When first detonated a nuclear bomb releases about 90% of its energy in less than one millionth of a second, mostly in the form of heat and shock waves.
There are very high concentrations of radiation in the blast areas. The force of the blast throws irradiated dust and debris as high as the stratosphere where it can remain in the air for years, where it will eventually fall to all areas of the Earth. Fallout at lower altitudes is carried by winds and falls back tothe ground relatively quickly, much of it within minutes. It is theorized that if enough dust and debris is thrown into the atmosphere in combination with the massive quantities of soot and smoke from burning cities and forests, sunlight could be blocked for weeks causing a “nuclear winter” in which fallout is constantly falling and temperatures fall dramatically around the world.
Radiation from fallout can affect large numbers of people even In countries not directly attacked with nuclear weapons. It has been estimated that if the U.S. were attacked with 1,000 nuclear bombs with a yield of 1 megaton, slightly over 1/3 of the U.S. population would die over time as a result of the fallout
Radiation from fallout could cause severe physical damage (many forms of cancer and body degeneration), genetic damage (birth defects and degenerative diseases), and mental and
physical development problems especially in infants and young children (weakness and mental retardation). Radiation from fallout could affect wild plants and animals as well as crops and livestock, causing mutations, defects, and death.
Fallout can contaminate soil and water, making them unsuitable for human use.
Large amounts of fallout and radioactivity spread over a large area (i.e. an entire region or the whole planet) would be nearly impossible to clean in any reasonable amount of time, especially
in a post-war world.