Definition 1:
Let's analyze the definition of "nuclear reaction" in the original text: "The process in which a nucleus produces a new nucleus under the bombardment of other particles is called a nuclear reaction". This definition is obviously limited, but it actually describes another kind of nuclear reaction-artificial modification of the nucleus (including heavy nuclear fission, etc. ).
Definition 2:
For radioactive elements such as uranium, bismuth and radium, protons and neutrons in the nucleus can be arranged continuously from high energy to low energy. This phenomenon is called "nuclear reaction" and the excess energy released is called "nuclear energy".
Definition 3:
When the nucleus is bombarded by an artificial nucleus with certain energy, the change of the nucleus is caused by the interaction between them. This process is called nuclear reaction. The first artificial nuclear reaction in history was produced by Rutherford with natural radioactive source (expanded Po) in 19 19.
Definition 4:
The so-called nuclear reaction refers to the process of releasing one or several particles when the nucleus is hit by a particle [1]. In the process of studying it, experimenters often use static laboratory coordinate system to actually measure data.
Definition 5:
The process of nuclear reaction and equation nuclear transformation is called nuclear reaction. The basis of writing the nuclear reaction equation is that the charge number and mass number remain unchanged before and after the reaction.
Nuclear reaction is qualitative change in essence, but it is different from general chemical reaction. A chemical reaction is just a rearrangement of atoms or ions, while the nucleus remains unchanged. Therefore, in a chemical reaction, one atom cannot be changed into another. Nuclear reaction is the transfer of nuclear gap points, which makes one atom change into other atoms, and the atom changes qualitatively. The energy effect of nuclear reaction is far greater than that of chemical reaction. Nuclear reaction energy is often measured in trillion electron volts, while chemical reaction energy is generally only a few electron volts. For example, nuclear reaction can't be realized by general chemical methods, and it needs many experimental techniques and theories of modern physics. First of all, high-energy nuclear "shells", such as helium nuclei, hydrogen nuclei and deuteron, should be artificially made, and these "shells" should be used to violently hit other nuclei, thus causing nuclear reactions. All kinds of accelerators are designed to artificially produce charged high-energy particles, which are used as nuclear "shells" of nuclear reactions. When the neutron was discovered in 1932, people not only had a correct understanding of the structure of the nucleus, but also found that the neutron was a new nuclear "shell". Because the neutron has no charge and there is no electric repulsion between it and the nucleus, it is much better than other charged high-energy particles to produce nuclear reactions. Some factories have nuclear reactors.
(Nuclear) Reactor: A device capable of maintaining a controlled and self-sustaining chain nuclear fission reaction.
Refers to any structure containing nuclear fuel, which is arranged in such a way that a self-sustaining chain nuclear fission process can occur in it without adding a neutron source.
Note: In a broad sense, the term reactor should include fission reactors, fusion reactors and fission-fusion hybrid reactors, but generally it only refers to fission reactors.
Nuclear reactor, also known as atomic reactor or reactor, is a device filled with nuclear fuel to realize large-scale controllable fission chain reaction.
Types of nuclear reactors
Nuclear reactors can be divided into the following types according to their uses: ① experiments with neutron beams or nuclear reactions with neutron beams, including research reactors and material experiments. (2) Nuclear reactors producing radioisotopes. (3) The nuclear reactor producing fissile materials is called production reactor. (4) Nuclear reactors that provide heat for heating, seawater desalination and chemical industry, such as multipurpose reactors. ⑤ Thermal nuclear reaction used for power generation is called power generation reactor. 6. Nuclear reactors used to propel ships, planes, rockets, etc. It's called a propulsion reactor.
