Nuclear fission
It all starts inside atomic nuclei. The nuclear fission process begins when a neutron hits a uranium nucleus. The uranium nucleus splits, releasing heat and new neutrons, which in turn can split other uranium nuclei. A chain reaction has started!
Inside the reactor containment is the reactor vessel. In it, the fuel elements are packed tightly to create a so-called critical mass. But that's not enough - the neutrons released during nuclear fission are moving too fast and their speed needs to be slowed down. That's why the fuel elements are enclosed in water. The water acts both as a moderator, which means it slows down the speed of the neutrons, and as a coolant. Only then can the neutrons be trapped in new atomic nuclei and cause fission of the uranium-235 nucleus.
Fission also releases heat energy. This helps to boil the water in the reactor vessel while the released neutrons, in turn, fission other nuclei creating heat and releasing more neutrons and so on. A chain reaction has started.
Water and boron are used to control the number of nuclear fissions, i.e. the reactor power. Boron is a substance that traps neutrons. By varying the flow of water through the reactor core - and by inserting or withdrawing control rods between the fuel elements - the reactor power is controlled. Varying the position of the control rods affects the power distribution in the core, as boron traps neutrons and interrupts the chain reaction. The control rods are operated from below by electric motors, but can be inserted by actuators, known as quick stops, if necessary.