Definition: Nuclear fuel is material that uses nuclear reactions to convert the fuel into energy. There are 2 types of nuclear fuel, uranium and plutonium. There is a third nuclear fuel but it is rarely used. Nuclear fuels are non-renewable energy resources, so when they are used up in nuclear reactions there is none left, they cannot be used again, which is bad for the environment. Nuclear reactions give us energy for light, electricity and various other uses. They are useful in that they can give off a lot of energy for the size.