Uranium is not the only material used for making A-bombs. Another material is the element plutonium, in its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from uranium. This can be done by putting U-238 in a nuclear reactor. After a while, the intense radioactivity causes it to pick up the extra particles, so that more and more of its atoms turn into plutonium.

Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the plutonium itself.