Elements used in nuclear reactors are called fissile material.
Nuclear energy, despite controversial issues surrounding it, is still a major part of alternative energy strategy for most countries. There are two types of nuclear reaction, fusion and fission. Fusion is what the sun uses to generate energy — the process in which light atoms combine and form heavier atoms. Fission is used in nuclear reactors and requires the atoms to be split, forming smaller atoms. There are four radioactive fuels used in nuclear reactors, although uranium is by far the most commonly used.
Uranium is the most commonly used reactor fuel as it is the most common usable element. Different isotopes of uranium can be used, with uranium 235 the most widely used, although uranium 233 is also available for this task. Uranium 233 does not exist in nature and is created in reactors using another fission material called thorium.
Plutonium is much rarer in nature than uranium and is more commonly used for bombs rather than as reactor fuel. Like uranium, it comes in various isotope forms, with plutonium 239 the most commonly used. It is mixed with the much more unstable plutonium 240 to generate power.
Neptunium is most commonly found in uranium ore and is normally seen as a by-product of uranium and plutonium reactors. It can also be combined with plutonium to make nuclear weapons. It is also used to detect radioactive neutrons in a Geiger counter.
Named after physicists Marie and Pierre Curie, who discovered radioactive gas radon. It is found chemically attached to uranium and plutonium and is separated in the reactor after neutron bombardment. Like neptunium, it is more commonly associated with nuclear waste than nuclear fuel. A more common use for curium is in X-ray spectrometers as it is a powerful alpha wave emitter.