Nuclear fission is the process that produces the atomic bomb, a weapon of mass destruction that utilizes the energy released by the division of atomic nuclei. When a nuclear device is detonated, a large fireball is created, vaporizing anything inside it, including soil and water. This creates the mushroom-shaped cloud that is associated with a nuclear explosion. The radioactive material from the nuclear device mixes with the material vaporized in the cloud in the form of a mushroom.
As this vaporized radioactive material cools, it condenses and forms particles, such as dust. This mixture of debris and soil with radionuclides is sent to the air and then falls back to Earth, known as catarrhal rain. This rain can travel long distances in wind currents and end up miles from the explosion site. It is highly radioactive and can contaminate food and water supplies.
The highest levels of outdoor rain radiation occur immediately after the arrival of rain and then decrease over time. While a serious event, such as a plane crash against a nuclear power plant, could result in the release of radioactive material into the air, a nuclear power plant would not explode like a nuclear weapon. A nuclear explosion caused by a nuclear bomb involves the joining or splitting of atoms (fusion and fission) to produce an intense pulse or wave of heat, light, air pressure, and radiation. A bomb case would produce a very destructive nuclear explosion, but not as large as that of a nuclear weapon developed for strategic military purposes.
In the 1980s, scientists evaluated the possible effects of nuclear war (many nuclear bombs explode in different parts of the world) and proposed the theory that a nuclear winter could occur. On the other hand, a nuclear explosion that occurs at or near the Earth's surface can cause serious pollution from radioactive fallout.