All the substances around us, such as our bodies and food, air, water, etc., are made up of the association of "atoms". An atom is made up of "nucleus" and "electronic" which moves around the nucleus.Furthermore, the nucleus is made up of "protons" and "neutrons". Nature of atom differs depending on the number of protons (atomic number). Also, atoms are very small, about one hundred millionths of a centimeter. Furthermore, the nucleus is small, and it is only about ten trillionth of centimeter. The kind of atoms that make up the substance is called "element", and about 110 kinds exist in the world.
Some nuclei emit naturally radiation to turn into another nucleus, and the radiation emitted from the nucleus is an alpha (α) ray and a beta (β) ray, which are particle beams of high speed particles, and a gamma (γ) which is an electromagnetic wave.
If a neutron hits the nucleus of uranium 235, it causes nuclear fission. At that time, thermal energy and neutrons are released together with the production of fission products, and nuclear fission occurs if this neutron hits further another uranium 235. This is called chain reaction of fission. Nuclear power plants use control rods and water to control nuclear fission stably by controlling the number and speed of neutrons. On the other hand, uranium 238 does not nuclear fission mostly, however it changes to plutonium 239 which absorbs neutrons and cause nuclear fission.