A meltdown doesn't necessarily put any radioative particles into the air. If it did, that would be a very unusual event (such as Chernobyl). On the other hand, a bomb has a tendency to put an ultra-fine dust of uranium, various transuranics, and quite a few fission products into the atmosphere.

I suspect that you may be confusing a radioactive isotope (such as U235) with the particles associated with nuclear radiation (such as alpha and beta particles).

A meltdown will generally involve a lot of radioactive material. It will usually be localized and only very rarely pulverized into a dust. Chernobyl's steam explosion did this. The meltdown will probably also emit a lot of radiation, but that never "contaminates" the atmosphere, unless you consider electrons or helium "contamination".

Unfortunately, conflating radioactive isotopes with the radiation those isotopes emit when they decay is very common misunderstanding.