How does fission work?

Everything is made up of tiny atoms. Atoms are made up of protons, neutrons and electrons. Some elements, like helium are small, composed of only one proton. Some are large. Uranium, the heaviest element in nature, has 92 protons.
Because uranium is so heavy, the atoms break apart easily. This is why uranium is used as a fuel in nuclear reactors. Fisson happens when a neutron crashes into a uranium atoms and spilt it. The neutrons from that atoms then crash into other atoms, and so on. When huge numbers of neutrons are crashing into atoms, it is called a chain reaction. A chain reaction produces heat.
At the centre of the nuclear power plant is reactor, where fission takes place. Inside the reactor is a supply of uranium, called the core. If the uranium reactor were left on its own, the chain reaction would get out of control. It would became so hot that that the reactor would melt, and then it would pass through the floor. Dangerous radiation would escape in the air. This would be a terrible disaster called the meltdown. To prevent this, scientist have used cooling rods or control rod, that are lowered into the nuclear reactor. This keep the reactor from getting too hot,and control the change reactions.