from the open ocean.
Let’s take a look at Reactor Pressure vessel.
The neutrons start to collide, the fission reaction, the chain reaction begins.
The heat energy release by the reaction will raise the temperature.
When the temperature and pressure in the Reactor Pressure vessel exceed a certain threshold
The control rod will drop to block the neutrons from hitting the Uranium-235 material in the next row.
Then when the temperature and pressure drop or fall below a certain threshold,
The control rod will rise to allow the neutrons to hit the Uranium-235 material in the next row.
Let the fission reaction and the chain reaction continue to occur. This is the role of the control rod
The fission reaction, the chain reaction, releases a lot of heat energy.
The fresh water in the Reactor Pressure vessel began to heat up.
When the temperature of fresh water reaches the boiling point, it turns into steam.
At this time, the pressure in the Reactor Pressure vessel is higher than on the main turbine side.
The steam will go from the Reactor Pressure vessel to the side of the Main turbine.
The steam power is able to turn the turbine
So when the turbine turns, the AC generator is starting to generate electricity
This power is provided for people’s daily use, and also provides power to Pump A and Pump B
After the steam passes through the Main turbine, it will advance into the Condenser.
Pump B is responsible for guiding the seawater into the Condenser, the temperature of the seawater is low.
When steam passes through or hits the sea water pipe, it will be cool.
When the steam is cooled, it becomes liquid water.
These water pipes are very good heat conduction water pipes.
Pump A is responsible for guiding the liquid water from the Condenser back to the Reactor Pressure vessel.
This forms a complete circular chain
When the earthquake happened. The safety mechanism of the nuclear power plant is automatically activated.
The control rod in the Reactor Pressure vessel had come down completely.
To stop the nuclear fission reaction and the chain reaction.
All power supply was stopped. Note that Pump A and Pump B also stop rotating
Because they require electricity to operate.
The next safety mechanism of the nuclear power plants was also activated.
The Emergency Diesel generator started. This is a generator that uses diesel to generate electricity.
It is to provide electricity to the entire Residual Heat Removal System, including Pump C and Pump D.
To continue the heat removal function to cool down the Reactor Pressure vessel.
However, they never expected that the power of the tsunami
The seawater flooded the generator room of the Emergency Diesel generator.
The Emergency Diesel generator had spoiled. Without electricity, Pump C and Pump D stop functioning
The entire Residual Heat Removal System cooling function had failed.
So when there was a scene in the movie saying SBO (station blackout).
Those employees began to be extremely nervous. They knew that things were extremely bad.
Then they called the fire truck and rushed to the nuclear power plant to plug in the water pipe.
Pouring fresh water directly to the Reactor Pressure vessel.
But when the fresh water in the fire truck runs out, the fire truck needs to leave the Reactor Pressure vessel to be replenished with water.
According to the report of TEPCO Nuclear Power Plant, this come-and-go traveling time can sometimes last as long as an hour.
When the fresh water ran out, they decided to use sea water
On the other hand, the staff desperately tried to restore the power supply
They called the power supply car, but it was already at night
because of the earthquake, the ground was full of debris
Therefore, it was a difficult task to make cable connection from the power supply car to the main power supply of the nuclear power plant.
According to the report of TEPCO Nuclear Power Plant,
The power supply car arrived at 10 o'clock in the evening when the incident occurred
The connection was only completed by 3 pm noon the next day.
The cable used to link the power car to the nuclear power plant weighs 5 tonnes.
But at this moment. The temperature inside the Reactor Pressure vessel had overheated.
Causing nuclear meltdown, including the dissolution of the container.
With the high temperature, the water and the dissolved matter have a chemical reaction to produce hydrogen
So a large amount of hydrogen began to accumulate in the Reactor Pressure vessel.
It was at 3:36 pm. UNIT 1 building exploded
Okay, after hearing this, we understand the importance of restoring electricity
It is because all the heat removal system in a nuclear power plant requires electricity to operate.
Without electricity, there is no way to cool down the Reactor Pressure vessel.
High temperature will lead to nuclear meltdown and a series of disasters.
There is a scene in the movie where a senior officer orders the supervisor of a nuclear power plant to stop using sea water
The supervisor asked why not. That senior officer answered a lot of illogical reasons
The supervisor just laughed
The reason why it is not possible for them to use sea water to pour over the nuclear reactor 
is because the Reactor Pressure vessel is tightly sealed.
It is mainly not to let the air inside leak out.
Therefore, it cannot be directly opened and filled with sea water.
So they still need to call a fire truck
Another reason why sea water cannot be used for heat removal is
If the parts of the Reactor Pressure vessel come into contact with seawater, the entire vessel will spoil. 
So at the beginning, fire trucks always try to fill with fresh water first.
At that time, they thought they could still save the Reactor Pressure vessel
But when the fresh water ran out, they decided to use sea water to pour over the reactor vessel.
They knew that the entire Reactor Pressure vessel couldn’t be saved
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