In the previous issue, we talked about the pressurized water reactor, and correspondingly, there is a boiling water reactor.
boiling water allows coolant to boil in the core, which means it is closer to "heating faster". The Fukushima nuclear power plant in Japan, built in the 1970s, belongs to the boiling water reactor.
boiling water reservoir has only one circuit, the structure is simple, and it saves steam generator , but the boiling water reservoir is even more fucking.
Since the top of the core needs to be installed with soda separator and other equipment, the control rod of the boiling water stack needs to be inserted upward from the bottom of the core.
If a mechanical or electrical failure occurs, the control rod of the boiling water reactor may not be able to be inserted, and the control rod of the pressurized water reactor can also fall by gravity.
Of course, the Fukushima nuclear accident is the result of a series of factors, including natural disasters and human disasters.
But you can’t give up your enthusiasm for nuclear power generation just because of a nuclear accident.
How to ensure the safety of nuclear power plants requires us to design it carefully. After all, tigers still eat people, but we can watch it outside the cage.
China Nuclear Safety
Take my country's pressurized water reactor nuclear power plant as an example, it sets up four safety barriers.
The first channel is the fuel core pellet, which is this uranium dioxide ceramic crystal, with most of its micropores not communicating with the outside. Under normal circumstances, more than 98% of radioactive substances will remain in these micropores.
The second barrier is the fuel cladding, that is, the fuel rod that contains the fuel core pellets. It can seal the fission product in the zirconium alloy cladding;
The third barrier is a circuit boundary, and the cooling water circulates and flows in the reactor pressure vessel, main pipe and steam generator. This is a completely sealed system.
Even if the fuel rod is damaged, radioactive substances will remain in pressure vessel and will not leak outside.
The fourth barrier is the containment shell.
This is a dome of 1 meter thick reinforced concrete building. It can ensure that the radioactive substances in nuclear reactor do not leak out, and can also withstand earthquakes, tornados and even large aircraft impacts.
Fukushima nuclear accident
Some of the safety design of my country's nuclear power plant is to draw experience from the Fukushima nuclear accident. So how did the disaster that year happen?
Speaking of the Fukushima nuclear accident in 2011, you may think it was caused by a major earthquake, but nuclear power plants are not afraid of earthquakes. The site selection of nuclear power plants is very strict, and they are usually built on a whole strata.
At the same time, during construction, a large amount of concrete ensures that the bottom of the reactor is an unbreakable whole.
When the earthquake just occurred, Fukushima Nuclear Power Plant was actually stress-free. Units 1, 2 and 3 automatically shut down the reactor according to the preset procedures, while Units 4, 5 and 6 are undergoing overhaul repairs, and everything seems to be normal.
However, an hour later, the tsunami came.
The tsunami that head-on attacked the Fukushima Daiichi Nuclear Power Plant reached 15 meters, while its own tsunami defense capability was only more than 5 meters. Soon, the nuclear power plant was almost soaked in 10 meters of water.
What's a pity is that the emergency diesel generators of units 1, 2 and 3 are actually arranged in the basement, that is, below sea level. In this way, all the diesel generators located in the basement have failed.
Even without emergency power, the original designers considered the nearby power grid. However, in the tsunami with a wave height of 39.8 meters, the towers near of the nuclear power plant collapsed one after another, and the power grid was destroyed.
After losing the external power supply, the reactor core cannot provide enough cooling water, and the decay heat of the fuel rod cannot be exported, causing the temperature of the cooling water in the core to continue to rise and eventually boil and evaporate, which makes the fuel rod directly exposed to the air, and Pandora's magic box is opened.
The huge heat first melts the fuel rod itself, and then the zirconium alloy wrapped in the outer layer. The melted zirconium alloy reacts with high-temperature water vapor, producing a large amount of hydrogen , which eventually leads to a hydrogen explosion, causing all the concrete at the top of the reactor factory to blow up, leaving only an empty steel structure rack.
But even so, the public relations of Dongpin Company came out and said: "Although a hydrogen explosion occurred, the reactor containment is still intact."
What they call containment is nothing more than the steel structure frame that wraps the reactor, and the outer concrete wall had already exploded in the explosion.
In this accident, if the culprit of the accident is to be selected and the tsunami is ranked first, then the management of TEDT will be ranked second.
Shortly after the accident, the engineers on the scene suggested using seawater to cool the reactor, but no one dared to make a decision from the Fukushima Nuclear Power Plant to Tektronix.
Because cooling the reactor with seawater means that these nuclear power units will be completely scrapped, and the interests have destroyed the conscience.
Later, the mobile power supply car sent electricity, and the staff quickly injected cooling water into the reactor, but due to the high pressure in the reactor, the cooling water could not be injected at all.
So Telco thought of a better way for them, which is to discharge the radioactive steam in the reactor into the air to reduce the internal pressure. For them, this operation is as simple as bowing and apologizing.
Only after the explosion of Unit 1 did Tel.P.T.P.C. had to use seawater to cool the reactor. However, at this time, it would not have much effect, but instead brought greater trouble.
Due to an explosion, there was a ruin on the ground. After the sea water flowed through the ruins, it carried a large amount of radioactive material to the Pacific Ocean.
Hualong No. 1
In my opinion, the Fukushima nuclear accident is actually not complicated. It is like a domino. The first piece is overturned, and the defense line behind it is all broken down. The key is how to design the safety protection of the nuclear power plant.
For example, my country's third-generation nuclear power technology - Hualong One, it has non-kinetic energy safety technology, and is equipped with a water tank on the top of the containment shell, and its valves require electricity to remain closed.
After losing power, the coolant in the water tank relies on its own gravity to pour into the reactor and directly cool the core.
And the Fukushima Nuclear Power Plant also has a water tank, which is a circular sink surrounding the reactor. There is an electric-driven valve between the reactor and the sink. In the event of loss of power, the valve is closed.
Let’s take a look at the “Hualong One” containment shell. It adopts a double-layer concrete design scheme. The inner containment thickness is 1.3 meters and the outer containment thickness reaches 1.8 meters. It can withstand the impact of large aircraft and the combustion of aviation fuel . Negative pressure is formed between the inner shell and the outer shell. Even if the inner shell is damaged, radioactive materials will not leak into the environment. In addition, it can resist super strong typhoons and magnitude 9 earthquakes of magnitude 17.
It can be said that "Hualong No. 1" is currently the most secure reactor in the world. It is almost impossible to have a nuclear accident that will have an impact on the outside world. From design to construction, and even to safety guarantees, nuclear power plants have undergone layers of strict review to ensure that there is no accident.
So in a sense, it also requires technology and courage to drag a nuclear power plant accident into Fukushima .
Next episode, I hope your nuclear power plant is already running. Let’s talk about how nuclear waste is processed?
