Shulou(Shulou.com)11/24 Report--

The long-standing "talk about Nuclear Color change" on April 26, 1986, in the Soviet Union and Ukraine, due to design defects, inappropriate testing experiments, and "honey" operation after problems, a serious explosion occurred at the No. 4 reactor. this is one of the most serious nuclear power plant accidents in history-the Chernobyl nuclear power plant accident. In the decade since then, nearly 200000 people have been killed.

Chernobyl nuclear power plant accident | Source: Baidu Encyclopedia on March 11, 2011, an earthquake measuring 9 on the Richter scale struck the Pacific Ocean in northeastern Japan and triggered a huge tsunami. There was a failure at the Fukushima Daiichi nuclear power plant, the control rods could not be inserted, the core temperature rose, and the reaction between cladding and water led to the production and accumulation of a large amount of hydrogen, which eventually led to the explosion of the plant of several units and the release of a large amount of radioactive gas into the atmosphere.

Identified by the Japanese Atomic Power Safety and Security Institute (NISA), the Fukushima nuclear power plant accident and the 1986 Chernobyl nuclear power plant accident are the highest level of nuclear accident: level 7 (major accident). It has been more than a decade since the accident at the Fukushima nuclear power plant, and according to statistics, more than 100 people have been affected by the radiation from the accident.

In fact, since the birth of nuclear power plants in the 1950s, there have been many large or small nuclear power plant accidents or anomalies all over the world. Including China's Daya Bay Nuclear Power Station in Guangdong Province, there was an abnormal phenomenon in 2010. Although this incident may not be regarded as an "accident", it still caused the staff at that time to sweat.

Now, Japan's attempt to discharge nuclear waste water from the Fukushima nuclear accident into the sea has caused heated discussion around the world, as well as great panic to billions of people around the world. You may ask, since nuclear power plants are "so dangerous", why build nuclear power plants vigorously?

Powerful nuclear energy Xiangning heard that every boy has a dream of building an atomic bomb. Of course, some girls also have it, such as Xiangning herself. But in times of peace, no one wants a beautiful blue star to grow a huge "🍄 mushroom cloud". Therefore, it is also very good to make some contribution to the nuclear power industry. after all, both are fission chain reactions of uranium-235.

Speaking of uranium-235, it is the 92nd element. There are 92 protons and 143 neutrons in a uranium-235 nucleus, which is a radioactive element in actinides. There are also two common isotopes of uranium-235 in nature, they are U-234 and U-238, but these two isotopes basically absorb neutrons and do not participate in fission reactions.

Nuclear fission | the fission reaction of source uranium-235 is mainly induced by neutron bombardment. When a neutron quickly bombards the nucleus of U-235, the original U-235 splits to produce two other elements, 2-3 neutrons, and about 200 MeV of energy. The generated 2-3 neutrons are not idle, but will continue to bombard other uranium-235, which in turn leads to more fission. When this reaction occurs one after another, it is called a chain reaction.

Chain reaction | it has been calculated here that the energy generated by the complete fission of 1g of pure uranium-235 can not only cook hundreds of tons of beef, but also equivalent to the explosion of 20 tons of TNT explosives, which is powerful enough to blow up buildings within a radius of 200m. Youdao is "there is no difference between good and evil". The important thing is how to use it. Is it good to use such a huge amount of energy to generate electricity? Of course!

Are nuclear power plants really clean and safe? To put it simply, the process of power generation in a nuclear power plant is to use the heat generated by the fission of uranium-235 to heat hot water, and the water vapor produced by boiling the hot water drives the turbine to rotate, which in turn drives the generator to generate electricity. After the transmission of power transmission and substation network, clean nuclear power will be used in your home.

You may ask, is nuclear power really safe? Don't panic, let's first look at the main components of the nuclear power plant, which includes the nuclear island, the conventional island, the supporting facilities of the nuclear power plant and the safety protection part of the nuclear power plant. Among them, the nuclear island is the main component of the nuclear power plant.

