Plutonium is the other readily fissile material. It requires that it is hit by faster moving neutrons than Uranium-235 for it to fission.

It can be said that plutonium does not occur naturally on earth as it halves in its quantity every 24400 years. Since the earth is about 4.5 billion years old it is hard to imagine that there is any left if there ever was any.

The plutonium used in nuclear reactors and weapons is therefore man-made. It comes from a nuclear reactor. It is created under these equations:

Breeder reactors use this process to produce their plutonium and can produce more plutonium than they actually use.