The binding energy of a nucleus is the amount of energy needed to separate a nucleus into its constituent neutrons and protons. The mass defect is the difference in mass between the mass of the separated nucleons and the mass of the nucleus. The energy released can be calculated by Einstein's energy mass equation of E = mc^2.
Nuclear fission can occur by bombarding Uranium 235 nucleus with a neutron, causing a chain reaction of releasing more thermal neutrons with collide with more uranium 235 nuclei. Within a thermal nuclear reactor, fuel rods are handled remotely, the moderator slows down neutrons with elastic collisions and control rods can absorb excess neutrons for a controlled reaction.