Let's take the boiling water pile as an example. Looking at the leftmost part of the picture below, the nuclear island consists of a reactor and a shell. The reactor consists of nuclear fuel rods, control rods and reaction furnaces, where the fission of uranium 235 takes place. Among them, the content of the radioactive element uranium-235 in each fuel rod is about 2% to 4%, which is much lower than its content in nuclear weapons (about 90%), and the uranium here is mainly in the form of uranium dioxide and is sealed in a zirconium alloy cladding tube. The low concentration of uranium-235 only has a low reaction rate and will not produce huge energy in an instant to cause an explosion.

Boiling water reactor structure | Source: Baijia / pupil science cloud control rods? As the name implies, it is used to control the speed of nuclear reactions. The control rods are mainly made of silver, indium and cadmium, with stainless steel cladding on the outside, which can absorb neutrons from the reactor. When the control rod is inserted into the gap between the fuel rods, it will slow down or even stop the nuclear reaction.

On the other hand, the conventional island is composed of turbines, generators and other parts, which is mainly used to convert heat energy into mechanical energy and then into electrical energy; the supporting facilities of nuclear power plants include some control systems and emergency control systems. The safety protection measures of the nuclear power plant mainly include the fuel rod cladding made of zirconium alloy tube or stainless steel pipe, the reactor pressure vessel shell composed of thick alloy steel plate and the containment shell (or reactor building) made of reinforced concrete. This kind of protection strategy, which is almost "redundant", is called "defense in depth". Enough to see that if there are no natural and man-made disasters, there is no doubt about the safety of the nuclear power plant!

But even so, the neutrons produced by the fission of uranium-235 still have a high speed, and the fastidious uranium-235 does not like to absorb such fast neutrons, so what should we do? At this point, the moderator comes out to play a role. Commonly used moderators are very common, which is the source of human life-water, which can slow down the speed of neutrons without absorbing neutrons.

See here, is there a little friend who wants to ask: is the nuclear waste water from Japan's Fukushima nuclear power plant used to slow down neutrons? Bingo, that's right! Under normal circumstances, the water of these moderators will not be leaked when the nuclear power plant is in a safe working condition. However, after the Fukushima earthquake, the lack of cooling water caused the core fuel rods to heat up and there was an uncontrollable explosion. Later, in order to cool the core, a large amount of water was poured into the reactor, resulting in a large amount of radioactive nuclear waste water.

Therefore, these nuclear waste water contains a large number of nuclear reaction products, including a large number of unused proliferative materials uranium-238 or thorium-232, unburned and newly generated fissionable materials plutonium-239, uranium-235 and transuranic elements such as neptunium, americium and curium produced by nuclear fuel during irradiation, as well as fission elements strontium-90, cesium-137, technetium-99 and so on. If it is discharged into the sea without treatment, it is bound to cause great harm!

108 positions of boiling water at present, worldwide, the reactors used in nuclear power plants are light water reactors, heavy water reactors, improved gas-cooled reactors and fast reactors. Among these types, LWR is the most widely used. If you don't understand, ask: what is light water? In fact, light water is an important binary compound: dihydrogen monoxide (HoloO)!

According to different ways of "boiling water", LWR also includes boiling water reactor nuclear power plant and pressurized water reactor nuclear power plant and so on. Here the editor asks the question: what kind of reactor is the Fukushima nuclear power plant? What about Chernobyl?

1. The structure of boiling water reactor is very simple, there is only one cooling water loop. Moreover, the water in this water loop is not only a coolant, but also a neutron moderator. In the boiling water reactor, the pressure in the cooling system is basically maintained at 70 atmospheric pressure, and the water will boil at about 270 ℃.

In the reactor, the steam-water mixture filters out the liquid water through the steam-water separator and steam dryer above the core and then sends it directly to the turbine to propel the generator to run and generate electricity. The steam leaving the turbine condenses into liquid water through the condenser and is sent back to the reactor to complete a cycle.

Boiling water reactor | Source: wiki compared with pressurized water reactor, the structure of boiling water reactor is simpler, and the working pressure and core temperature of the reactor are much lower, so the reactor is safer and cheaper. However, the circulation system of the boiling water reactor is directly connected to the core and the turbine, which may not only cause radioactive contamination of the turbine, but also adversely affect the stability of the turbine and other equipment in the reactor.

