Well, just like any gas it first goes up and if it cools enough then it comes down.
IF on the other hand it does not cool down and gets hotter, then some of it may never come back to Earth.

I will refrain from explaining the physics of the disappearance or disassociation of H2O molecules at this moment due to the adverse reaction it may cause among the HZ population.

Instead I'll leave some links here that you might want to visit.
If you are really interested and intelligent, you might figure out what I am referring to.

Google search came up with a total of 355000 nuclear mishaps.
http://www.google.com/search?q=Nuclear+power+station+mishaps&hl=en&rls=DDCA,DDCA:2007-11,DDCA:en&start=10&sa=N

List of some accidents here:
http://www.marketingtheworld.com/nuclearpower/nuclearaccidents.php

More Nuclear accidents here:
http://archive.greenpeace.org/comms/nukes/chernob/rep02.html

http://www.cdi.org/Issues/NukeAccidents/accidents.htm

Some video clips of nuclear explosions here:
http://www.nv.doe.gov/library/films/film.aspx?ID=52

High-Altitude Nuclear Weapon Effects Part One
http://www.nv.doe.gov/library/films/film.aspx?ID=53

More info
http://en.wikipedia.org/wiki/Nuclear_testing

"It is safe to walk on all of the islands...The Advisory Group reaffirmed: although the residual radioactivity on islands in Bikini Atoll is still higher than on other atolls in the Marshall islands,

"Provided that certain remedial measures are taken, Bikini Island could be permanently reinhabited.