Nuclear power is produced in a nuclear reactor in a special chamber called the reactor core. Within the core are large atoms of uranium or plutonium, which when struck by a single neutron, split and make smaller atoms, releasing other neutrons. They collide with other atoms, their neutrons are released and a chain reaction races through the rest of the material as more and more atoms are split. This process, called nuclear fission, releases heat (or radiation). If this process were left unchecked, a nuclear explosion could occur. So, the reactor core is surrounded by a jacket of water, and rods of graphite (controllers called neutron modulators) are raised or lowered inside the water to control the stream of neutrons.
The heat energy produced by this controlled chain reaction within the reactor core converts water to steam, and the steam is used to turn the power blades of a generator.