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.
A bomb cannot be made simply by putting a piece of uranium larger than critical mass into a casing, because this would cause it to go off immediately. Instead, two or more pieces are inserted a safe distance apart and assembled, or shot together, to start a chain reaction.
The simplest possible atomic bomb is one of the type dropped on Hiroshima. It is known as a gun-type bomb since it actually contains a type of gun. At one end of the barrel there is a target, a piece of U-235 slightly smaller than critical mass and shaped like a sphere with a conical wedge removed from it to form a tapering gap. At the other end of the barrel there is another, smaller, piece of U-235 in the shape of a cone with its apex pointing towards the gap in the target. It is the exact shape of the piece missing from the sphere. Together, the two pieces exceed the critical mass.
The smaller piece is backed by a charge of ordinary HIGH EXPLOSIVE. When this is set off, the cone is shot into the sphere and the force of impact welds the two pieces together solidly. The explosion follows instantly.
Plutonium bombs are slightly more sophisticated. Plutonium is even more easily fissionable than U-235, and its critical mass is lower: 35.2 lb (16 kg) for pure Pu-239. The mass can be reduced further, to 22lb (10 kg), by making a sphere of this weight of plutonium and surrounding it with nonfissionable U-238, which reflects neutrons back into the center of the sphere and minimizes loss to the outside.
Plutonium cannot be exploded so easily by a gun-type device. It has to be assembled with much greater speed or it will not explode properly.
Plutonium is therefore assembled by a technique known as IMPLOSION. A number of wedge-shaped pieces of plutonium, which together will build up into a sphere, are arranged at equal intervals around a neutron source. Explosive charges of exactly equal weight are placed behind each wedge and all are detonated together. The wedges shoot towards the centre and touch each other at the same moment. This technique was used for the second American atomic bomb, which was dropped on the city of Nagasaki.