Fission Induced Principle
Under the bombardment of foreign particles, the incident particles and the target core form a composite core. When the excitation energy of the composite core exceeds the height of the fission barrier, the probability of fission increases significantly. This fission is called induced fission. Among the induced fission, neutron fission is the most important. There is no Coulomb barrier between the neutron and the target nucleus, and neutrons with very low energy can enter the nucleus and cause the nucleus to excite and fission. After fission, neutrons are emitted, forming a chain reaction.
Nuclides that can cause fission by thermal neutrons are called fissile nuclei, also known as nuclear fuel. 235U, 239Pu and 233U are all nuclear fuel. 235U occurs naturally, but its abundance is small. 239Pu and 233U are produced by nuclear reactions.