The hazardous rain zone of a nuclear explosion can easily extend 10 to 20 miles (15 to 30 kilometers) from detonation, depending on explosive performance and weather conditions. The physical explosion effect is created by coupling immense amounts of energy, spanning the electromagnetic spectrum, with the environment. This energy is initially released in several forms of penetrating radiation, such as gamma radiation and X-rays. When there is surrounding material, such as air, rock, or water, this radiation interacts with the material and rapidly heats it to an equilibrium temperature.
This causes vaporization of the surrounding material, resulting in its rapid expansion and the formation of a shock wave that expands spherically from the center. Intense thermal radiation at the hypocenter forms a nuclear fireball, which is often associated with a mushroom-shaped cloud. Nuclear weapons also emit large amounts of thermal radiation in the form of visible, infrared and ultraviolet light. The main hazards are burns and eye injuries, which can occur well beyond the range of the explosion, depending on the performance of the weapon.
Fires can also be initiated by initial thermal radiation, but strong winds due to the shock wave can extinguish almost all such fires. Neutron radiation serves to transmute surrounding matter, often making it radioactive. This form of radioactive contamination is known as Nuclear Rain and represents the primary risk of exposure to ionizing radiation for a large nuclear weapon. The dangers of nuclear fallout are not limited to an increased risk of cancer and radiation sickness, but also include the presence of radionuclides in human organs from food.
For high-altitude nuclear explosions, electrons are captured in the Earth's magnetic field at altitudes between twenty and forty kilometers, where they interact with the Earth's magnetic field to produce a coherent nuclear electromagnetic pulse (NEMP). All nuclear explosions produce fission products, unfissioned nuclear material and weapon residues vaporized by the heat of the fireball. Radioactive fallout has occurred all over the world; for example, people have been exposed to iodine-131 from atmospheric nuclear tests. The consequences can also relate to nuclear accidents, even though a nuclear reactor does not explode like a nuclear weapon.
The 1963 Limited Nuclear-Test-Ban Treaty ended atmospheric testing for the United States, Great Britain and the Soviet Union, but two major non-signatories continued nuclear testing at a rate of approximately 5 megatons per year. Sensors may fail and results of lack of preventive measures would cause local nuclear fallout. A nuclear weapon detonated in the air produces less rain than a comparable explosion near the ground. Nuclear explosions also produce clouds of dust and sand-like radioactive particles that disperse into the atmosphere, known as nuclear rain.
Even in the midst of the Cold War, groundwater supplies such as aquifers would initially remain uncontaminated in the event of a nuclear fallout.