E9 Formation, deposition and fallout contamination

Topic

When the near-surface detonation—already described by the second model in this series—sucks soil and debris into the fireball, that material becomes incorporated into dust or ash particles that become radioactive; this is how fallout is created. The size of each particle determines how long it remains in the atmosphere before falling, and consequently how far it travels before settling: larger particles fall nearby within minutes or hours, while finer particles remain suspended and can travel much greater distances before reaching the ground.

Deposition occurs via two distinct pathways. Dry deposition is the gradual settling of particles under their own weight—a process more concentrated near the source and involving larger particles—whereas wet deposition occurs when these same particles are washed down by rain, which can concentrate contamination at greater distances and much more intensely than simple gravitational settling.

Once deposited, the material contaminates soil and bodies of water, entering the food chain through bioaccumulation: each link in the chain concentrates what it absorbed from the previous one, meaning an organism at a high trophic level can end up accumulating contamination levels far exceeding those found in its surrounding environment. For this reason, this model—which begins the moment the sucked-up soil is incorporated into the fireball—serves as the physical basis for determining, later in this series, which areas require prolonged sheltering, evacuation, or agricultural restrictions following an event.

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