The nucleus of the heavy metal element uranium -235 absorbs a neutron and produces a nuclear reaction, which splits the heavy nucleus into two (rarely three) lighter nuclei and 2-3 free neutrons, as well as β and γ rays and neutrinos, releasing huge energy. This process is called nuclear fission.
When neutrons bombard uranium -235 nuclei, some uranium -235 nuclei absorb neutrons and fission occurs. If neutrons produced by nuclear fission of uranium -235 bombard another uranium -235, it will cause new fission, which is a chain reaction of fission. The condition (or state) for this nuclear fission chain reaction to maintain itself, or to maintain its self-sustaining nuclear fission chain reaction, is that there is at least one neutron and no more than one neutron at each fission to another fission. This state is called "critical state".
The self-sustaining chain reaction between neutrons and uranium -235 nucleus can be controlled artificially. At present, the most commonly used control method is to put or take out materials that can absorb neutrons from fissile materials (such as uranium -235) that produce chain reactions. During normal operation, the fissile material is in a critical state, thus maintaining a stable nuclear fission chain reaction and thus maintaining a stable release of nuclear energy. If it is necessary to stop the chain reaction, more neutron absorbing materials should be put in; If more nuclear energy needs to be released, some neutron absorbing materials can be removed. This device, which can maintain and control nuclear fission so as to realize nuclear energy-heat energy conversion, is called a reactor.
1. Composition of atoms
Atoms consist of protons, neutrons and electrons. Everything in the world is made up of atoms, and any atom is made up of positively charged nuclei and negatively charged electrons rotating around the nuclei. A uranium -235 atom has 92 electrons, and its nucleus consists of 92 protons and 143 neutrons. 500,000 atoms are arranged to be equivalent to the diameter of a hair. If an atom is compared to a huge palace, its nucleus is only the size of a soybean, and electrons are equivalent to the tip of a needle. A thermal power plant with a capacity of 1 10,000 kilowatts will burn about 3.3 million tons of coal every year, and it needs many trains to transport it. A nuclear power plant with the same capacity only uses 30 tons of fuel a year.
2. The structure of the nucleus
Nuclei are usually composed of protons and neutrons. The simplest hydrogen nucleus has only one proton. The number of protons in the nucleus (that is, the atomic number) determines which element the atom belongs to. The total number of protons and neutrons is called the mass number of atoms.
isotope
Some atoms with the same proton number p but different neutron number n, or some atoms with the same atomic number z but different atomic mass numbers occupy the same position in the periodic table of chemical elements, which are called isotopes. Therefore, the term "isotope" is used to refer to various atoms of an element, which have the same chemical properties. Isotopes are usually divided into heavy isotopes (such as uranium -238, uranium -235, uranium -234 and uranium -233) and light isotopes (such as hydrogen isotopes such as deuterium and tritium) by mass.
4. Nuclear energy
More than 50 years ago, scientists found that the nucleus of uranium -235 can split after absorbing a neutron, and release 2-3 neutrons and a lot of energy at the same time, which is much greater than the energy released by chemical reaction. This is nuclear fission energy, which is what we call nuclear energy.
Atomic bombs use the energy released by nuclear fission to kill and destroy, and nuclear power reactors also use this principle to obtain energy. The difference is that it can be controlled.
5. Light nuclear fusion
Two lighter nuclei merge into a heavier nucleus, and at the same time, huge energy is released. This reaction is called light nuclear fusion reaction. It is one of the important ways to obtain nuclear energy. In the sun and other stars, due to extremely high pressure and temperature, light nuclei have enough kinetic energy to overcome electrostatic repulsion and continue to fuse. Self-sustaining nuclear fusion reaction must be carried out at extremely high pressure and temperature, so it is called "thermonuclear fusion reaction".
Hydrogen bombs use the fusion reaction of deuterium and tritium nuclei to instantly release huge energy to kill and destroy. Controlled thermonuclear fusion reactors are also applying this basic principle. The biggest difference between hydrogen bomb and hydrogen bomb is that the energy it releases can be controlled.
6. Characteristics of uranium and its energy release
Uranium is the element with the largest atomic number in nature. Natural uranium consists of several isotopes: except 0.7 1% uranium -235(235 is the mass number) and a small amount of uranium -234, the rest are uranium -238. The energy released by the complete fission of uranium -235 nucleus is 2700000 times that released by the complete combustion of the same amount of coal. In other words, the energy released by 1g U-235 is equivalent to the energy released by 2 tons of semi-high quality coal.
