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

Li Cundong

School of Physics, University of Chinese Academy of Sciences

Training unit: Institute of Physics, Chinese Academy of Sciences

Today, on August 24, 2023, the Japanese government was scheduled to formally discharge nuclear sewage into the Pacific Ocean (the Japanese side also indicated that it would cancel the discharge because of inappropriate weather conditions). This is a disaster for all mankind. The Japanese government has openly transferred the risk of nuclear pollution to the world, putting its own self-interest above the long-term well-being of all mankind, which is extremely selfish and irresponsible!

It should be said that the Fukushima nuclear accident originated from natural disasters, but today it comes more from man-made disasters, but nuclear power, as a green and stable clean energy technology, can be said to be a necessary science and technology tree for us to go to the sea of stars in the future. Therefore, we must not give up eating for fear of choking or talk about "nuclear" discoloration.

In fact, we have seen a lot of nuclear power plant accidents, including the Chernobyl nuclear power plant accident in the former Soviet Union, the three Mile Island nuclear accident in the United States, and the Fukushima nuclear power plant accident. If there is an accident in the nuclear power plant, it will produce a large number of radioactive nuclear pollutants, such as nuclear waste water, which will bring great disaster to human society. At present, there is no good way to deal with these nuclear pollutants, generally through deep burial waiting for their own decay. But we should see that the development of nuclear power plants is also advancing by leaps and bounds. Well, a safer fourth-generation nuclear power plant is coming soon.

At present, countries around the world are making great efforts to develop safer fourth-generation nuclear power plant technology, isn't it? when I went to Baidu to check (touch) Wen (fish) two days ago, I ate a whale. China's nuclear power plant technology has reached this point?

It's reasonable to eat a whale while fishing. The picture is from the Internet. As for why I'm surprised, please let me sell it first.

When it comes to the fourth generation of nuclear power, one of the first questions to bear the brunt is: what is the fourth generation of nuclear power? why is it the fourth generation? what are the first three generations? is nuclear fusion coming soon?

Don't worry, in fact, the fourth generation of nuclear power is still nuclear fission, and there is still a long way to go before nuclear fusion.

The concept of fourth-generation nuclear power was put forward by Americans. In 1999, the U.S. Department of Energy first put forward the initiative of fourth-generation nuclear power, and then organized a meeting of senior government representatives from Argentina, Brazil, Canada, France, Japan, South Korea, South Africa, the United Kingdom and the United States. the fourth-generation nuclear power was discussed and a consensus was reached: the world needs to develop nuclear power, but the existing third-generation nuclear power technology still needs to be improved, and the new nuclear power technology must improve economy and safety. To reduce nuclear waste and prevent nuclear proliferation, new nuclear power technologies should be considered together with the nuclear fuel cycle.

Pexels, in fact, the intergenerational upgrading of nuclear power technology may not be as great as we think. To put it simply, the first generation of nuclear power technology is the early prototype reactor plant, that is, the light water reactor nuclear power plant developed in the 1950s and early 1960s; the second generation nuclear power plant is the large commercial nuclear power plant developed in the late 1960s and early 1990s. As for the third-generation nuclear power plants, they refer to the more advanced LWR nuclear power plants, which adopt passive safety systems with standardized, optimized design and high safety.

The main development objectives of the fourth generation nuclear power plants are: the sustainable development of nuclear energy, that is, to provide sustainable energy and minimize nuclear waste through the effective use of nuclear fuel, and to improve safety and reliability. greatly reduce the probability integration of core damage, and have the ability to quickly restore reactor operation, eliminate the need to take emergency measures outside the plant site, improve economy and prevent nuclear proliferation.

Comparison of goals and requirements between third-generation and fourth-generation nuclear power plants | the figure is derived from references, that is, safety improvement is one of the main features of fourth-generation nuclear power. Let's focus on security. The fourth generation of nuclear power plants should be able to prove that there will be no serious damage to the core and ensure that they will not be released from out-of-plant release and do not require off-site response. In other words, even in the event of natural disasters such as earthquakes, tsunamis and hurricanes, fourth-generation nuclear power plants should be able to ensure their safety, that is, there will be no serious damage to the core and no leakage of radioactive material.

