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

Most chemical disasters involve chemicals which are highly reactive in the environment. As such, they are almost immediately reacted to form less harmful chemicals than their original forms. Upon contact with water, some chemicals may also dissolve into concentrations which are not deemed as harmful to the humans or environment.

Also, chemicals tend to be more dense than air. As such, upon exposure, they sink to the bottom rather than remain in the atmosphere. This reduces the radius in which the chemical can spread as containment measures come into place.

In contrast, radiation from nuclear plants cannot be easily dispersed by the environment as they have a very long half life. As such, they are able to spread over large distances.

Furthermore, while containment measures for chemical accidents are usually straight-forward (through neutralisation, chemical reaction, precipitation etc), the only way to contain radiation is to physically contain them in lead barrels; an impossible task once a nuclear plant has leaked. That is why nuclear plants have a containment vessel made up of lead which will melt upon an accident so as to contain the nuclear waste.