7. How to release nuclear energy?
There are two main ways to acquire nuclear energy, namely heavy nuclear fission and light nuclear fusion. One characteristic of U-235 is that when a neutron bombards its nucleus, it can split into two smaller nuclei, producing 2-3 neutrons and β, γ rays at the same time, releasing about 200 mev of energy.
If a newly generated neutron bombards another uranium -235 nucleus, it will cause new fission, and so on, so that the fission reaction will continue. This is the fission chain reaction, and nuclear energy is constantly released.
8. Release of nuclear fusion energy
Light nuclear fusion equal to uranium releases several times as much energy as uranium. For example, the energy produced by the complete reaction of 1g lithium deuteride (Li-6) is about three times that of 1g uranium -235. The conditions of nuclear fusion are very harsh, that is, to keep the hydrogen nucleus at a high temperature of tens of millions of degrees, so that a considerable nucleus can have kinetic energy to realize polymerization.
9. Nuclear power plants are clean energy.
At present, most environmental pollution problems are caused by the use of fossil fuels. The combustion of fossil fuels will release a lot of smoke, carbon dioxide, sulfur dioxide, nitrogen oxides and so on. The "greenhouse effect" caused by harmful gases such as carbon dioxide will raise the earth's temperature, cause climate anomalies, accelerate the process of land desertification, and bring disastrous effects on the sustainable development of social economy. Nuclear power plants do not emit these harmful substances and will not cause "greenhouse effect". Compared with thermal power plants, nuclear power plants will not cause "greenhouse effect".
Around foreign nuclear power plants, people live, swim, graze cattle and sheep and fish. Some nuclear power plants are located near big cities and some are located in tourist areas. Nuclear power plants are safe, economical and clean energy, which is more conducive to environmental protection than thermal power plants.
10. Nuclear power plants are economic energy.
The statistical data of countries with nuclear power in the world for many years show that although the specific investment of nuclear power plants is higher than that of coal-fired power plants, the total power generation cost of nuclear power plants is lower than that of coal-fired power plants because the cost of nuclear fuel is obviously lower than that of coal-fired power plants and fuel is a long-term factor.
1 1. Nuclear energy is an energy source for sustainable development.
The proven uranium reserves in the world are about 4.9 million tons and thorium reserves are about 2.75 million tons. These fission fuels are enough to be used in the era of fusion energy. Fusion fuels are mainly deuterium and lithium. The content of deuterium in seawater is 0.034 g/L. It is estimated that the total amount of water on the earth is about 654.38+038 billion cubic meters, including 4 trillion tons of deuterium and more than 200 billion tons of lithium. Lithium can be used to make tritium, which is enough for human use in the era of fusion energy. According to the current level of energy consumption in the world, deuterium and tritium available for nuclear fusion on the earth can be used by human beings for hundreds of billions of years. Therefore, some energy experts believe that as long as the nuclear fusion technology is solved, mankind will fundamentally solve the energy problem.
fission chain reaction
Neutron-mediated self-sustaining fission reaction. For example, 235U nuclear fission occurs after absorbing one neutron, and at the same time, 2 ~ 3 neutrons are released on average to remove the loss. If another neutron can cause another 235U nuclear fission, the fission can proceed on its own. The energy released by only one nuclear fission is only 3.4×10-1joule (j), which is insignificant compared with the commonly used energy. In order to make nuclear fission reach the practical level, a large number of nuclear fission must occur, which must maintain the fission chain reaction. In fission reaction, neutrons have three possible destinations: fission, absorption and escape from nuclear fuel. The main components of natural uranium are 238U and 235U, and 235U only accounts for 0.7 1%. 235U is a good fission fuel, which can be split by neutrons of any energy and has a large fission cross section for thermal neutrons; However, thermal neutrons can not cause 238U fission, and 238U can absorb thermal neutrons to produce (n, γ) reaction. Neutrons absorbed and escaped from nuclear fuel are losses. In order to realize the fission chain reaction, it is necessary to reduce these two losses, so that the ratio of the number of neutrons obtained in the later fission process to the number of neutrons caused by the previous generation, that is, the neutron multiplication coefficient k≥ 1.