In addition, nuclear reactors are divided into natural gas uranium reactors, enriched uranium reactors and thorium reactors according to fuel types; According to neutron energy, it is divided into fast neutron reactor and thermal neutron reactor; According to the material of coolant (heat carrier), it can be divided into water-cooled reactor, gas-cooled reactor, organic liquid-cooled reactor and liquid metal-cooled reactor. According to moderator, it can be divided into graphite reactor, heavy water reactor, pressurized water reactor, boiling water reactor, organic reactor, molten salt reactor reactor and beryllium reactor. According to neutron flux, it can be divided into high flux reactor and general energy reactor. According to thermal state, it can be divided into boiling reactor, non-boiling reactor and pressurized water reactor. According to the operation mode, it can be divided into pulse reactor and steady reactor. There are more than 900 designs of nuclear reactors in concept, but in fact they are very limited.
Working principle of nuclear reactor
Nuclear reactor is the heart of nuclear power plant, and its working principle is as follows:
Atoms consist of nuclei and extranuclear electrons. The nucleus consists of protons and neutrons. When the nucleus of uranium -235 is bombarded by foreign neutrons, one nucleus will absorb a neutron, split into two nuclei with smaller mass, and release 2-3 neutrons at the same time. Neutrons produced by this fission bombard another uranium -235 nucleus, causing new fission. This continuation is the chain reaction of fission. Chain reaction produces a lot of heat energy. Only by using circulating water (or other substances) to take away heat can the reactor be prevented from overheating. The derived heat can turn water into steam and drive gas turbines to generate electricity. Therefore, the most basic composition of a nuclear reactor is fission core+heat carrier. But only these two can't do it. Because high-speed neutrons will scatter a lot, it is necessary to slow down the neutron speed and increase the chance of collision with the nucleus; The working state of nuclear reactor should be decided according to people's wishes, which requires control facilities; Uranium and fission products are highly radioactive and will cause harm to people, so reliable protective measures must be taken. To sum up, the reasonable structure of nuclear reactor should be: nuclear fuel+moderator+heat carrier+control facilities+protection device.
It should also be pointed out that uranium ore cannot be directly used as nuclear fuel. Uranium ore must be selected, crushed, acid leached and concentrated to make uranium rods with certain uranium content and certain geometry before it can participate in the reactor work.
Nuclear reactor use
Nuclear fission releases a lot of energy and neutrons. Nuclear reactors have many uses, but simply put, they use fission nuclear energy and fission neutrons.
Nuclear energy is mainly used for power generation, but it is also widely used in other aspects. Such as nuclear heating and nuclear energy.
Nuclear heating is a new technology developed in 1980s. It is an economical, safe and clean heat source, so it has been widely valued in the world. In the energy structure, the heat source for low temperature (such as heating) accounts for about half of the total heat consumption, which is mostly obtained by direct coal burning, causing serious pollution to the environment. In China's energy structure, nearly 70% of energy is consumed in the form of heat energy, of which about 60% is low-temperature heat energy below 120℃. Therefore, the development of nuclear reactor low-temperature heating is of great significance to alleviate the shortage of supply and transportation, purify the environment and reduce pollution. Nuclear heating is a promising way to use nuclear energy. Nuclear heating can be used not only for residential heating in winter, but also for industrial heating. In particular, high-temperature gas-cooled reactor can provide high-temperature heat source, which can be used in coal gasification, ironmaking and other industries with huge heat consumption. Since nuclear energy can be used for heating, it can also be used for cooling. Tsinghua University has successfully tested on a 5 MW low-temperature heating reactor. Another potential use of nuclear heating is seawater desalination. Among various desalination schemes, nuclear heating is the most economical one. In the Middle East and North Africa, due to the lack of fresh water, there is a great demand for seawater desalination.
Nuclear energy is also a kind of energy with unique advantages. Because it does not need air to support combustion, it can be used as a special power in underground, underwater and airless space. Because it consumes less materials and has high energy, it is a special power supply that can supply energy for a long time after one charge. For example, it can be used as a special power for rockets, spaceships, satellites, submarines, aircraft carriers and so on. Nuclear power may be used for interstellar travel in the future. At present, human space exploration is limited to the solar system, so the energy required by the aircraft is not large, and solar cells can be used. If you want to explore other galaxies outside our solar system, nuclear power may be the only option. The United States, Russia and other countries have been engaged in the research and development of nuclear-powered satellites, aiming at installing power generation equipment with a power generation capacity of hundreds of kilowatts on satellites. Due to the adoption of high-power power supply, the power of satellites in communication and military affairs will be greatly enhanced. 15 10 15, NASA launched the nuclear-powered space exploration spacecraft Cassini. It will fly to Saturn for 7 years and travel 3.5 billion kilometers.
