In which section of a nuclear reactor does nuclear fission occur?

A process wherein the atom’s nucleus is further divided into two nuclei, that is, fission products, is called nuclear fission. This happens when the nuclei of radioactive elements (e.g. Uranium or Plutonium isotopes) are able to attract neutrons. Because these isotopes’ nuclei are unstable, an addition to their energy causes them to divide into new pieces of neutron. In nuclear reactors, fission occurs when the power plant gathers heat from a steam in a physical process. Nuclear reactors are designed to sustain nuclear chain reactions.