Basic Introduction Chinese Name: Fission Element mbth: Fission Element Discipline: Nuclear Physical Elements: Uranium, Polonium, Thorium Definition: Elements with Fission Reaction Related Terms: Introduction to Nuclear Fission, Related Elements, Fission Reaction, Related Background, Brief Introduction Fission Elements are simply chemical elements that can undergo nuclear fission reactions, mainly thorium 232, uranium 233, uranium 235, uranium 238 and uranium 238. Fission elements are divided into two categories according to whether their nuclei are easy to fission. Fissionable nuclides that can cause nuclear fission of their atoms when bombarded by neutrons of any energy are called fissionable nuclides. The energy of neutrons used to bombard the nucleus of fissionable nuclides can cause different fission reactions; For fissile nuclide nuclei, neutrons with arbitrary energy can be bombarded to cause fission. In practice, according to the size of neutron energy (that is, neutron velocity), neutrons are roughly divided into fast neutrons, intermediate neutrons and thermal neutrons. Uranium is the heaviest metal in nature. It is silvery white and has the characteristics of high hardness, high density, good ductility and strong radioactivity. Uranium usually exists in the combination of uranium with oxygen, oxide or silicate. The fission of uranium atomic energy releases a lot of energy, which can generate electricity and make nuclear weapons. Thorium is a natural radioactive metal element. Because it is more abundant than uranium and does not produce polonium -239, it may be used as fuel for nuclear reactors. Thorium oxide (thorium dioxide) is one of the substances with the highest melting point (3300℃). It turns white when heated, so it is used as a lampshade for gas lamps. Atomic number 90, atomic weight 232.038, melting point 1755℃, boiling point 4788℃. Polonium is the most important transuranic element. Polonium (atomic number 94) is manufactured artificially. In nuclear reactors, natural uranium absorbs neutrons in a series of nuclear reactions. Unlike uranium isotope 238, which constitutes 99.3% of natural uranium, the most important polonium isotope (polonium 239) is a combustible substance, which can be used in nuclear bullets and as raw materials for nuclear reactors. The large-scale production of polonium and its separation from uranium began at the end of World War II. The first bomb containing polonium exploded in Mexico on 1945. Fast breeder reactors not only provide energy, but also produce excessive polonium. Fission reaction nuclear fission is simply the phenomenon that a heavy nuclear fission splits into two or three nuclei with medium mass, and at the same time releases a lot of energy, accompanied by the release of two or three neutrons. Fission reaction is a nuclear reaction in which a heavy nucleus splits into two or more lighter nuclei with different masses. There are two kinds of spontaneous fission and induced fission. Spontaneous fission is the expression of heavy nuclear instability; When heavy nuclei are bombarded by neutrons, charged particles or photons, induced fission is a splitting reaction. Nuclear bullets are designed according to the principle of induced fission reaction. The energy released during fission is quite huge, and the energy released by 1 kg uranium fission exceeds the heat released when 2000 tons of coal are completely burned. 1 uranium -235 may undergo the following fission: u-235+n-1= = kr-89+ba-144+3n.
Fission produces an average of 2.4 neutrons with an energy of 2 15 MeV. Fission releases energy because the storage mode of mass and energy in the nucleus is based on iron and related elements. From the heaviest element to iron, the energy storage efficiency basically changes continuously. Therefore, the process that any heavy nucleus can split into a lighter nucleus (until iron) is beneficial in the energy relationship. If the nuclei of heavier elements can split to form lighter nuclei, energy will be released. However, once many nuclei of these heavy elements are formed inside the star, they are very stable, even though they need input energy (from supernova explosion) when they are formed. Unstable heavy nuclei, such as uranium -235, can spontaneously fission. Fast-moving neutrons can also cause fission when they hit unstable nuclei. Because fission itself will release neutrons from the divided nuclei, if a sufficient number of radioactive materials (such as uranium -235) are piled together, the spontaneous fission of one nucleus will trigger the fission of two or more nearby nuclei, and each nucleus will trigger the fission of at least two other nuclei, and so on, the so-called chain reaction will occur. This is the so-called nuclear bullet (actually a nuclear bomb) and nuclear reactor (controlled slow fission) used for power generation.
For a nuclear bomb, the chain reaction is an explosion caused by uncontrolled fission, because the fission of each nucleus will cause the fission of several adjacent nuclei. For nuclear reactors, the reaction rate is controlled by neutron absorbing substances (usually graphite and cadmium rods) inserted into uranium (or other radioactive substances), so that the fission of each core just triggers the fission of another core on average. The fission of 1 kg uranium -235 will generate 20,000 megawatt hours of energy (enough to run a 20 megawatt power station 1000 hours), which is equivalent to the energy released by burning 3 million tons of coal. The result of nuclear fission reaction is to produce a number of fission fragments of medium quality and their decay products. There are many possible nuclear fission modes, most of which split into two fission fragments. For the fission of 235U caused by thermal neutrons, about 30 different fission modes, that is, about 60 kinds of fission fragments, have been found. The mass number of fission fragments is mostly between 72 ~ 158. Almost all fission fragments are unstable, and they undergo a series of β and γ decay. In this way, the final fission products may include various radioactive and stable nuclear isotopes of more than 300 different nuclides. Some nuclides in fission products have a long half-life or strong radioactivity, which will bring a series of special problems to their transportation and final safe storage (see radioactive waste disposal). This is also one of the important issues that must be considered when using fission energy. Some fission products, such as 135Xe and 149Sm, have considerable thermal neutron absorption cross sections, which will absorb thermal neutrons in the reactor, thus affecting the neutron balance of the reactor. Therefore, the process of generation, decay and disappearance of these fission products should be carefully studied. BACKGROUND From the end of 19 to the beginning of the 20th century, scientists have made in-depth and meticulous research on the wonderful atomic world and made many great achievements. They found that electrons are smaller than atoms; The existence of atomic nuclei was further discovered. Isotopes are determined by accurate measurements. Nuclear characteristics (including nuclear charge, nuclear mass and nuclear volume, etc.). ) conducted a preliminary exploration. 19 19 Rutherford successfully realized the first artificial nuclear reaction in human history, and a new particle, proton, was observed during the nuclear reaction. Since then, people not only know that protons do exist in the nucleus, but also can change one element into another new element through nuclear reaction. 1932 chadwick tong discovered neutrons, which freed scientists from some assumptions about the nuclear structure at that time. German physicist Heisenberg put forward the theory that the nucleus is composed of protons and neutrons. According to this theory, the nuclear structures of various elements in the periodic table can be easily explained. 1934, Iorio and Curie correctly confirmed the existence of artificial radioactivity through chemical analysis, and obtained the first batch of artificial radionuclides from this great discovery. The discovery of artificial radionuclides has given new content to the theory of nuclear structure. It tells us that in addition to naturally occurring radioactive elements such as polonium and radium, stable elements can also be bombarded with alpha particles and neutrons, thus producing a variety of artificial radionuclides. This great discovery by Aurio Curie has opened up a very broad prospect for the artificial manufacture of radionuclides.