At present, nuclear propulsion is mainly used in nuclear submarines, nuclear aircraft carriers and nuclear icebreakers. Because of the high energy density of nuclear energy, it can run for a long time with only a small amount of nuclear fuel, which has great advantages in the military. Especially nuclear fission energy does not need oxygen, so nuclear submarines can sail underwater for a long time. It is precisely because nuclear power propulsion has such great advantages that in the past few decades, there have been hundreds of nuclear reactors used for ship propulsion in the world, exceeding the number of nuclear power plants (of course, its power is far less than that of nuclear power plants). Now nuclear carriers, nuclear destroyers, nuclear cruisers and nuclear submarines constitute a powerful maritime nuclear force.
The second main purpose of nuclear reactor is to use a large number of neutrons released by nuclear fission chain reaction. There are many uses in this field, and here are just a few examples. We know that the nuclei of many stable elements will become radioactive isotopes if they absorb another neutron. Therefore, the reactor can be used for mass production of various radioisotopes. The wide application of radioisotopes in industry, agriculture and medicine is now almost well known. In addition, membranes with tiny pores are often used in industrial, medical and scientific research to filter and remove tiny impurities or bacteria in the solution. Neutron bombardment of thin film materials in the reactor can produce tiny holes, which can meet the above technical requirements. Using neutrons in reactors can also produce high-quality semiconductor materials. We know that monocrystalline silicon must be doped with a small amount of other materials to become a semiconductor, such as phosphorus. Generally, diffusion method is adopted, and phosphorus vapor permeates the surface of silicon wafer in the furnace. However, the effect of doing so is not ideal. The concentration of phosphorus in silicon is uneven, and the internal concentration becomes lower when the surface concentration is high. Neutron doping technology can now be used. Monocrystalline silicon is irradiated by neutrons in the reactor. After capturing a neutron, silicon decays into phosphorus. Because neutrons are uncharged and easily enter the silicon wafer, the silicon semiconductor produced by this method has excellent performance. Neutrons produced by reactors can be used to treat cancer. Because many cancer tissues have a high absorption ratio of boron, and boron has a strong neutron absorption ability. Boron will become lithium after being absorbed by cancer tissue, and alpha rays will be emitted after neutron irradiation. Alpha rays can effectively kill cancer cells, and the therapeutic effect is much better than that of gamma rays irradiated from the outside. Neutrons in the reactor can also be used for neutron photography or neutron imaging. Neutrons are easily scattered by light substances, so neutron photography is particularly effective for detecting light substances (such as explosives and drugs), but it can't be detected if X-rays or ultrasonic imaging are used.
Development process of nuclear reactor
As early as 1929, Cockcroft successfully transformed the nucleus with protons. However, the nuclear reaction with protons requires a lot of energy, and the chance of proton colliding with the target nucleus is very small.
1938, Germans otto hahn and Hugh Trass successfully collided neutrons with uranium atoms. This experiment is of great significance. It not only makes uranium atoms simply split, but also reduces the total mass after fission and releases energy at the same time. It is particularly important that when uranium atoms fission, apart from fission fragments, two or three neutrons will be emitted, which can cause the fission of the next uranium atom, thus causing a chain reaction.
1939 65438+ 10, the news that neutrons cause nuclear fission of uranium atoms reached Fermi's ears. At that time, he had fled to Columbia University in the United States. Fermi is a talented scientist. As soon as he heard the news, he intuitively imagined the possibility of an atomic reactor and began to work hard for its realization. Fermi organized a research group to thoroughly study the establishment of atomic reactors. Fermi and his assistants often stay up all night doing theoretical calculations and thinking about the shape design of the reactor.
Sometimes you have to personally solve the procurement problem of graphite materials.
