Nuclear fallout

The detonation of a nuclear weapon releases a tremendous amount of energy. While the majority of that released is blast and thermal energy, 15 percent of it is in the form of nuclear radiation. Nuclear radiation is categorized as initial or delayed, according to the time after detonation that it is produced.

Initial radiation, in the form of gamma rays, is produced within the first 60 seconds after detonation. Within that 60 seconds, the fireball reaches an estimated height of 2 miles, the estimated effective range of gamma rays in air. Delayed radiation is that produced after the first 60 seconds and includes induced radiation and fallout.

Process and product

The term "fallout" was used to describe radioactive material "falling out" of the mushroom cloud produced by the explosion of the first nuclear device, the TRINITY, detonated in Alamogordo, New Mexico, on July 16, 1946. Today, the term refers both to the process by which radioactively contaminated particles fall back to Earth and to the airborne, radioactively contaminated dust and debris. Fallout is classified as early fallout and delayed fallout.

Early fallout, that which returns to Earth within 24 hours of detonation, tends to be particles that have a high concentration of radioactivity and that vary from 0.01 to 1 centimeter. The particles travel a few hundred miles, with the larger particles returning closer than smaller particles to ground zero.

Delayed fallout, that which returns to Earth later than 24 hours after a detonation, consists of particles from approximately 0.01 centimeter to a few micrometers, or roughly from a fine sand to a very fine sand. Because they are smaller than early fallout particles, delayed fallout particles tend to contain less radioactivity. Their small size also means very little mass, causing delayed fallout particles to descend slowly, often reaching Earth at locations far from the point of the detonation. This makes delayed fallout a global concern.

Particle formation

The energy released during a nuclear detonation is greater than 4 billion BTU per kiloton yield. This energy elevates the temperature inside the fireball to phenomenally high levels, causing the materials trapped in the fireball to change almost instantaneously from a solid state to a plasma state.

The extreme temperatures cause the fireball to rise quickly and expand, engulfing and vaporizing tiny particles suspended in the environment. As the fireball rises, a vacuum is created, resulting in a tremendous updraft. The surrounding atmosphere fills the void created by the updraft, creating afterwinds.