What is nuclear energy and how is it controlled?

Nuclear energy is energy released by splitting (fission) atomic nuclei. In a reactor, heavy atoms (like U-235) absorb a neutron and split into smaller nuclei, releasing heat and more neutrons in a chain reaction. The heat is used to produce steam and drive turbines for electricity. Control of the reaction is achieved with control rods made of neutron-absorbing materials (e.g. boron). Inserting these rods into the reactor core captures excess neutrons, slowing or stopping the chain reaction; withdrawing rods accelerates it. Thus operators regulate reactor power (and heat output) by adjusting control rods and moderator conditions, keeping the fission process steady and safe while harnessing the released energy.