1942,1February 2, 942, Fermi's research team gathered in front of a huge graphite reactor in the football field of the University of Chicago, USA. At this time, Fermi gave a signal, and then the control rod was slowly pulled out of the huge reactor composed of 7 tons of uranium fuel buried between graphite. As the counter clicks until the control lever rises to a certain extent, the sound of the counter becomes a piece, which shows that the chain reaction has begun. This is the first time that mankind has released and controlled atomic energy.
1954 the former Soviet union built the world's first atomic power station, using enriched uranium as fuel and graphite water-cooled reactor, with a power output of 5000 kilowatts. 1956, Britain also built an atomic power station. The development of atomic power plants is not smooth sailing. Many people are worried and afraid of radioactive pollution in nuclear power plants, so there has been an anti-nuclear power movement. In fact, under strict scientific management, atomic energy is a safe energy source. The radioactive level around the atomic power station is actually not much different from the natural background.
1in March, 979, three mile island atomic power station in the United States caused an unprecedented serious accident in the history of atomic energy development due to operational errors and equipment failures. However, due to safety measures such as reactor shutdown system, emergency cooling system and containment, the amount of radioactive escape is very small, and people and the environment are not affected, which fully shows that the development of modern science and technology can ensure the safe use of atomic energy.
In a word, because the reactor is a huge neutron source, it is an effective tool for basic and applied scientific research. At present, its application field is expanding day by day, and its application potential is also great, which needs further development.
In the peaceful use of atomic energy, the most typical generation is atomic energy, also known as nuclear energy. If the explosion of the atomic bomb is the result of the instantaneous and uncontrolled nuclear fission chain reaction of uranium -235 or plutonium -239, then the energy used by the atomic power station is the nuclear fission chain reaction of uranium -235 or plutonium -239 under control for a long time. A device that can artificially control the speed of nuclear fission reaction and maintain the chain nuclear fission reaction is called a reactor. Fermi invented the reactor to produce plutonium -239. This kind of reactor is called production reactor. The core of atomic power station is also the reactor, which uses the huge heat generated by nuclear fission reaction in the reactor to generate saturated steam and drive the gas turbine to generate electricity. This kind of reactor is called a power reactor. The difference between atomic power generation and coal and oil power generation is only the device that generates heat. The former is an atomic reactor and the latter is a coal-fired oil-fired boiler.
There are many types of reactors, but they are mainly composed of active region, reflective layer, external pressure shell and shielding layer. The active zone consists of nuclear fuel, moderator, coolant and control rod. Among the reactors used in nuclear power plants, pressurized water reactor is the most competitive type (about 6 1%), boiling water reactor accounts for a certain proportion (about 24%), and heavy water reactor is less used (about 5%). The main characteristics of PWR are: 1) using low-cost and widely available ordinary water as moderator and coolant; 2) In order to keep the cooling water with high temperature in the reactor in liquid state, the reactor runs under high pressure (the water pressure is about 15.5 MPa), so it is called pressurized water reactor; 3) Because the water in the reactor is in liquid state, the steam that drives the turbo-generator set must be generated outside the reactor; This is achieved by a steam generator. Cooling water from the reactor, that is, primary water, flows into one side of the heat transfer tube of the steam generator to transfer heat to secondary water on the other side of the heat transfer tube and convert it into steam (secondary steam pressure is 6-7 MPa, steam temperature is 275-290℃). 4) Because ordinary water is used as moderator and coolant, the thermal neutron absorption cross section is large, so natural uranium cannot be used as nuclear fuel, and enriched uranium (2-4% uranium -235) must be used as nuclear fuel. Boiling water reactor and pressurized water reactor belong to light water reactor. Like PWR, it uses ordinary water as moderator and coolant. The difference is that steam (pressure is about 7 MPa) is generated in the boiling water reactor and directly enters the gas turbine to generate electricity. There is no need for a steam generator, and there is no difference between the primary circuit and the secondary circuit. The system is particularly simple, and its working pressure is lower than that of PWR. However, the steam of boiling water reactor is radioactive, so shielding measures should be taken to prevent radioactive leakage. Heavy water reactor uses heavy water as moderator and coolant. Because its thermal neutron absorption cross section is much smaller than ordinary water, natural uranium can be used as nuclear fuel for heavy water reactor. The so-called thermal neutron refers to the fast neutron emitted by uranium -235 atom, which slows down to 2200 m/s and has an energy of about 1/40 eV. Thermal neutrons are more likely to cause fission of uranium -235 than to be captured by uranium -238 nuclei 190 times. In this way, the nuclear fission chain reaction can continue in the heavy water reactor fueled by natural uranium. Because the efficiency of heavy water moderated neutrons is not as good as that of ordinary water, the core of heavy water reactor is much larger than that of light water reactor, which brings difficulties to the manufacture of pressure vessels. The heavy water reactor still needs to be equipped with a steam generator, and the heavy water in the primary loop brings heat to the steam generator, which is transferred to the ordinary water in the secondary loop to generate steam. The biggest advantage of heavy water reactor is to use natural uranium instead of enriched uranium as nuclear fuel, but an important reason that hinders its development is that it is difficult to obtain heavy water, because it only accounts for 1/6500 in natural water.
