Fission is a kind of radioactive decay – the splitting of an atom – and the process that occurs inside a nuclear power plant’s reactor core. It results in the release of a large amount of energy – usually heat. In the case of the Fukushima Daiichi plant, the heat from the reactor is used to boil water. The resulting steam is used to power turbines, which generate electricity.
When fission occurs, in addition to the energy released, gamma radiation is emitted. Gamma radiation falls under the category of ionizing radiation, which, when it comes in contact with living tissue, can deposit enough energy to change living cells. In high enough concentrations, gamma radiation can cause radiation sickness.