Why do radiation disasters affect a larger area than chemical disaster?

Why do radiation disasters affect a larger area than chemical disaster?

A radiation disaster spreads gaseous radioactive particles in the atmosphere, which can travel far and wide due to wind. Even a few radioactive particles can cause severe radiation effects as they work by a chain reaction. In a chemical disaster, there is no chain reaction and it is limited only to the area where the chemical has spread. Due to this reason, a radiation disaster affects a larger area than a chemical disaster.