The former Soviet Union built the world's first atomic power station in 1954, which opened a new page for the peaceful use of atomic energy by mankind. Britain and the United States built atomic power stations in 1956 and 1959 respectively. By September 28th, 2004, there were 439 nuclear reactors for power generation in 3/kloc-0 countries and regions in the world, with a total capacity of 364.6 million kilowatts, accounting for 16% of the world's total power generation capacity. Among them, France has built 59 nuclear reactors for power generation, and nuclear power generation accounts for 78% of its total power generation; Japan has built 54 buildings, and atomic power generation accounts for 25% of its total power generation; The United States has built 104 building, and the atomic power generation accounts for 20% of its total power generation; Russia has built 29 buildings, and the atomic power generation accounts for 15% of its total power generation. China built its first nuclear power plant in 199 1, including this one, and now there are nine nuclear power reactors in operation with a total capacity of 6.6 million kilowatts. China is building two other reactors. China has also built an atomic power station for Pakistan.
The main advantages of atomic power generation over conventional power generation are: 1) high concentration of energy, low fuel cost and good comprehensive economic benefits. 1 kg uranium -235 or plutonium -239 provides energy equivalent to 2300 tons of anthracite in theory. In the current practical application, 1 kg of natural uranium can replace 20-30 tons of coal. Although the one-time capital investment of atomic power generation is large, the cost of nuclear fuel is much cheaper than that of coal and oil. Therefore, the total cost of atomic power generation has been lower than that of conventional power generation. 2) Due to the small amount of fuel required, it is not limited by transportation and storage. For example, a conventional power plant with a capacity of 6.5438+100,000 kilowatts needs to burn 3 million tons of coal a year, with an average daily demand of 1 10,000 tons of coal. Using atomic energy to generate electricity requires only 30 tons of nuclear fuel a year. 3) Light pollution to the environment. Atomic power generation does not emit harmful gases and solid particles such as CO, SO2 and NOX, nor does it emit carbon dioxide that produces greenhouse effect. Atomic power stations emit little radioactive waste gas and waste liquid every day, and the radiation dose received by surrounding residents is less than 1% of the natural background. The probability of a serious accident that releases a large amount of radioactive materials is extremely low. In the operating history of the world 10000 reactor year, there was only one Chernobyl accident outside the plant, which was caused by the illegal operation of the operators and the design defects of the reactor itself (lack of necessary safety barriers). You may have heard of the accident at Three Mile Island Nuclear Power Station in the United States, which was caused by human negligence and equipment failure. Because the reactor has several safety barriers, no one died in the accident, and the average radiation dose of 2 million people within 80 kilometers is less than that of wearing night-light watches for one year.