Chernobyl is now partially open as a tourist attraction. The picture comes from the network. As for the reactor type of fourth-generation nuclear power, there are actually six options. They are: gas-cooled fast reactor; lead alloy liquid gold cooled fast reactor; molten salt reactor; liquid sodium cooled fast reactor; ultra-high temperature gas-cooled reactor and supercritical water-cooled reactor. All six can be thought of as fourth-generation reactors, but obviously, three of them are fast reactors and the other three are not. It seems that the intergenerational division standard of nuclear power is very unique (bushi

As for do not know what is fast pile of children's shoes, to find a high school physics teacher to make up lessons!

In these six kinds of four-generation reactors, our country has studied them more or less, and some of them have achieved the leading position in the world. Space is limited, so this time we will mainly talk about ultra-high temperature gas-cooled reactors and molten salt reactors. As for the rest, we'll talk when we have time.

Ultra-high temperature gas-cooled reactor how to boil water in nuclear power plants. We have already written an article about nuclear power, a more fancy way to boil water.

It's really well written, but it's a pity I didn't write it. It talks not only about the principle of nuclear power plants, but also about the backward pressurized water reactors and boiling water reactors that we have mainly built, which can be described by our predecessors. I am not going to repeat them and save me a lot of work.

After reading the above article, I think you can understand the moderators and coolants in nuclear reactors. In ultra-high temperature gas-cooled reactor (Very High Temperature Reactor,VHTR). The moderator is graphite and the coolant is the inert gas helium, which is the source of the name, gas cooling.

Schematic diagram of ultra-high temperature gas-cooled reactor | the diagram is from the reference material, which is the schematic diagram of the principle of ultra-high temperature gas-cooled reactor. The core of the pebble bed reactor is the place where the nuclear reaction takes place. The discharge tube can remove the spent fuel after the reaction in time to ensure that the fuel is continuously added to the reactor. The heat generated by the reaction generates steam in the steam generator, emmm, or boiled water, and the resulting steam is recycled to generate electricity in the conventional island.

VHTR is a derivative of the high temperature reactor developed in the last century, which is characterized by all-ceramic coated particulate fuel. It is known as the "reactor that will never melt down". Even in the event of a magnitude 9 earthquake and a severe tsunami, it is impossible for the core to melt due to the loss of core heat, and there will be no nuclear leakage, which is very safe.

China built the world's first commercial-scale ultra-high temperature gas-cooled reactor-Shidao Bay high-temperature gas-cooled reactor nuclear power plant demonstration project-- was connected to the grid last year, which is the world's first grid-connected fourth-generation high-temperature gas-cooled reactor nuclear power project. it marks that China has achieved a world leading position in the field of this advanced nuclear energy technology.

The ultra-high temperature gas-cooled reactor in China uses a spherical core, and its nuclear fuel element is a high-temperature resistant all-ceramic coated particle fuel ball with a diameter of 6 cm. The outermost layer is graphite layer and the matrix graphite powder is inside, which is the moderator of nuclear reaction. 12000 0.9mm nuclear fuel particles coated with four-layer all-ceramic materials are dispersed in the graphite powder.

Schematic diagram of fuel elements for ultra-high temperature gas-cooled reactors in China | from references, the secret of its non-melting lies in this graphite ball, this fuel element can still effectively prevent the leakage of radioactivity at a high temperature of 1650 ℃, and the design shows that even in extreme accidents, the temperature in the reactor can not reach this temperature. Therefore, the ultra-high temperature gas-cooled reactor has very high safety.

In addition to the inherent high safety, the power generation efficiency of the ultra-high temperature gas-cooled reactor is higher; the working temperature of its core outlet is high, which can produce hydrogen without producing carbon dioxide; high temperature can also supply heat to the industrial field, which can be used in petrochemical, coal gasification and other fields.

The molten salt reactor is now recycled at the beginning of the article, and the news that excites me is that the thorium-based molten salt reactor experimental reactor in our country has been put into trial operation.

The picture is from thepaper.cn. Let's start with the molten salt pile. Of the six candidate reactor types of the fourth generation reactor, the molten salt reactor (Molten Salt Reactor,MSR) is the only liquid fuel reactor. The coolant used in the molten salt reactor is molten salt, usually fluorine salt, which is also the carrier of nuclear fuel, which is dissolved in fluorine salt. The fuel used is usually fluoride salts of uranium or thorium. Now the concept of solid fuel molten salt reactor has also been developed, which only uses fluorinated molten salt as coolant to transfer heat, and uses silicon carbide sealed and graphite coated fuel particles as nuclear fuel.

