Buy Potassium Iodide if you want, it's cheap and doesn't hurt anything. Don't take it until you see the flash.
I still think it's pretty unlikely that the US gets nuked. Ukraine, maybe, but probably not us. Improbable =/= impossible though.
What this explosion does, I think, signify is that Ukrane is planning on taking the Crimean peninsula back, and is making sure they can cut it off from easy resupply from Russia.