Some people may ask, will the reactor explode like an atomic bomb? There are at least three reasons for this: 1) More than 90% of the nuclear fuel used in atomic bombs is fission uranium -235, while only 2-4% of the nuclear fuel used in power generation reactors is fission uranium-235; 2) The reactor is equipped with control rods made of materials that are easy to absorb neutrons, and the speed of nuclear fission reaction is controlled by adjusting the position of the control rods; 3) The coolant continuously brings out the huge heat generated by the nuclear fission reaction in the reactor, so as to control the temperature in the reactor within the required range.
Some people may ask, why don't some countries transfer atomic power generation technology easily? This is because when the reactor is used for power generation, a certain amount of plutonium -239 will also be produced in the reactor (except that most neutrons bombard the uranium -235 nucleus to make it fission, some neutrons are still captured by the uranium -238 nucleus to make the latter become plutonium -239). More than 50% of plutonium-239 produced in the reactor is fission by neutron bombardment, releasing energy and proliferating nuclear fuel; Less than 50% of the remaining plutonium -239 remains in the reactor. ), after reprocessing, plutonium -239 can be extracted and used to make atomic bombs. Heavy water reactors produce about twice as much plutonium -239 as pressurized water reactors.
thrust power
The heat generated by the reactor is brought to the steam generator, and the saturated steam generated by the steam generator drives the turbine to provide propulsion power. The familiar nuclear submarines, nuclear-powered aircraft carriers and atomic icebreakers are all propelled by atomic energy.
Due to the incomparable advantages of conventional submarines, nuclear submarines have become the main warships in the modern navy. The main advantages of nuclear submarines are: 1) long endurance. Endurance refers to the distance that a fuel can sail continuously. For nuclear submarines, the underwater endurance can reach 75,000 nautical miles; However, the underwater endurance of a conventional submarine is only 100-400 nautical miles (related to the speed), because it is propelled underwater by a battery, and it needs to surface or float to the depth of the snorkel every once in a while, and the battery is charged by a diesel generator set. 2) High speed. The underwater speed of nuclear submarines can reach more than 30 knots (1 knot is 1 knot), and they often sail at the maximum speed; The maximum underwater speed of a conventional submarine is 15-20 knots, but due to the limitation of batteries, it generally does not sail at the maximum speed. 3) Good concealment performance. Nuclear submarines stay underwater for about 2500 hours, while conventional submarines only stay 10-20 hours. About 500 nuclear submarines have been built in the world, equipped with nearly 700 reactors, exceeding the total number of reactors built for nuclear power generation. 197 1 year, China built the first nuclear submarine, and the trial flight was successful. From 65438 to 0988, China successfully completed the test of launching ballistic missiles from underwater nuclear submarines.
Nuclear-powered aircraft carriers also have the advantage of high-speed endurance. You can keep the speed above 30 knots for a long time without worrying about fuel consumption. It not only does not need to refuel the logistics fleet, but also carries twice as much aviation fuel and weapons as a conventional aircraft carrier of the same class. Its endurance is 654.38+00,000 nautical miles. The first nuclear-powered aircraft carrier in the world was an aircraft carrier enterprise built by the United States in 1960. In addition, France has a nuclear-powered aircraft carrier.
The world's first atomic icebreaker was built by the former Soviet Union in 1959. Compared with the conventional power icebreaker, it has outstanding advantages: 1) Because there is no need to reserve a large amount of fuel, the load capacity of the ship will not be reduced due to fuel consumption, and the ice breaking capacity will always remain unchanged; 2) The shaft power can reach 75,000 horsepower, and it can sail in the Arctic region with ice thickness of 2.0-2.5 meters; However, the shaft power of conventional icebreakers is about 25,000 horsepower, and generally it can only sail where the ice thickness is 0.7-0.9 meters; 3) Endurance is unlimited.
supply heat
There is a very broad market for direct heating by using the energy generated by reactors. For example, building a 200,000-kilowatt low-temperature heating reactor consumes only 1 ton of uranium dioxide every year, which can provide heat for 5 million square meters of buildings. Boilers heating the same building area need to burn 300,000 tons of coal every year. Compared with the period of 15, the cost of nuclear heating is cheaper than that of coal-fired heating. The former Soviet Union, Canada, Sweden and China all built low-temperature heating reactors for cold regions.