Controlled fission of U-235 in the nuclear rector

The neutrons in the reactor are absorbed (1) by the nuclei of Uranium which causes it to break up (1) releasing energy (1) and neutrons (1)

The released neutrons cause further fission (1) leading to a chain reaction (1)

The moderator is used to slow down the neutrons (1) to increase the rate of fission. The control rods absorb the neutrons (1) and can be inserted to decrease the rate of fission or removed to increase the rate of fission (1)

The energy released is used to heat water (1)