The World of Nuclear Science

Warfare - Fission Weapons

Many nuclear weapons are based on fission - the splitting of a heavy atom by a neutron into two smaller atoms, releasing energy.

a neutron colliding with a heavy atom may break it up into smaller atoms, releasing neutrons and energy

However, not all samples of uranium-235 will be able to sustain a chain reaction. Samples may contain uranium-238 impurities. These uranium-238 atoms simply absorb neutrons rather than splitting up and releasing more neutrons. Consequently, since no additional neutrons are released, a chain reaction cannot occur via that path.

If the mass of a radioactive material is increased above the critical mass (for example, by joining two small pieces together), the assembly is supercritical, and f will be greater than, or equal to 1. A chain reaction will be sustained. (When the mass is less than the critical mass, the assembly is subcritical.)

In a nuclear bomb, the fissile material (uranium-235) must be kept subcritical during transport, but be made supercritical once it reaches its target. The simplest way to achieve this is to transport two pieces of material to the target, each under the critical mass. Once at the target, the pieces are forced together to create a supercritical mass, fissioning and creating an explosion. Nuclear weapons like this are known as gun-type weapons. The bomb used by the United States over Hiroshima in Japan was a gun-type bomb, and released as much energy as about 20 kilotonnes of TNT.

a schematic diagram of a gun-type bomb. The two pieces each are under the critical mass, and are kept separate by metal casing (not shown). On impact, the detonating rod forces the small chunk of uranium into the bigger chunk, joining them together to create one larger uranium sample that is supercritical. The device then explodes.

the mushroom cloud produced over Hiroshima when the United States dropped a nuclear bomb over the city, instantly killing many people

Another method of nuclear weapon detonation is called implosion. In this type of bomb, chunks of explosives called lenses are placed around a subcritical mass. An electrical impulse detonates these explosives simultaneously, converging onto the mass and compressing it. The compression makes the mass supercritical, creating an explosion. The bomb detonated over Nagasaki in Japan was of this type.

The chain reaction occurs for a fraction of a second, and releases a huge amount of energy within this short time. This causes the temperature to rise by several million degrees, resulting in the vapourisation of the bomb material - an explosion.