how does a nuclear reactor works ?

When a heavy nucleus like uranium-235 is hit with a neutron, it undergoes nuclear fission. The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons. The released kinetic energy is converted to thermal energy.

This thermal energy is used to boil water to produce pressurized steam, which will drive steam turbines to generate electricity. This is the working of nuclear reactor.