Reactor Meltdown

The core wouldn’t explode if its turned of, the steam side of the reactor would explode, but the turbines are most likely some distance from the reactor. Let say that the core is split open when still running, then it would explode, sort of! Contaminated fuel rods would be in critical condition and still working after the explosion. The airborne contamination would normally be severe. There are some different types of reactors in use in the world and some are better built than others. With a quick enough warning, the engineers would cut the reactor of. Almost all (if not all) reactors have a safety measure that can shut down the reactor in seconds.

Regarding nuclear pollutants, as I see it there are three things to consider:

Inert Gas Isotopes. Gases that would most likely disperse immediately; and mixed particles like:

Fenon
Xeon
Krypton 14
and others

Contaminated Dust spread by the wind, mixed particles:

Jod 131
Cesium 137
and others

Place of the Meltdown (split core), half life in the thousands of years. It would probably spread and contaminate the locale soil hundreds of miles wide:

Plutonium 239, 240, 241
Uranium 235
Cesium 137

Offered by Geson.