The atomic nuclei of certain substances can undergo decay and emit rays that are invisible to the naked eye and cannot be felt, and can only be detected by specialized instruments; this property of a substance is called radioactivity. Radioactive substances are those that can naturally radiate energy outward and emit rays. They are usually metals with high atomic mass, such as plutonium and uranium. There are three types of rays emitted by radioactive substances and they are alpha rays, beta rays and gamma rays.
In the reactor of a nuclear power plant, the nuclear fuel used is mainly containing about 3% of uranium-235, in the event of fission, uranium-235 absorbs a neutron to form a composite nucleus, the composite nucleus is unstable, after a very short period of time (about 10-14 seconds), and then split into two major fragments, while emitting several neutrons and a certain amount of energy.
U-235+neutrons --→ X1+X2+ 2.43 neutrons+energy
X1 and X2 represent fission fragments. When U-235 fissions, more than 60 different fragments are formed, which decay through beta (beta) decay to produce about 250 different nuclides called fission products. Among these fission products, the mass numbers are concentrated around 95 and 140, such as strontium-90, iodine-131, and cesium-137.
Fission fragments are radionuclides that undergo a series of decays and have strong radioactivity, mainly beta rays and gamma (gamma) rays, and some nuclides have a shorter half-life (the time it takes to halve the activity), such as iodine-131 (8 days) and so on, and some have a longer half-life, such as cesium-137 (more than 30 years) and so on. 137 (more than 30 years), etc.
The energy in the reactor is mainly released by the fission of uranium-235, including the instantaneous release of fission energy (accounting for more than 90%) and the fission products in its subsequent decay is the slow release of energy (less than 10%) of the two parts of the instantaneous release of energy, including the kinetic energy of the fission fragments, fission neutron kinetic energy, the instantaneous γ-ray energy, and so on, the slow release of energy including the fission products γ-ray energy. release of energy including fission products γ-ray decay energy and β-ray decay energy, etc..
Fukushima Daiichi nuclear power plant in the earthquake, the reactor through the automatic control system, the fission reaction will automatically stop, so the reactor's main source of energy to stop generating, but because there are a large number of fission fragments will continue to decay, produce a certain amount of energy, so the reactor after the shutdown, as well as from the reactor removed from the spent fuel, in its subsequent period of a long period of time will also continue to produce heat, which needs to be cooled down by water.