Schematic diagram of molten salt reactor the nuclear reaction of the molten salt reactor is carried out on the left, and the lower freezing plug and emergency storage tank are important guarantees for the safety of the reactor. With the heat exchanger as the boundary, the nuclear island on the left and the conventional island on the right, the heat released by the reaction is absorbed by cooling salt and is used to heat the conventional island to generate high temperature steam and drive the steam turbine to generate electricity. So, although the conventional island is still boiling water, the nuclear island is burning salt?

The inherent safety of the molten salt reactor is obvious. The fuel of the molten salt reactor itself is melted, and there is no core melting problem at all; the low vapor pressure of the molten salt reduces the occurrence of the breach accident, and even if the breach accident occurs, the molten salt will solidify rapidly at the ambient temperature to prevent the further expansion of the accident. When the core temperature rises above the early warning value, the freezing plug at the bottom can melt automatically, and the molten salt will flow into the emergency tank and cool without causing nuclear pollution.

The molten salt reactor can use thorium-based nuclear fuel. The reserves of thorium-based nuclear fuel are higher than that of uranium-based nuclear fuel. at the same time, thorium-based nuclear energy is not easy to be used in weapons and is more suitable for civilian purposes.

Characteristic structure diagram of thorium-based molten salt reactor. In addition to safety, molten salt reactor can make more efficient use of nuclear resources and prevent nuclear proliferation. Without picking fuel, molten salt reactors can directly use all nuclear fuels such as uranium, plutonium and thorium without special treatment, and can even use plutonium obtained by nuclear weapons. There is very little plutonium in the product, which can prevent nuclear proliferation.

The molten salt reactor has high thermal power density and can be miniaturized and modular design. There is no need for control rods in operation, it can be refueled continuously, the service life is long, and the power is easy to adjust.

Molten salt reactor can run at high temperature, and it can also be used in hydrogen production, ammonia production, coal gasification and other industrial fields.

Molten salt piles can be built underground and have no demand for water, so they can be built in arid inland areas. Molten salt is solid at room temperature, will not cause a lot of nuclear pollution caused by leakage, the protection of the biosphere and groundwater table is less stringent, can be built underground, underground construction can also better protect nuclear power plants. The nuclear island of molten salt reactor does not need cooling water, so it can be built in arid areas without water.

Schematic diagram of the future prospect of thorium-based molten salt reactor. Nuclear power, as a typical clean energy, has no pollution and stable output, so it is one of the most ideal ways to generate electricity at present. The vigorous development of nuclear power will not only give us access to cheaper energy, but also help us achieve our goal of carbon peak and carbon neutrality at an early date. Therefore, China has the largest nuclear power capacity under construction in the world. The development of nuclear power is also by leaps and bounds, economy and safety have been significantly improved. It is believed that we will use more, better, safer and cheaper nuclear power in the future.

Source of the cover picture: the second Bureau of China Construction won the bid for the first phase of the expansion project of Shandong Shidao Bay Nuclear Power Plant.

References:

[1] Wang Shiheng. The fourth Generation Nuclear Power Station and the Future of Nuclear Power in China [J]. Science, 2005, 57 (01): 18-21. 4.

[2] Deng Hui, Zhan Meng. Why non-meltdown nuclear reactors are inherently safe [N]. Guangming Daily, 2022-01-06.

Li Xuefeng, Lei Meifang. The emergence and development of the fourth generation nuclear energy system [J]. China Nuclear Industry, 2018 (02): 29-32.

[4] GE Weiwei. High temperature gas-cooled reactor: be the leader of "going out" [J]. China Nuclear Industry, 2018 (02): 15-17.

[5] with the announcement of the National highest Technology Award, you still don't know what the high-temperature gas-cooled reactor is all about.

Cai Xiangzhou, Dai Zhimin, Xu Hongjie. Nuclear energy system of thorium-based molten salt reactor [J]. Physics, 2016. 45 (09): 578-590.

[7] Hu Chunmei. The Renaissance of molten salt heap [J]. China Nuclear Industry, 2018 (02): 26-28.

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

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