Pressurized water reactors, however, pressurized water reactors are the most widely used reactor types in the world, accounting for almost 60% of the total nuclear power capacity. At present, this type of stack is also vigorously developed in our country. Why is this?

First, take a look at the structure of the pressurized water reactor. The structure of pressurized water reactor is more complex than that of boiling water reactor, and its cooling system consists of two circulation loops. Look at the picture, there is the yellow first loop and the blue second loop. The primary circuit connects the reactor core with the steam boiler in the secondary circuit. The pressure in this loop is relatively high, usually about 150 atmospheric pressure. At such a high pressure, the cooling water can be heated to about 343 ℃ without boiling.

Pressurized water reactor | Source: Baidu then, the circulating cooling water in the primary loop transfers heat to the secondary circuit after passing through the steam boiler, because the pressure in the secondary circuit is about 70 atmospheric pressure, and the water in the primary circuit heats the water in the secondary circuit to about 270 ℃ boiling, and after purification, the water vapor is sent to the turbine to drive the generator to run and generate electricity. After heat transfer, the water in the primary circuit lowers the temperature and flows back to the reactor core to complete the primary loop cycle; the secondary loop water from the turbine is condensed through the condenser and also flows back to the steam boiler to complete the secondary loop cycle.

Although compared with the boiling water reactor, the pressurized water reactor is more complex, and the working pressure and temperature in the core are higher than that of the boiling water reactor, so the material performance of the reactor is higher than that of the boiling water reactor. However, the core volume of PWR is smaller and the core power density is higher (the core power density of large PWR can reach 100 kW / L), so the power generation efficiency of PWR is higher than that of boiling water reactor.

Going back to the question that Sangning has just thrown out, it is not difficult to speculate on the reactor type of the Fukushima nuclear power plant. Bingo! Boiling water pile! This is also an important reason why it produced a large amount of nuclear waste water with radioactive material after the accident. The Chernobyl nuclear power plant was built earlier and belongs to the second generation of graphite boiling water reactor, which uses graphite as moderator and boiling light water as coolant.

Did you find out?

Isn't the core meaning of a nuclear power plant

Fancy boiled water!

Why vigorously develop nuclear power? If asked: why should nuclear power be developed? Of course, it has many outstanding advantages.

Here, we can not generalize, although the accident of the nuclear power plant is very serious, but from the probability of accidents, nuclear power is a safe, environmentally friendly and sustainable energy. Because of the large reserves of nuclear fuel, it can be used for much longer than primary energy, and nuclear power plants do not produce large amounts of greenhouse gases like conventional thermal power plants. Therefore, from the perspective of long-term development, nuclear power is very beneficial.

Of course, there are some troubles in nuclear power. In nuclear waste, there are a large number of unfinished radionuclides that are difficult to deal with; moreover, nuclear waste and contaminated equipment and clothing are also a headache.

Finally, we propose again--

For nuclear power

Safe, reliable and foolproof design and operation is critical!

Hope that the future of mankind can live in harmony with nuclear power!

References:

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Feng Xianling, Xue Guangyu. Working principle and development prospect of nuclear power plant [J]. Industry and Technology Forum, 2021 Journal 20 (07): 75-76.

[3] Hu Anbang. The riddle of nuclear power plant wastewater [J]. Inside and outside the classroom (Science Fans), 2021 (06): 24-25.

[4] Altman spoke on how to dispose of nuclear waste

[5] the past life and present life of nuclear wastewater.

Wang Qun, Geng Yunling. Analysis and consideration of Fukushima Nuclear accident in Japan [J]. Defense Science and Technology, 2012 Jing 33 (06): 11-19.

Wang Yongqiang, Yang Bin. Talking about nuclear power and its safety [J]. Metal World, 2011 (03): 10-15.

[8] Motion Picture decryption the manufacture of Atomic bomb

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). Author: Yan Xiangning

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