Controlled experiments reveal how nuclear fallout particles form‎

In less than a millionth of a second after a nuclear detonation or a severe nuclear reactor accident, an enormous burst of energy heats the surrounding air and materials. Everything in the vicinity is vaporized into a hot, glowing cloud of gas and plasma. As that nuclear fireball expands, it mixes with air, begins to cool and condenses into tiny solid particles—creating nuclear fallout.